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1========================
2ftrace - Function Tracer
3========================
4
5Copyright 2008 Red Hat Inc.
6
7:Author: Steven Rostedt <srostedt@redhat.com>
8:License: The GNU Free Documentation License, Version 1.2
9 (dual licensed under the GPL v2)
10:Original Reviewers: Elias Oltmanns, Randy Dunlap, Andrew Morton,
11 John Kacur, and David Teigland.
12
13- Written for: 2.6.28-rc2
14- Updated for: 3.10
15- Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt
16- Converted to rst format - Changbin Du <changbin.du@intel.com>
17
18Introduction
19------------
20
21Ftrace is an internal tracer designed to help out developers and
22designers of systems to find what is going on inside the kernel.
23It can be used for debugging or analyzing latencies and
24performance issues that take place outside of user-space.
25
26Although ftrace is typically considered the function tracer, it
27is really a framework of several assorted tracing utilities.
28There's latency tracing to examine what occurs between interrupts
29disabled and enabled, as well as for preemption and from a time
30a task is woken to the task is actually scheduled in.
31
32One of the most common uses of ftrace is the event tracing.
33Throughout the kernel is hundreds of static event points that
34can be enabled via the tracefs file system to see what is
35going on in certain parts of the kernel.
36
37See events.rst for more information.
38
39
40Implementation Details
41----------------------
42
43See Documentation/trace/ftrace-design.rst for details for arch porters and such.
44
45
46The File System
47---------------
48
49Ftrace uses the tracefs file system to hold the control files as
50well as the files to display output.
51
52When tracefs is configured into the kernel (which selecting any ftrace
53option will do) the directory /sys/kernel/tracing will be created. To mount
54this directory, you can add to your /etc/fstab file::
55
56 tracefs /sys/kernel/tracing tracefs defaults 0 0
57
58Or you can mount it at run time with::
59
60 mount -t tracefs nodev /sys/kernel/tracing
61
62For quicker access to that directory you may want to make a soft link to
63it::
64
65 ln -s /sys/kernel/tracing /tracing
66
67.. attention::
68
69 Before 4.1, all ftrace tracing control files were within the debugfs
70 file system, which is typically located at /sys/kernel/debug/tracing.
71 For backward compatibility, when mounting the debugfs file system,
72 the tracefs file system will be automatically mounted at:
73
74 /sys/kernel/debug/tracing
75
76 All files located in the tracefs file system will be located in that
77 debugfs file system directory as well.
78
79.. attention::
80
81 Any selected ftrace option will also create the tracefs file system.
82 The rest of the document will assume that you are in the ftrace directory
83 (cd /sys/kernel/tracing) and will only concentrate on the files within that
84 directory and not distract from the content with the extended
85 "/sys/kernel/tracing" path name.
86
87That's it! (assuming that you have ftrace configured into your kernel)
88
89After mounting tracefs you will have access to the control and output files
90of ftrace. Here is a list of some of the key files:
91
92
93 Note: all time values are in microseconds.
94
95 current_tracer:
96
97 This is used to set or display the current tracer
98 that is configured. Changing the current tracer clears
99 the ring buffer content as well as the "snapshot" buffer.
100
101 available_tracers:
102
103 This holds the different types of tracers that
104 have been compiled into the kernel. The
105 tracers listed here can be configured by
106 echoing their name into current_tracer.
107
108 tracing_on:
109
110 This sets or displays whether writing to the trace
111 ring buffer is enabled. Echo 0 into this file to disable
112 the tracer or 1 to enable it. Note, this only disables
113 writing to the ring buffer, the tracing overhead may
114 still be occurring.
115
116 The kernel function tracing_off() can be used within the
117 kernel to disable writing to the ring buffer, which will
118 set this file to "0". User space can re-enable tracing by
119 echoing "1" into the file.
120
121 Note, the function and event trigger "traceoff" will also
122 set this file to zero and stop tracing. Which can also
123 be re-enabled by user space using this file.
124
125 trace:
126
127 This file holds the output of the trace in a human
128 readable format (described below). Opening this file for
129 writing with the O_TRUNC flag clears the ring buffer content.
130 Note, this file is not a consumer. If tracing is off
131 (no tracer running, or tracing_on is zero), it will produce
132 the same output each time it is read. When tracing is on,
133 it may produce inconsistent results as it tries to read
134 the entire buffer without consuming it.
135
136 trace_pipe:
137
138 The output is the same as the "trace" file but this
139 file is meant to be streamed with live tracing.
140 Reads from this file will block until new data is
141 retrieved. Unlike the "trace" file, this file is a
142 consumer. This means reading from this file causes
143 sequential reads to display more current data. Once
144 data is read from this file, it is consumed, and
145 will not be read again with a sequential read. The
146 "trace" file is static, and if the tracer is not
147 adding more data, it will display the same
148 information every time it is read.
149
150 trace_options:
151
152 This file lets the user control the amount of data
153 that is displayed in one of the above output
154 files. Options also exist to modify how a tracer
155 or events work (stack traces, timestamps, etc).
156
157 options:
158
159 This is a directory that has a file for every available
160 trace option (also in trace_options). Options may also be set
161 or cleared by writing a "1" or "0" respectively into the
162 corresponding file with the option name.
163
164 tracing_max_latency:
165
166 Some of the tracers record the max latency.
167 For example, the maximum time that interrupts are disabled.
168 The maximum time is saved in this file. The max trace will also be
169 stored, and displayed by "trace". A new max trace will only be
170 recorded if the latency is greater than the value in this file
171 (in microseconds).
172
173 By echoing in a time into this file, no latency will be recorded
174 unless it is greater than the time in this file.
175
176 tracing_thresh:
177
178 Some latency tracers will record a trace whenever the
179 latency is greater than the number in this file.
180 Only active when the file contains a number greater than 0.
181 (in microseconds)
182
183 buffer_percent:
184
185 This is the watermark for how much the ring buffer needs to be filled
186 before a waiter is woken up. That is, if an application calls a
187 blocking read syscall on one of the per_cpu trace_pipe_raw files, it
188 will block until the given amount of data specified by buffer_percent
189 is in the ring buffer before it wakes the reader up. This also
190 controls how the splice system calls are blocked on this file::
191
192 0 - means to wake up as soon as there is any data in the ring buffer.
193 50 - means to wake up when roughly half of the ring buffer sub-buffers
194 are full.
195 100 - means to block until the ring buffer is totally full and is
196 about to start overwriting the older data.
197
198 buffer_size_kb:
199
200 This sets or displays the number of kilobytes each CPU
201 buffer holds. By default, the trace buffers are the same size
202 for each CPU. The displayed number is the size of the
203 CPU buffer and not total size of all buffers. The
204 trace buffers are allocated in pages (blocks of memory
205 that the kernel uses for allocation, usually 4 KB in size).
206 A few extra pages may be allocated to accommodate buffer management
207 meta-data. If the last page allocated has room for more bytes
208 than requested, the rest of the page will be used,
209 making the actual allocation bigger than requested or shown.
210 ( Note, the size may not be a multiple of the page size
211 due to buffer management meta-data. )
212
213 Buffer sizes for individual CPUs may vary
214 (see "per_cpu/cpu0/buffer_size_kb" below), and if they do
215 this file will show "X".
216
217 buffer_total_size_kb:
218
219 This displays the total combined size of all the trace buffers.
220
221 buffer_subbuf_size_kb:
222
223 This sets or displays the sub buffer size. The ring buffer is broken up
224 into several same size "sub buffers". An event can not be bigger than
225 the size of the sub buffer. Normally, the sub buffer is the size of the
226 architecture's page (4K on x86). The sub buffer also contains meta data
227 at the start which also limits the size of an event. That means when
228 the sub buffer is a page size, no event can be larger than the page
229 size minus the sub buffer meta data.
230
231 Note, the buffer_subbuf_size_kb is a way for the user to specify the
232 minimum size of the subbuffer. The kernel may make it bigger due to the
233 implementation details, or simply fail the operation if the kernel can
234 not handle the request.
235
236 Changing the sub buffer size allows for events to be larger than the
237 page size.
238
239 Note: When changing the sub-buffer size, tracing is stopped and any
240 data in the ring buffer and the snapshot buffer will be discarded.
241
242 free_buffer:
243
244 If a process is performing tracing, and the ring buffer should be
245 shrunk "freed" when the process is finished, even if it were to be
246 killed by a signal, this file can be used for that purpose. On close
247 of this file, the ring buffer will be resized to its minimum size.
248 Having a process that is tracing also open this file, when the process
249 exits its file descriptor for this file will be closed, and in doing so,
250 the ring buffer will be "freed".
251
252 It may also stop tracing if disable_on_free option is set.
253
254 tracing_cpumask:
255
256 This is a mask that lets the user only trace on specified CPUs.
257 The format is a hex string representing the CPUs.
258
259 set_ftrace_filter:
260
261 When dynamic ftrace is configured in (see the
262 section below "dynamic ftrace"), the code is dynamically
263 modified (code text rewrite) to disable calling of the
264 function profiler (mcount). This lets tracing be configured
265 in with practically no overhead in performance. This also
266 has a side effect of enabling or disabling specific functions
267 to be traced. Echoing names of functions into this file
268 will limit the trace to only those functions.
269 This influences the tracers "function" and "function_graph"
270 and thus also function profiling (see "function_profile_enabled").
271
272 The functions listed in "available_filter_functions" are what
273 can be written into this file.
274
275 This interface also allows for commands to be used. See the
276 "Filter commands" section for more details.
277
278 As a speed up, since processing strings can be quite expensive
279 and requires a check of all functions registered to tracing, instead
280 an index can be written into this file. A number (starting with "1")
281 written will instead select the same corresponding at the line position
282 of the "available_filter_functions" file.
283
284 set_ftrace_notrace:
285
286 This has an effect opposite to that of
287 set_ftrace_filter. Any function that is added here will not
288 be traced. If a function exists in both set_ftrace_filter
289 and set_ftrace_notrace, the function will _not_ be traced.
290
291 set_ftrace_pid:
292
293 Have the function tracer only trace the threads whose PID are
294 listed in this file.
295
296 If the "function-fork" option is set, then when a task whose
297 PID is listed in this file forks, the child's PID will
298 automatically be added to this file, and the child will be
299 traced by the function tracer as well. This option will also
300 cause PIDs of tasks that exit to be removed from the file.
301
302 set_ftrace_notrace_pid:
303
304 Have the function tracer ignore threads whose PID are listed in
305 this file.
306
307 If the "function-fork" option is set, then when a task whose
308 PID is listed in this file forks, the child's PID will
309 automatically be added to this file, and the child will not be
310 traced by the function tracer as well. This option will also
311 cause PIDs of tasks that exit to be removed from the file.
312
313 If a PID is in both this file and "set_ftrace_pid", then this
314 file takes precedence, and the thread will not be traced.
315
316 set_event_pid:
317
318 Have the events only trace a task with a PID listed in this file.
319 Note, sched_switch and sched_wake_up will also trace events
320 listed in this file.
321
322 To have the PIDs of children of tasks with their PID in this file
323 added on fork, enable the "event-fork" option. That option will also
324 cause the PIDs of tasks to be removed from this file when the task
325 exits.
326
327 set_event_notrace_pid:
328
329 Have the events not trace a task with a PID listed in this file.
330 Note, sched_switch and sched_wakeup will trace threads not listed
331 in this file, even if a thread's PID is in the file if the
332 sched_switch or sched_wakeup events also trace a thread that should
333 be traced.
334
335 To have the PIDs of children of tasks with their PID in this file
336 added on fork, enable the "event-fork" option. That option will also
337 cause the PIDs of tasks to be removed from this file when the task
338 exits.
339
340 set_graph_function:
341
342 Functions listed in this file will cause the function graph
343 tracer to only trace these functions and the functions that
344 they call. (See the section "dynamic ftrace" for more details).
345 Note, set_ftrace_filter and set_ftrace_notrace still affects
346 what functions are being traced.
347
348 set_graph_notrace:
349
350 Similar to set_graph_function, but will disable function graph
351 tracing when the function is hit until it exits the function.
352 This makes it possible to ignore tracing functions that are called
353 by a specific function.
354
355 available_filter_functions:
356
357 This lists the functions that ftrace has processed and can trace.
358 These are the function names that you can pass to
359 "set_ftrace_filter", "set_ftrace_notrace",
360 "set_graph_function", or "set_graph_notrace".
361 (See the section "dynamic ftrace" below for more details.)
362
363 available_filter_functions_addrs:
364
365 Similar to available_filter_functions, but with address displayed
366 for each function. The displayed address is the patch-site address
367 and can differ from /proc/kallsyms address.
368
369 dyn_ftrace_total_info:
370
371 This file is for debugging purposes. The number of functions that
372 have been converted to nops and are available to be traced.
373
374 enabled_functions:
375
376 This file is more for debugging ftrace, but can also be useful
377 in seeing if any function has a callback attached to it.
378 Not only does the trace infrastructure use ftrace function
379 trace utility, but other subsystems might too. This file
380 displays all functions that have a callback attached to them
381 as well as the number of callbacks that have been attached.
382 Note, a callback may also call multiple functions which will
383 not be listed in this count.
384
385 If the callback registered to be traced by a function with
386 the "save regs" attribute (thus even more overhead), a 'R'
387 will be displayed on the same line as the function that
388 is returning registers.
389
390 If the callback registered to be traced by a function with
391 the "ip modify" attribute (thus the regs->ip can be changed),
392 an 'I' will be displayed on the same line as the function that
393 can be overridden.
394
395 If a non ftrace trampoline is attached (BPF) a 'D' will be displayed.
396 Note, normal ftrace trampolines can also be attached, but only one
397 "direct" trampoline can be attached to a given function at a time.
398
399 Some architectures can not call direct trampolines, but instead have
400 the ftrace ops function located above the function entry point. In
401 such cases an 'O' will be displayed.
402
403 If a function had either the "ip modify" or a "direct" call attached to
404 it in the past, a 'M' will be shown. This flag is never cleared. It is
405 used to know if a function was every modified by the ftrace infrastructure,
406 and can be used for debugging.
407
408 If the architecture supports it, it will also show what callback
409 is being directly called by the function. If the count is greater
410 than 1 it most likely will be ftrace_ops_list_func().
411
412 If the callback of a function jumps to a trampoline that is
413 specific to the callback and which is not the standard trampoline,
414 its address will be printed as well as the function that the
415 trampoline calls.
416
417 touched_functions:
418
419 This file contains all the functions that ever had a function callback
420 to it via the ftrace infrastructure. It has the same format as
421 enabled_functions but shows all functions that have every been
422 traced.
423
424 To see any function that has every been modified by "ip modify" or a
425 direct trampoline, one can perform the following command:
426
427 grep ' M ' /sys/kernel/tracing/touched_functions
428
429 function_profile_enabled:
430
431 When set it will enable all functions with either the function
432 tracer, or if configured, the function graph tracer. It will
433 keep a histogram of the number of functions that were called
434 and if the function graph tracer was configured, it will also keep
435 track of the time spent in those functions. The histogram
436 content can be displayed in the files:
437
438 trace_stat/function<cpu> ( function0, function1, etc).
439
440 trace_stat:
441
442 A directory that holds different tracing stats.
443
444 kprobe_events:
445
446 Enable dynamic trace points. See kprobetrace.rst.
447
448 kprobe_profile:
449
450 Dynamic trace points stats. See kprobetrace.rst.
451
452 max_graph_depth:
453
454 Used with the function graph tracer. This is the max depth
455 it will trace into a function. Setting this to a value of
456 one will show only the first kernel function that is called
457 from user space.
458
459 printk_formats:
460
461 This is for tools that read the raw format files. If an event in
462 the ring buffer references a string, only a pointer to the string
463 is recorded into the buffer and not the string itself. This prevents
464 tools from knowing what that string was. This file displays the string
465 and address for the string allowing tools to map the pointers to what
466 the strings were.
467
468 saved_cmdlines:
469
470 Only the pid of the task is recorded in a trace event unless
471 the event specifically saves the task comm as well. Ftrace
472 makes a cache of pid mappings to comms to try to display
473 comms for events. If a pid for a comm is not listed, then
474 "<...>" is displayed in the output.
475
476 If the option "record-cmd" is set to "0", then comms of tasks
477 will not be saved during recording. By default, it is enabled.
478
479 saved_cmdlines_size:
480
481 By default, 128 comms are saved (see "saved_cmdlines" above). To
482 increase or decrease the amount of comms that are cached, echo
483 the number of comms to cache into this file.
484
485 saved_tgids:
486
487 If the option "record-tgid" is set, on each scheduling context switch
488 the Task Group ID of a task is saved in a table mapping the PID of
489 the thread to its TGID. By default, the "record-tgid" option is
490 disabled.
491
492 snapshot:
493
494 This displays the "snapshot" buffer and also lets the user
495 take a snapshot of the current running trace.
496 See the "Snapshot" section below for more details.
497
498 stack_max_size:
499
500 When the stack tracer is activated, this will display the
501 maximum stack size it has encountered.
502 See the "Stack Trace" section below.
503
504 stack_trace:
505
506 This displays the stack back trace of the largest stack
507 that was encountered when the stack tracer is activated.
508 See the "Stack Trace" section below.
509
510 stack_trace_filter:
511
512 This is similar to "set_ftrace_filter" but it limits what
513 functions the stack tracer will check.
514
515 trace_clock:
516
517 Whenever an event is recorded into the ring buffer, a
518 "timestamp" is added. This stamp comes from a specified
519 clock. By default, ftrace uses the "local" clock. This
520 clock is very fast and strictly per cpu, but on some
521 systems it may not be monotonic with respect to other
522 CPUs. In other words, the local clocks may not be in sync
523 with local clocks on other CPUs.
524
525 Usual clocks for tracing::
526
527 # cat trace_clock
528 [local] global counter x86-tsc
529
530 The clock with the square brackets around it is the one in effect.
531
532 local:
533 Default clock, but may not be in sync across CPUs
534
535 global:
536 This clock is in sync with all CPUs but may
537 be a bit slower than the local clock.
538
539 counter:
540 This is not a clock at all, but literally an atomic
541 counter. It counts up one by one, but is in sync
542 with all CPUs. This is useful when you need to
543 know exactly the order events occurred with respect to
544 each other on different CPUs.
545
546 uptime:
547 This uses the jiffies counter and the time stamp
548 is relative to the time since boot up.
549
550 perf:
551 This makes ftrace use the same clock that perf uses.
552 Eventually perf will be able to read ftrace buffers
553 and this will help out in interleaving the data.
554
555 x86-tsc:
556 Architectures may define their own clocks. For
557 example, x86 uses its own TSC cycle clock here.
558
559 ppc-tb:
560 This uses the powerpc timebase register value.
561 This is in sync across CPUs and can also be used
562 to correlate events across hypervisor/guest if
563 tb_offset is known.
564
565 mono:
566 This uses the fast monotonic clock (CLOCK_MONOTONIC)
567 which is monotonic and is subject to NTP rate adjustments.
568
569 mono_raw:
570 This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
571 which is monotonic but is not subject to any rate adjustments
572 and ticks at the same rate as the hardware clocksource.
573
574 boot:
575 This is the boot clock (CLOCK_BOOTTIME) and is based on the
576 fast monotonic clock, but also accounts for time spent in
577 suspend. Since the clock access is designed for use in
578 tracing in the suspend path, some side effects are possible
579 if clock is accessed after the suspend time is accounted before
580 the fast mono clock is updated. In this case, the clock update
581 appears to happen slightly sooner than it normally would have.
582 Also on 32-bit systems, it's possible that the 64-bit boot offset
583 sees a partial update. These effects are rare and post
584 processing should be able to handle them. See comments in the
585 ktime_get_boot_fast_ns() function for more information.
586
587 tai:
588 This is the tai clock (CLOCK_TAI) and is derived from the wall-
589 clock time. However, this clock does not experience
590 discontinuities and backwards jumps caused by NTP inserting leap
591 seconds. Since the clock access is designed for use in tracing,
592 side effects are possible. The clock access may yield wrong
593 readouts in case the internal TAI offset is updated e.g., caused
594 by setting the system time or using adjtimex() with an offset.
595 These effects are rare and post processing should be able to
596 handle them. See comments in the ktime_get_tai_fast_ns()
597 function for more information.
598
599 To set a clock, simply echo the clock name into this file::
600
601 # echo global > trace_clock
602
603 Setting a clock clears the ring buffer content as well as the
604 "snapshot" buffer.
605
606 trace_marker:
607
608 This is a very useful file for synchronizing user space
609 with events happening in the kernel. Writing strings into
610 this file will be written into the ftrace buffer.
611
612 It is useful in applications to open this file at the start
613 of the application and just reference the file descriptor
614 for the file::
615
616 void trace_write(const char *fmt, ...)
617 {
618 va_list ap;
619 char buf[256];
620 int n;
621
622 if (trace_fd < 0)
623 return;
624
625 va_start(ap, fmt);
626 n = vsnprintf(buf, 256, fmt, ap);
627 va_end(ap);
628
629 write(trace_fd, buf, n);
630 }
631
632 start::
633
634 trace_fd = open("trace_marker", O_WRONLY);
635
636 Note: Writing into the trace_marker file can also initiate triggers
637 that are written into /sys/kernel/tracing/events/ftrace/print/trigger
638 See "Event triggers" in Documentation/trace/events.rst and an
639 example in Documentation/trace/histogram.rst (Section 3.)
640
641 trace_marker_raw:
642
643 This is similar to trace_marker above, but is meant for binary data
644 to be written to it, where a tool can be used to parse the data
645 from trace_pipe_raw.
646
647 uprobe_events:
648
649 Add dynamic tracepoints in programs.
650 See uprobetracer.rst
651
652 uprobe_profile:
653
654 Uprobe statistics. See uprobetrace.txt
655
656 instances:
657
658 This is a way to make multiple trace buffers where different
659 events can be recorded in different buffers.
660 See "Instances" section below.
661
662 events:
663
664 This is the trace event directory. It holds event tracepoints
665 (also known as static tracepoints) that have been compiled
666 into the kernel. It shows what event tracepoints exist
667 and how they are grouped by system. There are "enable"
668 files at various levels that can enable the tracepoints
669 when a "1" is written to them.
670
671 See events.rst for more information.
672
673 set_event:
674
675 By echoing in the event into this file, will enable that event.
676
677 See events.rst for more information.
678
679 available_events:
680
681 A list of events that can be enabled in tracing.
682
683 See events.rst for more information.
684
685 timestamp_mode:
686
687 Certain tracers may change the timestamp mode used when
688 logging trace events into the event buffer. Events with
689 different modes can coexist within a buffer but the mode in
690 effect when an event is logged determines which timestamp mode
691 is used for that event. The default timestamp mode is
692 'delta'.
693
694 Usual timestamp modes for tracing:
695
696 # cat timestamp_mode
697 [delta] absolute
698
699 The timestamp mode with the square brackets around it is the
700 one in effect.
701
702 delta: Default timestamp mode - timestamp is a delta against
703 a per-buffer timestamp.
704
705 absolute: The timestamp is a full timestamp, not a delta
706 against some other value. As such it takes up more
707 space and is less efficient.
708
709 hwlat_detector:
710
711 Directory for the Hardware Latency Detector.
712 See "Hardware Latency Detector" section below.
713
714 per_cpu:
715
716 This is a directory that contains the trace per_cpu information.
717
718 per_cpu/cpu0/buffer_size_kb:
719
720 The ftrace buffer is defined per_cpu. That is, there's a separate
721 buffer for each CPU to allow writes to be done atomically,
722 and free from cache bouncing. These buffers may have different
723 size buffers. This file is similar to the buffer_size_kb
724 file, but it only displays or sets the buffer size for the
725 specific CPU. (here cpu0).
726
727 per_cpu/cpu0/trace:
728
729 This is similar to the "trace" file, but it will only display
730 the data specific for the CPU. If written to, it only clears
731 the specific CPU buffer.
732
733 per_cpu/cpu0/trace_pipe
734
735 This is similar to the "trace_pipe" file, and is a consuming
736 read, but it will only display (and consume) the data specific
737 for the CPU.
738
739 per_cpu/cpu0/trace_pipe_raw
740
741 For tools that can parse the ftrace ring buffer binary format,
742 the trace_pipe_raw file can be used to extract the data
743 from the ring buffer directly. With the use of the splice()
744 system call, the buffer data can be quickly transferred to
745 a file or to the network where a server is collecting the
746 data.
747
748 Like trace_pipe, this is a consuming reader, where multiple
749 reads will always produce different data.
750
751 per_cpu/cpu0/snapshot:
752
753 This is similar to the main "snapshot" file, but will only
754 snapshot the current CPU (if supported). It only displays
755 the content of the snapshot for a given CPU, and if
756 written to, only clears this CPU buffer.
757
758 per_cpu/cpu0/snapshot_raw:
759
760 Similar to the trace_pipe_raw, but will read the binary format
761 from the snapshot buffer for the given CPU.
762
763 per_cpu/cpu0/stats:
764
765 This displays certain stats about the ring buffer:
766
767 entries:
768 The number of events that are still in the buffer.
769
770 overrun:
771 The number of lost events due to overwriting when
772 the buffer was full.
773
774 commit overrun:
775 Should always be zero.
776 This gets set if so many events happened within a nested
777 event (ring buffer is re-entrant), that it fills the
778 buffer and starts dropping events.
779
780 bytes:
781 Bytes actually read (not overwritten).
782
783 oldest event ts:
784 The oldest timestamp in the buffer
785
786 now ts:
787 The current timestamp
788
789 dropped events:
790 Events lost due to overwrite option being off.
791
792 read events:
793 The number of events read.
794
795The Tracers
796-----------
797
798Here is the list of current tracers that may be configured.
799
800 "function"
801
802 Function call tracer to trace all kernel functions.
803
804 "function_graph"
805
806 Similar to the function tracer except that the
807 function tracer probes the functions on their entry
808 whereas the function graph tracer traces on both entry
809 and exit of the functions. It then provides the ability
810 to draw a graph of function calls similar to C code
811 source.
812
813 "blk"
814
815 The block tracer. The tracer used by the blktrace user
816 application.
817
818 "hwlat"
819
820 The Hardware Latency tracer is used to detect if the hardware
821 produces any latency. See "Hardware Latency Detector" section
822 below.
823
824 "irqsoff"
825
826 Traces the areas that disable interrupts and saves
827 the trace with the longest max latency.
828 See tracing_max_latency. When a new max is recorded,
829 it replaces the old trace. It is best to view this
830 trace with the latency-format option enabled, which
831 happens automatically when the tracer is selected.
832
833 "preemptoff"
834
835 Similar to irqsoff but traces and records the amount of
836 time for which preemption is disabled.
837
838 "preemptirqsoff"
839
840 Similar to irqsoff and preemptoff, but traces and
841 records the largest time for which irqs and/or preemption
842 is disabled.
843
844 "wakeup"
845
846 Traces and records the max latency that it takes for
847 the highest priority task to get scheduled after
848 it has been woken up.
849 Traces all tasks as an average developer would expect.
850
851 "wakeup_rt"
852
853 Traces and records the max latency that it takes for just
854 RT tasks (as the current "wakeup" does). This is useful
855 for those interested in wake up timings of RT tasks.
856
857 "wakeup_dl"
858
859 Traces and records the max latency that it takes for
860 a SCHED_DEADLINE task to be woken (as the "wakeup" and
861 "wakeup_rt" does).
862
863 "mmiotrace"
864
865 A special tracer that is used to trace binary module.
866 It will trace all the calls that a module makes to the
867 hardware. Everything it writes and reads from the I/O
868 as well.
869
870 "branch"
871
872 This tracer can be configured when tracing likely/unlikely
873 calls within the kernel. It will trace when a likely and
874 unlikely branch is hit and if it was correct in its prediction
875 of being correct.
876
877 "nop"
878
879 This is the "trace nothing" tracer. To remove all
880 tracers from tracing simply echo "nop" into
881 current_tracer.
882
883Error conditions
884----------------
885
886 For most ftrace commands, failure modes are obvious and communicated
887 using standard return codes.
888
889 For other more involved commands, extended error information may be
890 available via the tracing/error_log file. For the commands that
891 support it, reading the tracing/error_log file after an error will
892 display more detailed information about what went wrong, if
893 information is available. The tracing/error_log file is a circular
894 error log displaying a small number (currently, 8) of ftrace errors
895 for the last (8) failed commands.
896
897 The extended error information and usage takes the form shown in
898 this example::
899
900 # echo xxx > /sys/kernel/tracing/events/sched/sched_wakeup/trigger
901 echo: write error: Invalid argument
902
903 # cat /sys/kernel/tracing/error_log
904 [ 5348.887237] location: error: Couldn't yyy: zzz
905 Command: xxx
906 ^
907 [ 7517.023364] location: error: Bad rrr: sss
908 Command: ppp qqq
909 ^
910
911 To clear the error log, echo the empty string into it::
912
913 # echo > /sys/kernel/tracing/error_log
914
915Examples of using the tracer
916----------------------------
917
918Here are typical examples of using the tracers when controlling
919them only with the tracefs interface (without using any
920user-land utilities).
921
922Output format:
923--------------
924
925Here is an example of the output format of the file "trace"::
926
927 # tracer: function
928 #
929 # entries-in-buffer/entries-written: 140080/250280 #P:4
930 #
931 # _-----=> irqs-off
932 # / _----=> need-resched
933 # | / _---=> hardirq/softirq
934 # || / _--=> preempt-depth
935 # ||| / delay
936 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
937 # | | | |||| | |
938 bash-1977 [000] .... 17284.993652: sys_close <-system_call_fastpath
939 bash-1977 [000] .... 17284.993653: __close_fd <-sys_close
940 bash-1977 [000] .... 17284.993653: _raw_spin_lock <-__close_fd
941 sshd-1974 [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
942 bash-1977 [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
943 bash-1977 [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
944 bash-1977 [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
945 bash-1977 [000] .... 17284.993657: filp_close <-__close_fd
946 bash-1977 [000] .... 17284.993657: dnotify_flush <-filp_close
947 sshd-1974 [003] .... 17284.993658: sys_select <-system_call_fastpath
948 ....
949
950A header is printed with the tracer name that is represented by
951the trace. In this case the tracer is "function". Then it shows the
952number of events in the buffer as well as the total number of entries
953that were written. The difference is the number of entries that were
954lost due to the buffer filling up (250280 - 140080 = 110200 events
955lost).
956
957The header explains the content of the events. Task name "bash", the task
958PID "1977", the CPU that it was running on "000", the latency format
959(explained below), the timestamp in <secs>.<usecs> format, the
960function name that was traced "sys_close" and the parent function that
961called this function "system_call_fastpath". The timestamp is the time
962at which the function was entered.
963
964Latency trace format
965--------------------
966
967When the latency-format option is enabled or when one of the latency
968tracers is set, the trace file gives somewhat more information to see
969why a latency happened. Here is a typical trace::
970
971 # tracer: irqsoff
972 #
973 # irqsoff latency trace v1.1.5 on 3.8.0-test+
974 # --------------------------------------------------------------------
975 # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
976 # -----------------
977 # | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
978 # -----------------
979 # => started at: __lock_task_sighand
980 # => ended at: _raw_spin_unlock_irqrestore
981 #
982 #
983 # _------=> CPU#
984 # / _-----=> irqs-off
985 # | / _----=> need-resched
986 # || / _---=> hardirq/softirq
987 # ||| / _--=> preempt-depth
988 # |||| / delay
989 # cmd pid ||||| time | caller
990 # \ / ||||| \ | /
991 ps-6143 2d... 0us!: trace_hardirqs_off <-__lock_task_sighand
992 ps-6143 2d..1 259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
993 ps-6143 2d..1 263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
994 ps-6143 2d..1 306us : <stack trace>
995 => trace_hardirqs_on_caller
996 => trace_hardirqs_on
997 => _raw_spin_unlock_irqrestore
998 => do_task_stat
999 => proc_tgid_stat
1000 => proc_single_show
1001 => seq_read
1002 => vfs_read
1003 => sys_read
1004 => system_call_fastpath
1005
1006
1007This shows that the current tracer is "irqsoff" tracing the time
1008for which interrupts were disabled. It gives the trace version (which
1009never changes) and the version of the kernel upon which this was executed on
1010(3.8). Then it displays the max latency in microseconds (259 us). The number
1011of trace entries displayed and the total number (both are four: #4/4).
1012VP, KP, SP, and HP are always zero and are reserved for later use.
1013#P is the number of online CPUs (#P:4).
1014
1015The task is the process that was running when the latency
1016occurred. (ps pid: 6143).
1017
1018The start and stop (the functions in which the interrupts were
1019disabled and enabled respectively) that caused the latencies:
1020
1021 - __lock_task_sighand is where the interrupts were disabled.
1022 - _raw_spin_unlock_irqrestore is where they were enabled again.
1023
1024The next lines after the header are the trace itself. The header
1025explains which is which.
1026
1027 cmd: The name of the process in the trace.
1028
1029 pid: The PID of that process.
1030
1031 CPU#: The CPU which the process was running on.
1032
1033 irqs-off: 'd' interrupts are disabled. '.' otherwise.
1034 .. caution:: If the architecture does not support a way to
1035 read the irq flags variable, an 'X' will always
1036 be printed here.
1037
1038 need-resched:
1039 - 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
1040 - 'n' only TIF_NEED_RESCHED is set,
1041 - 'p' only PREEMPT_NEED_RESCHED is set,
1042 - '.' otherwise.
1043
1044 hardirq/softirq:
1045 - 'Z' - NMI occurred inside a hardirq
1046 - 'z' - NMI is running
1047 - 'H' - hard irq occurred inside a softirq.
1048 - 'h' - hard irq is running
1049 - 's' - soft irq is running
1050 - '.' - normal context.
1051
1052 preempt-depth: The level of preempt_disabled
1053
1054The above is mostly meaningful for kernel developers.
1055
1056 time:
1057 When the latency-format option is enabled, the trace file
1058 output includes a timestamp relative to the start of the
1059 trace. This differs from the output when latency-format
1060 is disabled, which includes an absolute timestamp.
1061
1062 delay:
1063 This is just to help catch your eye a bit better. And
1064 needs to be fixed to be only relative to the same CPU.
1065 The marks are determined by the difference between this
1066 current trace and the next trace.
1067
1068 - '$' - greater than 1 second
1069 - '@' - greater than 100 millisecond
1070 - '*' - greater than 10 millisecond
1071 - '#' - greater than 1000 microsecond
1072 - '!' - greater than 100 microsecond
1073 - '+' - greater than 10 microsecond
1074 - ' ' - less than or equal to 10 microsecond.
1075
1076 The rest is the same as the 'trace' file.
1077
1078 Note, the latency tracers will usually end with a back trace
1079 to easily find where the latency occurred.
1080
1081trace_options
1082-------------
1083
1084The trace_options file (or the options directory) is used to control
1085what gets printed in the trace output, or manipulate the tracers.
1086To see what is available, simply cat the file::
1087
1088 cat trace_options
1089 print-parent
1090 nosym-offset
1091 nosym-addr
1092 noverbose
1093 noraw
1094 nohex
1095 nobin
1096 noblock
1097 nofields
1098 trace_printk
1099 annotate
1100 nouserstacktrace
1101 nosym-userobj
1102 noprintk-msg-only
1103 context-info
1104 nolatency-format
1105 record-cmd
1106 norecord-tgid
1107 overwrite
1108 nodisable_on_free
1109 irq-info
1110 markers
1111 noevent-fork
1112 function-trace
1113 nofunction-fork
1114 nodisplay-graph
1115 nostacktrace
1116 nobranch
1117
1118To disable one of the options, echo in the option prepended with
1119"no"::
1120
1121 echo noprint-parent > trace_options
1122
1123To enable an option, leave off the "no"::
1124
1125 echo sym-offset > trace_options
1126
1127Here are the available options:
1128
1129 print-parent
1130 On function traces, display the calling (parent)
1131 function as well as the function being traced.
1132 ::
1133
1134 print-parent:
1135 bash-4000 [01] 1477.606694: simple_strtoul <-kstrtoul
1136
1137 noprint-parent:
1138 bash-4000 [01] 1477.606694: simple_strtoul
1139
1140
1141 sym-offset
1142 Display not only the function name, but also the
1143 offset in the function. For example, instead of
1144 seeing just "ktime_get", you will see
1145 "ktime_get+0xb/0x20".
1146 ::
1147
1148 sym-offset:
1149 bash-4000 [01] 1477.606694: simple_strtoul+0x6/0xa0
1150
1151 sym-addr
1152 This will also display the function address as well
1153 as the function name.
1154 ::
1155
1156 sym-addr:
1157 bash-4000 [01] 1477.606694: simple_strtoul <c0339346>
1158
1159 verbose
1160 This deals with the trace file when the
1161 latency-format option is enabled.
1162 ::
1163
1164 bash 4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
1165 (+0.000ms): simple_strtoul (kstrtoul)
1166
1167 raw
1168 This will display raw numbers. This option is best for
1169 use with user applications that can translate the raw
1170 numbers better than having it done in the kernel.
1171
1172 hex
1173 Similar to raw, but the numbers will be in a hexadecimal format.
1174
1175 bin
1176 This will print out the formats in raw binary.
1177
1178 block
1179 When set, reading trace_pipe will not block when polled.
1180
1181 fields
1182 Print the fields as described by their types. This is a better
1183 option than using hex, bin or raw, as it gives a better parsing
1184 of the content of the event.
1185
1186 trace_printk
1187 Can disable trace_printk() from writing into the buffer.
1188
1189 annotate
1190 It is sometimes confusing when the CPU buffers are full
1191 and one CPU buffer had a lot of events recently, thus
1192 a shorter time frame, were another CPU may have only had
1193 a few events, which lets it have older events. When
1194 the trace is reported, it shows the oldest events first,
1195 and it may look like only one CPU ran (the one with the
1196 oldest events). When the annotate option is set, it will
1197 display when a new CPU buffer started::
1198
1199 <idle>-0 [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
1200 <idle>-0 [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
1201 <idle>-0 [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
1202 ##### CPU 2 buffer started ####
1203 <idle>-0 [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
1204 <idle>-0 [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
1205 <idle>-0 [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
1206
1207 userstacktrace
1208 This option changes the trace. It records a
1209 stacktrace of the current user space thread after
1210 each trace event.
1211
1212 sym-userobj
1213 when user stacktrace are enabled, look up which
1214 object the address belongs to, and print a
1215 relative address. This is especially useful when
1216 ASLR is on, otherwise you don't get a chance to
1217 resolve the address to object/file/line after
1218 the app is no longer running
1219
1220 The lookup is performed when you read
1221 trace,trace_pipe. Example::
1222
1223 a.out-1623 [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
1224 x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
1225
1226
1227 printk-msg-only
1228 When set, trace_printk()s will only show the format
1229 and not their parameters (if trace_bprintk() or
1230 trace_bputs() was used to save the trace_printk()).
1231
1232 context-info
1233 Show only the event data. Hides the comm, PID,
1234 timestamp, CPU, and other useful data.
1235
1236 latency-format
1237 This option changes the trace output. When it is enabled,
1238 the trace displays additional information about the
1239 latency, as described in "Latency trace format".
1240
1241 pause-on-trace
1242 When set, opening the trace file for read, will pause
1243 writing to the ring buffer (as if tracing_on was set to zero).
1244 This simulates the original behavior of the trace file.
1245 When the file is closed, tracing will be enabled again.
1246
1247 hash-ptr
1248 When set, "%p" in the event printk format displays the
1249 hashed pointer value instead of real address.
1250 This will be useful if you want to find out which hashed
1251 value is corresponding to the real value in trace log.
1252
1253 record-cmd
1254 When any event or tracer is enabled, a hook is enabled
1255 in the sched_switch trace point to fill comm cache
1256 with mapped pids and comms. But this may cause some
1257 overhead, and if you only care about pids, and not the
1258 name of the task, disabling this option can lower the
1259 impact of tracing. See "saved_cmdlines".
1260
1261 record-tgid
1262 When any event or tracer is enabled, a hook is enabled
1263 in the sched_switch trace point to fill the cache of
1264 mapped Thread Group IDs (TGID) mapping to pids. See
1265 "saved_tgids".
1266
1267 overwrite
1268 This controls what happens when the trace buffer is
1269 full. If "1" (default), the oldest events are
1270 discarded and overwritten. If "0", then the newest
1271 events are discarded.
1272 (see per_cpu/cpu0/stats for overrun and dropped)
1273
1274 disable_on_free
1275 When the free_buffer is closed, tracing will
1276 stop (tracing_on set to 0).
1277
1278 irq-info
1279 Shows the interrupt, preempt count, need resched data.
1280 When disabled, the trace looks like::
1281
1282 # tracer: function
1283 #
1284 # entries-in-buffer/entries-written: 144405/9452052 #P:4
1285 #
1286 # TASK-PID CPU# TIMESTAMP FUNCTION
1287 # | | | | |
1288 <idle>-0 [002] 23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
1289 <idle>-0 [002] 23636.756054: activate_task <-ttwu_do_activate.constprop.89
1290 <idle>-0 [002] 23636.756055: enqueue_task <-activate_task
1291
1292
1293 markers
1294 When set, the trace_marker is writable (only by root).
1295 When disabled, the trace_marker will error with EINVAL
1296 on write.
1297
1298 event-fork
1299 When set, tasks with PIDs listed in set_event_pid will have
1300 the PIDs of their children added to set_event_pid when those
1301 tasks fork. Also, when tasks with PIDs in set_event_pid exit,
1302 their PIDs will be removed from the file.
1303
1304 This affects PIDs listed in set_event_notrace_pid as well.
1305
1306 function-trace
1307 The latency tracers will enable function tracing
1308 if this option is enabled (default it is). When
1309 it is disabled, the latency tracers do not trace
1310 functions. This keeps the overhead of the tracer down
1311 when performing latency tests.
1312
1313 function-fork
1314 When set, tasks with PIDs listed in set_ftrace_pid will
1315 have the PIDs of their children added to set_ftrace_pid
1316 when those tasks fork. Also, when tasks with PIDs in
1317 set_ftrace_pid exit, their PIDs will be removed from the
1318 file.
1319
1320 This affects PIDs in set_ftrace_notrace_pid as well.
1321
1322 display-graph
1323 When set, the latency tracers (irqsoff, wakeup, etc) will
1324 use function graph tracing instead of function tracing.
1325
1326 stacktrace
1327 When set, a stack trace is recorded after any trace event
1328 is recorded.
1329
1330 branch
1331 Enable branch tracing with the tracer. This enables branch
1332 tracer along with the currently set tracer. Enabling this
1333 with the "nop" tracer is the same as just enabling the
1334 "branch" tracer.
1335
1336.. tip:: Some tracers have their own options. They only appear in this
1337 file when the tracer is active. They always appear in the
1338 options directory.
1339
1340
1341Here are the per tracer options:
1342
1343Options for function tracer:
1344
1345 func_stack_trace
1346 When set, a stack trace is recorded after every
1347 function that is recorded. NOTE! Limit the functions
1348 that are recorded before enabling this, with
1349 "set_ftrace_filter" otherwise the system performance
1350 will be critically degraded. Remember to disable
1351 this option before clearing the function filter.
1352
1353Options for function_graph tracer:
1354
1355 Since the function_graph tracer has a slightly different output
1356 it has its own options to control what is displayed.
1357
1358 funcgraph-overrun
1359 When set, the "overrun" of the graph stack is
1360 displayed after each function traced. The
1361 overrun, is when the stack depth of the calls
1362 is greater than what is reserved for each task.
1363 Each task has a fixed array of functions to
1364 trace in the call graph. If the depth of the
1365 calls exceeds that, the function is not traced.
1366 The overrun is the number of functions missed
1367 due to exceeding this array.
1368
1369 funcgraph-cpu
1370 When set, the CPU number of the CPU where the trace
1371 occurred is displayed.
1372
1373 funcgraph-overhead
1374 When set, if the function takes longer than
1375 A certain amount, then a delay marker is
1376 displayed. See "delay" above, under the
1377 header description.
1378
1379 funcgraph-proc
1380 Unlike other tracers, the process' command line
1381 is not displayed by default, but instead only
1382 when a task is traced in and out during a context
1383 switch. Enabling this options has the command
1384 of each process displayed at every line.
1385
1386 funcgraph-duration
1387 At the end of each function (the return)
1388 the duration of the amount of time in the
1389 function is displayed in microseconds.
1390
1391 funcgraph-abstime
1392 When set, the timestamp is displayed at each line.
1393
1394 funcgraph-irqs
1395 When disabled, functions that happen inside an
1396 interrupt will not be traced.
1397
1398 funcgraph-tail
1399 When set, the return event will include the function
1400 that it represents. By default this is off, and
1401 only a closing curly bracket "}" is displayed for
1402 the return of a function.
1403
1404 funcgraph-retval
1405 When set, the return value of each traced function
1406 will be printed after an equal sign "=". By default
1407 this is off.
1408
1409 funcgraph-retval-hex
1410 When set, the return value will always be printed
1411 in hexadecimal format. If the option is not set and
1412 the return value is an error code, it will be printed
1413 in signed decimal format; otherwise it will also be
1414 printed in hexadecimal format. By default, this option
1415 is off.
1416
1417 sleep-time
1418 When running function graph tracer, to include
1419 the time a task schedules out in its function.
1420 When enabled, it will account time the task has been
1421 scheduled out as part of the function call.
1422
1423 graph-time
1424 When running function profiler with function graph tracer,
1425 to include the time to call nested functions. When this is
1426 not set, the time reported for the function will only
1427 include the time the function itself executed for, not the
1428 time for functions that it called.
1429
1430Options for blk tracer:
1431
1432 blk_classic
1433 Shows a more minimalistic output.
1434
1435
1436irqsoff
1437-------
1438
1439When interrupts are disabled, the CPU can not react to any other
1440external event (besides NMIs and SMIs). This prevents the timer
1441interrupt from triggering or the mouse interrupt from letting
1442the kernel know of a new mouse event. The result is a latency
1443with the reaction time.
1444
1445The irqsoff tracer tracks the time for which interrupts are
1446disabled. When a new maximum latency is hit, the tracer saves
1447the trace leading up to that latency point so that every time a
1448new maximum is reached, the old saved trace is discarded and the
1449new trace is saved.
1450
1451To reset the maximum, echo 0 into tracing_max_latency. Here is
1452an example::
1453
1454 # echo 0 > options/function-trace
1455 # echo irqsoff > current_tracer
1456 # echo 1 > tracing_on
1457 # echo 0 > tracing_max_latency
1458 # ls -ltr
1459 [...]
1460 # echo 0 > tracing_on
1461 # cat trace
1462 # tracer: irqsoff
1463 #
1464 # irqsoff latency trace v1.1.5 on 3.8.0-test+
1465 # --------------------------------------------------------------------
1466 # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1467 # -----------------
1468 # | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
1469 # -----------------
1470 # => started at: run_timer_softirq
1471 # => ended at: run_timer_softirq
1472 #
1473 #
1474 # _------=> CPU#
1475 # / _-----=> irqs-off
1476 # | / _----=> need-resched
1477 # || / _---=> hardirq/softirq
1478 # ||| / _--=> preempt-depth
1479 # |||| / delay
1480 # cmd pid ||||| time | caller
1481 # \ / ||||| \ | /
1482 <idle>-0 0d.s2 0us+: _raw_spin_lock_irq <-run_timer_softirq
1483 <idle>-0 0dNs3 17us : _raw_spin_unlock_irq <-run_timer_softirq
1484 <idle>-0 0dNs3 17us+: trace_hardirqs_on <-run_timer_softirq
1485 <idle>-0 0dNs3 25us : <stack trace>
1486 => _raw_spin_unlock_irq
1487 => run_timer_softirq
1488 => __do_softirq
1489 => call_softirq
1490 => do_softirq
1491 => irq_exit
1492 => smp_apic_timer_interrupt
1493 => apic_timer_interrupt
1494 => rcu_idle_exit
1495 => cpu_idle
1496 => rest_init
1497 => start_kernel
1498 => x86_64_start_reservations
1499 => x86_64_start_kernel
1500
1501Here we see that we had a latency of 16 microseconds (which is
1502very good). The _raw_spin_lock_irq in run_timer_softirq disabled
1503interrupts. The difference between the 16 and the displayed
1504timestamp 25us occurred because the clock was incremented
1505between the time of recording the max latency and the time of
1506recording the function that had that latency.
1507
1508Note the above example had function-trace not set. If we set
1509function-trace, we get a much larger output::
1510
1511 with echo 1 > options/function-trace
1512
1513 # tracer: irqsoff
1514 #
1515 # irqsoff latency trace v1.1.5 on 3.8.0-test+
1516 # --------------------------------------------------------------------
1517 # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1518 # -----------------
1519 # | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
1520 # -----------------
1521 # => started at: ata_scsi_queuecmd
1522 # => ended at: ata_scsi_queuecmd
1523 #
1524 #
1525 # _------=> CPU#
1526 # / _-----=> irqs-off
1527 # | / _----=> need-resched
1528 # || / _---=> hardirq/softirq
1529 # ||| / _--=> preempt-depth
1530 # |||| / delay
1531 # cmd pid ||||| time | caller
1532 # \ / ||||| \ | /
1533 bash-2042 3d... 0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1534 bash-2042 3d... 0us : add_preempt_count <-_raw_spin_lock_irqsave
1535 bash-2042 3d..1 1us : ata_scsi_find_dev <-ata_scsi_queuecmd
1536 bash-2042 3d..1 1us : __ata_scsi_find_dev <-ata_scsi_find_dev
1537 bash-2042 3d..1 2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
1538 bash-2042 3d..1 2us : ata_qc_new_init <-__ata_scsi_queuecmd
1539 bash-2042 3d..1 3us : ata_sg_init <-__ata_scsi_queuecmd
1540 bash-2042 3d..1 4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
1541 bash-2042 3d..1 4us : ata_build_rw_tf <-ata_scsi_rw_xlat
1542 [...]
1543 bash-2042 3d..1 67us : delay_tsc <-__delay
1544 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
1545 bash-2042 3d..2 67us : sub_preempt_count <-delay_tsc
1546 bash-2042 3d..1 67us : add_preempt_count <-delay_tsc
1547 bash-2042 3d..2 68us : sub_preempt_count <-delay_tsc
1548 bash-2042 3d..1 68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1549 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1550 bash-2042 3d..1 71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1551 bash-2042 3d..1 72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1552 bash-2042 3d..1 120us : <stack trace>
1553 => _raw_spin_unlock_irqrestore
1554 => ata_scsi_queuecmd
1555 => scsi_dispatch_cmd
1556 => scsi_request_fn
1557 => __blk_run_queue_uncond
1558 => __blk_run_queue
1559 => blk_queue_bio
1560 => submit_bio_noacct
1561 => submit_bio
1562 => submit_bh
1563 => __ext3_get_inode_loc
1564 => ext3_iget
1565 => ext3_lookup
1566 => lookup_real
1567 => __lookup_hash
1568 => walk_component
1569 => lookup_last
1570 => path_lookupat
1571 => filename_lookup
1572 => user_path_at_empty
1573 => user_path_at
1574 => vfs_fstatat
1575 => vfs_stat
1576 => sys_newstat
1577 => system_call_fastpath
1578
1579
1580Here we traced a 71 microsecond latency. But we also see all the
1581functions that were called during that time. Note that by
1582enabling function tracing, we incur an added overhead. This
1583overhead may extend the latency times. But nevertheless, this
1584trace has provided some very helpful debugging information.
1585
1586If we prefer function graph output instead of function, we can set
1587display-graph option::
1588
1589 with echo 1 > options/display-graph
1590
1591 # tracer: irqsoff
1592 #
1593 # irqsoff latency trace v1.1.5 on 4.20.0-rc6+
1594 # --------------------------------------------------------------------
1595 # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
1596 # -----------------
1597 # | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0)
1598 # -----------------
1599 # => started at: free_debug_processing
1600 # => ended at: return_to_handler
1601 #
1602 #
1603 # _-----=> irqs-off
1604 # / _----=> need-resched
1605 # | / _---=> hardirq/softirq
1606 # || / _--=> preempt-depth
1607 # ||| /
1608 # REL TIME CPU TASK/PID |||| DURATION FUNCTION CALLS
1609 # | | | | |||| | | | | | |
1610 0 us | 0) bash-1507 | d... | 0.000 us | _raw_spin_lock_irqsave();
1611 0 us | 0) bash-1507 | d..1 | 0.378 us | do_raw_spin_trylock();
1612 1 us | 0) bash-1507 | d..2 | | set_track() {
1613 2 us | 0) bash-1507 | d..2 | | save_stack_trace() {
1614 2 us | 0) bash-1507 | d..2 | | __save_stack_trace() {
1615 3 us | 0) bash-1507 | d..2 | | __unwind_start() {
1616 3 us | 0) bash-1507 | d..2 | | get_stack_info() {
1617 3 us | 0) bash-1507 | d..2 | 0.351 us | in_task_stack();
1618 4 us | 0) bash-1507 | d..2 | 1.107 us | }
1619 [...]
1620 3750 us | 0) bash-1507 | d..1 | 0.516 us | do_raw_spin_unlock();
1621 3750 us | 0) bash-1507 | d..1 | 0.000 us | _raw_spin_unlock_irqrestore();
1622 3764 us | 0) bash-1507 | d..1 | 0.000 us | tracer_hardirqs_on();
1623 bash-1507 0d..1 3792us : <stack trace>
1624 => free_debug_processing
1625 => __slab_free
1626 => kmem_cache_free
1627 => vm_area_free
1628 => remove_vma
1629 => exit_mmap
1630 => mmput
1631 => begin_new_exec
1632 => load_elf_binary
1633 => search_binary_handler
1634 => __do_execve_file.isra.32
1635 => __x64_sys_execve
1636 => do_syscall_64
1637 => entry_SYSCALL_64_after_hwframe
1638
1639preemptoff
1640----------
1641
1642When preemption is disabled, we may be able to receive
1643interrupts but the task cannot be preempted and a higher
1644priority task must wait for preemption to be enabled again
1645before it can preempt a lower priority task.
1646
1647The preemptoff tracer traces the places that disable preemption.
1648Like the irqsoff tracer, it records the maximum latency for
1649which preemption was disabled. The control of preemptoff tracer
1650is much like the irqsoff tracer.
1651::
1652
1653 # echo 0 > options/function-trace
1654 # echo preemptoff > current_tracer
1655 # echo 1 > tracing_on
1656 # echo 0 > tracing_max_latency
1657 # ls -ltr
1658 [...]
1659 # echo 0 > tracing_on
1660 # cat trace
1661 # tracer: preemptoff
1662 #
1663 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1664 # --------------------------------------------------------------------
1665 # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1666 # -----------------
1667 # | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1668 # -----------------
1669 # => started at: do_IRQ
1670 # => ended at: do_IRQ
1671 #
1672 #
1673 # _------=> CPU#
1674 # / _-----=> irqs-off
1675 # | / _----=> need-resched
1676 # || / _---=> hardirq/softirq
1677 # ||| / _--=> preempt-depth
1678 # |||| / delay
1679 # cmd pid ||||| time | caller
1680 # \ / ||||| \ | /
1681 sshd-1991 1d.h. 0us+: irq_enter <-do_IRQ
1682 sshd-1991 1d..1 46us : irq_exit <-do_IRQ
1683 sshd-1991 1d..1 47us+: trace_preempt_on <-do_IRQ
1684 sshd-1991 1d..1 52us : <stack trace>
1685 => sub_preempt_count
1686 => irq_exit
1687 => do_IRQ
1688 => ret_from_intr
1689
1690
1691This has some more changes. Preemption was disabled when an
1692interrupt came in (notice the 'h'), and was enabled on exit.
1693But we also see that interrupts have been disabled when entering
1694the preempt off section and leaving it (the 'd'). We do not know if
1695interrupts were enabled in the mean time or shortly after this
1696was over.
1697::
1698
1699 # tracer: preemptoff
1700 #
1701 # preemptoff latency trace v1.1.5 on 3.8.0-test+
1702 # --------------------------------------------------------------------
1703 # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1704 # -----------------
1705 # | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1706 # -----------------
1707 # => started at: wake_up_new_task
1708 # => ended at: task_rq_unlock
1709 #
1710 #
1711 # _------=> CPU#
1712 # / _-----=> irqs-off
1713 # | / _----=> need-resched
1714 # || / _---=> hardirq/softirq
1715 # ||| / _--=> preempt-depth
1716 # |||| / delay
1717 # cmd pid ||||| time | caller
1718 # \ / ||||| \ | /
1719 bash-1994 1d..1 0us : _raw_spin_lock_irqsave <-wake_up_new_task
1720 bash-1994 1d..1 0us : select_task_rq_fair <-select_task_rq
1721 bash-1994 1d..1 1us : __rcu_read_lock <-select_task_rq_fair
1722 bash-1994 1d..1 1us : source_load <-select_task_rq_fair
1723 bash-1994 1d..1 1us : source_load <-select_task_rq_fair
1724 [...]
1725 bash-1994 1d..1 12us : irq_enter <-smp_apic_timer_interrupt
1726 bash-1994 1d..1 12us : rcu_irq_enter <-irq_enter
1727 bash-1994 1d..1 13us : add_preempt_count <-irq_enter
1728 bash-1994 1d.h1 13us : exit_idle <-smp_apic_timer_interrupt
1729 bash-1994 1d.h1 13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1730 bash-1994 1d.h1 13us : _raw_spin_lock <-hrtimer_interrupt
1731 bash-1994 1d.h1 14us : add_preempt_count <-_raw_spin_lock
1732 bash-1994 1d.h2 14us : ktime_get_update_offsets <-hrtimer_interrupt
1733 [...]
1734 bash-1994 1d.h1 35us : lapic_next_event <-clockevents_program_event
1735 bash-1994 1d.h1 35us : irq_exit <-smp_apic_timer_interrupt
1736 bash-1994 1d.h1 36us : sub_preempt_count <-irq_exit
1737 bash-1994 1d..2 36us : do_softirq <-irq_exit
1738 bash-1994 1d..2 36us : __do_softirq <-call_softirq
1739 bash-1994 1d..2 36us : __local_bh_disable <-__do_softirq
1740 bash-1994 1d.s2 37us : add_preempt_count <-_raw_spin_lock_irq
1741 bash-1994 1d.s3 38us : _raw_spin_unlock <-run_timer_softirq
1742 bash-1994 1d.s3 39us : sub_preempt_count <-_raw_spin_unlock
1743 bash-1994 1d.s2 39us : call_timer_fn <-run_timer_softirq
1744 [...]
1745 bash-1994 1dNs2 81us : cpu_needs_another_gp <-rcu_process_callbacks
1746 bash-1994 1dNs2 82us : __local_bh_enable <-__do_softirq
1747 bash-1994 1dNs2 82us : sub_preempt_count <-__local_bh_enable
1748 bash-1994 1dN.2 82us : idle_cpu <-irq_exit
1749 bash-1994 1dN.2 83us : rcu_irq_exit <-irq_exit
1750 bash-1994 1dN.2 83us : sub_preempt_count <-irq_exit
1751 bash-1994 1.N.1 84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1752 bash-1994 1.N.1 84us+: trace_preempt_on <-task_rq_unlock
1753 bash-1994 1.N.1 104us : <stack trace>
1754 => sub_preempt_count
1755 => _raw_spin_unlock_irqrestore
1756 => task_rq_unlock
1757 => wake_up_new_task
1758 => do_fork
1759 => sys_clone
1760 => stub_clone
1761
1762
1763The above is an example of the preemptoff trace with
1764function-trace set. Here we see that interrupts were not disabled
1765the entire time. The irq_enter code lets us know that we entered
1766an interrupt 'h'. Before that, the functions being traced still
1767show that it is not in an interrupt, but we can see from the
1768functions themselves that this is not the case.
1769
1770preemptirqsoff
1771--------------
1772
1773Knowing the locations that have interrupts disabled or
1774preemption disabled for the longest times is helpful. But
1775sometimes we would like to know when either preemption and/or
1776interrupts are disabled.
1777
1778Consider the following code::
1779
1780 local_irq_disable();
1781 call_function_with_irqs_off();
1782 preempt_disable();
1783 call_function_with_irqs_and_preemption_off();
1784 local_irq_enable();
1785 call_function_with_preemption_off();
1786 preempt_enable();
1787
1788The irqsoff tracer will record the total length of
1789call_function_with_irqs_off() and
1790call_function_with_irqs_and_preemption_off().
1791
1792The preemptoff tracer will record the total length of
1793call_function_with_irqs_and_preemption_off() and
1794call_function_with_preemption_off().
1795
1796But neither will trace the time that interrupts and/or
1797preemption is disabled. This total time is the time that we can
1798not schedule. To record this time, use the preemptirqsoff
1799tracer.
1800
1801Again, using this trace is much like the irqsoff and preemptoff
1802tracers.
1803::
1804
1805 # echo 0 > options/function-trace
1806 # echo preemptirqsoff > current_tracer
1807 # echo 1 > tracing_on
1808 # echo 0 > tracing_max_latency
1809 # ls -ltr
1810 [...]
1811 # echo 0 > tracing_on
1812 # cat trace
1813 # tracer: preemptirqsoff
1814 #
1815 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1816 # --------------------------------------------------------------------
1817 # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1818 # -----------------
1819 # | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1820 # -----------------
1821 # => started at: ata_scsi_queuecmd
1822 # => ended at: ata_scsi_queuecmd
1823 #
1824 #
1825 # _------=> CPU#
1826 # / _-----=> irqs-off
1827 # | / _----=> need-resched
1828 # || / _---=> hardirq/softirq
1829 # ||| / _--=> preempt-depth
1830 # |||| / delay
1831 # cmd pid ||||| time | caller
1832 # \ / ||||| \ | /
1833 ls-2230 3d... 0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1834 ls-2230 3...1 100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1835 ls-2230 3...1 101us+: trace_preempt_on <-ata_scsi_queuecmd
1836 ls-2230 3...1 111us : <stack trace>
1837 => sub_preempt_count
1838 => _raw_spin_unlock_irqrestore
1839 => ata_scsi_queuecmd
1840 => scsi_dispatch_cmd
1841 => scsi_request_fn
1842 => __blk_run_queue_uncond
1843 => __blk_run_queue
1844 => blk_queue_bio
1845 => submit_bio_noacct
1846 => submit_bio
1847 => submit_bh
1848 => ext3_bread
1849 => ext3_dir_bread
1850 => htree_dirblock_to_tree
1851 => ext3_htree_fill_tree
1852 => ext3_readdir
1853 => vfs_readdir
1854 => sys_getdents
1855 => system_call_fastpath
1856
1857
1858The trace_hardirqs_off_thunk is called from assembly on x86 when
1859interrupts are disabled in the assembly code. Without the
1860function tracing, we do not know if interrupts were enabled
1861within the preemption points. We do see that it started with
1862preemption enabled.
1863
1864Here is a trace with function-trace set::
1865
1866 # tracer: preemptirqsoff
1867 #
1868 # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1869 # --------------------------------------------------------------------
1870 # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1871 # -----------------
1872 # | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1873 # -----------------
1874 # => started at: schedule
1875 # => ended at: mutex_unlock
1876 #
1877 #
1878 # _------=> CPU#
1879 # / _-----=> irqs-off
1880 # | / _----=> need-resched
1881 # || / _---=> hardirq/softirq
1882 # ||| / _--=> preempt-depth
1883 # |||| / delay
1884 # cmd pid ||||| time | caller
1885 # \ / ||||| \ | /
1886 kworker/-59 3...1 0us : __schedule <-schedule
1887 kworker/-59 3d..1 0us : rcu_preempt_qs <-rcu_note_context_switch
1888 kworker/-59 3d..1 1us : add_preempt_count <-_raw_spin_lock_irq
1889 kworker/-59 3d..2 1us : deactivate_task <-__schedule
1890 kworker/-59 3d..2 1us : dequeue_task <-deactivate_task
1891 kworker/-59 3d..2 2us : update_rq_clock <-dequeue_task
1892 kworker/-59 3d..2 2us : dequeue_task_fair <-dequeue_task
1893 kworker/-59 3d..2 2us : update_curr <-dequeue_task_fair
1894 kworker/-59 3d..2 2us : update_min_vruntime <-update_curr
1895 kworker/-59 3d..2 3us : cpuacct_charge <-update_curr
1896 kworker/-59 3d..2 3us : __rcu_read_lock <-cpuacct_charge
1897 kworker/-59 3d..2 3us : __rcu_read_unlock <-cpuacct_charge
1898 kworker/-59 3d..2 3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1899 kworker/-59 3d..2 4us : clear_buddies <-dequeue_task_fair
1900 kworker/-59 3d..2 4us : account_entity_dequeue <-dequeue_task_fair
1901 kworker/-59 3d..2 4us : update_min_vruntime <-dequeue_task_fair
1902 kworker/-59 3d..2 4us : update_cfs_shares <-dequeue_task_fair
1903 kworker/-59 3d..2 5us : hrtick_update <-dequeue_task_fair
1904 kworker/-59 3d..2 5us : wq_worker_sleeping <-__schedule
1905 kworker/-59 3d..2 5us : kthread_data <-wq_worker_sleeping
1906 kworker/-59 3d..2 5us : put_prev_task_fair <-__schedule
1907 kworker/-59 3d..2 6us : pick_next_task_fair <-pick_next_task
1908 kworker/-59 3d..2 6us : clear_buddies <-pick_next_task_fair
1909 kworker/-59 3d..2 6us : set_next_entity <-pick_next_task_fair
1910 kworker/-59 3d..2 6us : update_stats_wait_end <-set_next_entity
1911 ls-2269 3d..2 7us : finish_task_switch <-__schedule
1912 ls-2269 3d..2 7us : _raw_spin_unlock_irq <-finish_task_switch
1913 ls-2269 3d..2 8us : do_IRQ <-ret_from_intr
1914 ls-2269 3d..2 8us : irq_enter <-do_IRQ
1915 ls-2269 3d..2 8us : rcu_irq_enter <-irq_enter
1916 ls-2269 3d..2 9us : add_preempt_count <-irq_enter
1917 ls-2269 3d.h2 9us : exit_idle <-do_IRQ
1918 [...]
1919 ls-2269 3d.h3 20us : sub_preempt_count <-_raw_spin_unlock
1920 ls-2269 3d.h2 20us : irq_exit <-do_IRQ
1921 ls-2269 3d.h2 21us : sub_preempt_count <-irq_exit
1922 ls-2269 3d..3 21us : do_softirq <-irq_exit
1923 ls-2269 3d..3 21us : __do_softirq <-call_softirq
1924 ls-2269 3d..3 21us+: __local_bh_disable <-__do_softirq
1925 ls-2269 3d.s4 29us : sub_preempt_count <-_local_bh_enable_ip
1926 ls-2269 3d.s5 29us : sub_preempt_count <-_local_bh_enable_ip
1927 ls-2269 3d.s5 31us : do_IRQ <-ret_from_intr
1928 ls-2269 3d.s5 31us : irq_enter <-do_IRQ
1929 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
1930 [...]
1931 ls-2269 3d.s5 31us : rcu_irq_enter <-irq_enter
1932 ls-2269 3d.s5 32us : add_preempt_count <-irq_enter
1933 ls-2269 3d.H5 32us : exit_idle <-do_IRQ
1934 ls-2269 3d.H5 32us : handle_irq <-do_IRQ
1935 ls-2269 3d.H5 32us : irq_to_desc <-handle_irq
1936 ls-2269 3d.H5 33us : handle_fasteoi_irq <-handle_irq
1937 [...]
1938 ls-2269 3d.s5 158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1939 ls-2269 3d.s3 158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1940 ls-2269 3d.s3 159us : __local_bh_enable <-__do_softirq
1941 ls-2269 3d.s3 159us : sub_preempt_count <-__local_bh_enable
1942 ls-2269 3d..3 159us : idle_cpu <-irq_exit
1943 ls-2269 3d..3 159us : rcu_irq_exit <-irq_exit
1944 ls-2269 3d..3 160us : sub_preempt_count <-irq_exit
1945 ls-2269 3d... 161us : __mutex_unlock_slowpath <-mutex_unlock
1946 ls-2269 3d... 162us+: trace_hardirqs_on <-mutex_unlock
1947 ls-2269 3d... 186us : <stack trace>
1948 => __mutex_unlock_slowpath
1949 => mutex_unlock
1950 => process_output
1951 => n_tty_write
1952 => tty_write
1953 => vfs_write
1954 => sys_write
1955 => system_call_fastpath
1956
1957This is an interesting trace. It started with kworker running and
1958scheduling out and ls taking over. But as soon as ls released the
1959rq lock and enabled interrupts (but not preemption) an interrupt
1960triggered. When the interrupt finished, it started running softirqs.
1961But while the softirq was running, another interrupt triggered.
1962When an interrupt is running inside a softirq, the annotation is 'H'.
1963
1964
1965wakeup
1966------
1967
1968One common case that people are interested in tracing is the
1969time it takes for a task that is woken to actually wake up.
1970Now for non Real-Time tasks, this can be arbitrary. But tracing
1971it none the less can be interesting.
1972
1973Without function tracing::
1974
1975 # echo 0 > options/function-trace
1976 # echo wakeup > current_tracer
1977 # echo 1 > tracing_on
1978 # echo 0 > tracing_max_latency
1979 # chrt -f 5 sleep 1
1980 # echo 0 > tracing_on
1981 # cat trace
1982 # tracer: wakeup
1983 #
1984 # wakeup latency trace v1.1.5 on 3.8.0-test+
1985 # --------------------------------------------------------------------
1986 # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1987 # -----------------
1988 # | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
1989 # -----------------
1990 #
1991 # _------=> CPU#
1992 # / _-----=> irqs-off
1993 # | / _----=> need-resched
1994 # || / _---=> hardirq/softirq
1995 # ||| / _--=> preempt-depth
1996 # |||| / delay
1997 # cmd pid ||||| time | caller
1998 # \ / ||||| \ | /
1999 <idle>-0 3dNs7 0us : 0:120:R + [003] 312:100:R kworker/3:1H
2000 <idle>-0 3dNs7 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
2001 <idle>-0 3d..3 15us : __schedule <-schedule
2002 <idle>-0 3d..3 15us : 0:120:R ==> [003] 312:100:R kworker/3:1H
2003
2004The tracer only traces the highest priority task in the system
2005to avoid tracing the normal circumstances. Here we see that
2006the kworker with a nice priority of -20 (not very nice), took
2007just 15 microseconds from the time it woke up, to the time it
2008ran.
2009
2010Non Real-Time tasks are not that interesting. A more interesting
2011trace is to concentrate only on Real-Time tasks.
2012
2013wakeup_rt
2014---------
2015
2016In a Real-Time environment it is very important to know the
2017wakeup time it takes for the highest priority task that is woken
2018up to the time that it executes. This is also known as "schedule
2019latency". I stress the point that this is about RT tasks. It is
2020also important to know the scheduling latency of non-RT tasks,
2021but the average schedule latency is better for non-RT tasks.
2022Tools like LatencyTop are more appropriate for such
2023measurements.
2024
2025Real-Time environments are interested in the worst case latency.
2026That is the longest latency it takes for something to happen,
2027and not the average. We can have a very fast scheduler that may
2028only have a large latency once in a while, but that would not
2029work well with Real-Time tasks. The wakeup_rt tracer was designed
2030to record the worst case wakeups of RT tasks. Non-RT tasks are
2031not recorded because the tracer only records one worst case and
2032tracing non-RT tasks that are unpredictable will overwrite the
2033worst case latency of RT tasks (just run the normal wakeup
2034tracer for a while to see that effect).
2035
2036Since this tracer only deals with RT tasks, we will run this
2037slightly differently than we did with the previous tracers.
2038Instead of performing an 'ls', we will run 'sleep 1' under
2039'chrt' which changes the priority of the task.
2040::
2041
2042 # echo 0 > options/function-trace
2043 # echo wakeup_rt > current_tracer
2044 # echo 1 > tracing_on
2045 # echo 0 > tracing_max_latency
2046 # chrt -f 5 sleep 1
2047 # echo 0 > tracing_on
2048 # cat trace
2049 # tracer: wakeup
2050 #
2051 # tracer: wakeup_rt
2052 #
2053 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2054 # --------------------------------------------------------------------
2055 # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2056 # -----------------
2057 # | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
2058 # -----------------
2059 #
2060 # _------=> CPU#
2061 # / _-----=> irqs-off
2062 # | / _----=> need-resched
2063 # || / _---=> hardirq/softirq
2064 # ||| / _--=> preempt-depth
2065 # |||| / delay
2066 # cmd pid ||||| time | caller
2067 # \ / ||||| \ | /
2068 <idle>-0 3d.h4 0us : 0:120:R + [003] 2389: 94:R sleep
2069 <idle>-0 3d.h4 1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
2070 <idle>-0 3d..3 5us : __schedule <-schedule
2071 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
2072
2073
2074Running this on an idle system, we see that it only took 5 microseconds
2075to perform the task switch. Note, since the trace point in the schedule
2076is before the actual "switch", we stop the tracing when the recorded task
2077is about to schedule in. This may change if we add a new marker at the
2078end of the scheduler.
2079
2080Notice that the recorded task is 'sleep' with the PID of 2389
2081and it has an rt_prio of 5. This priority is user-space priority
2082and not the internal kernel priority. The policy is 1 for
2083SCHED_FIFO and 2 for SCHED_RR.
2084
2085Note, that the trace data shows the internal priority (99 - rtprio).
2086::
2087
2088 <idle>-0 3d..3 5us : 0:120:R ==> [003] 2389: 94:R sleep
2089
2090The 0:120:R means idle was running with a nice priority of 0 (120 - 120)
2091and in the running state 'R'. The sleep task was scheduled in with
20922389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
2093and it too is in the running state.
2094
2095Doing the same with chrt -r 5 and function-trace set.
2096::
2097
2098 echo 1 > options/function-trace
2099
2100 # tracer: wakeup_rt
2101 #
2102 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2103 # --------------------------------------------------------------------
2104 # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2105 # -----------------
2106 # | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
2107 # -----------------
2108 #
2109 # _------=> CPU#
2110 # / _-----=> irqs-off
2111 # | / _----=> need-resched
2112 # || / _---=> hardirq/softirq
2113 # ||| / _--=> preempt-depth
2114 # |||| / delay
2115 # cmd pid ||||| time | caller
2116 # \ / ||||| \ | /
2117 <idle>-0 3d.h4 1us+: 0:120:R + [003] 2448: 94:R sleep
2118 <idle>-0 3d.h4 2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
2119 <idle>-0 3d.h3 3us : check_preempt_curr <-ttwu_do_wakeup
2120 <idle>-0 3d.h3 3us : resched_curr <-check_preempt_curr
2121 <idle>-0 3dNh3 4us : task_woken_rt <-ttwu_do_wakeup
2122 <idle>-0 3dNh3 4us : _raw_spin_unlock <-try_to_wake_up
2123 <idle>-0 3dNh3 4us : sub_preempt_count <-_raw_spin_unlock
2124 <idle>-0 3dNh2 5us : ttwu_stat <-try_to_wake_up
2125 <idle>-0 3dNh2 5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
2126 <idle>-0 3dNh2 6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2127 <idle>-0 3dNh1 6us : _raw_spin_lock <-__run_hrtimer
2128 <idle>-0 3dNh1 6us : add_preempt_count <-_raw_spin_lock
2129 <idle>-0 3dNh2 7us : _raw_spin_unlock <-hrtimer_interrupt
2130 <idle>-0 3dNh2 7us : sub_preempt_count <-_raw_spin_unlock
2131 <idle>-0 3dNh1 7us : tick_program_event <-hrtimer_interrupt
2132 <idle>-0 3dNh1 7us : clockevents_program_event <-tick_program_event
2133 <idle>-0 3dNh1 8us : ktime_get <-clockevents_program_event
2134 <idle>-0 3dNh1 8us : lapic_next_event <-clockevents_program_event
2135 <idle>-0 3dNh1 8us : irq_exit <-smp_apic_timer_interrupt
2136 <idle>-0 3dNh1 9us : sub_preempt_count <-irq_exit
2137 <idle>-0 3dN.2 9us : idle_cpu <-irq_exit
2138 <idle>-0 3dN.2 9us : rcu_irq_exit <-irq_exit
2139 <idle>-0 3dN.2 10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
2140 <idle>-0 3dN.2 10us : sub_preempt_count <-irq_exit
2141 <idle>-0 3.N.1 11us : rcu_idle_exit <-cpu_idle
2142 <idle>-0 3dN.1 11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
2143 <idle>-0 3.N.1 11us : tick_nohz_idle_exit <-cpu_idle
2144 <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
2145 <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit
2146 <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
2147 <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit
2148 <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz
2149 <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock
2150 <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz
2151 <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update
2152 <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz
2153 <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock
2154 <idle>-0 3dN.1 15us : calc_load_nohz_stop <-tick_nohz_idle_exit
2155 <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
2156 <idle>-0 3dN.1 15us : hrtimer_cancel <-tick_nohz_idle_exit
2157 <idle>-0 3dN.1 15us : hrtimer_try_to_cancel <-hrtimer_cancel
2158 <idle>-0 3dN.1 16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
2159 <idle>-0 3dN.1 16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
2160 <idle>-0 3dN.1 16us : add_preempt_count <-_raw_spin_lock_irqsave
2161 <idle>-0 3dN.2 17us : __remove_hrtimer <-remove_hrtimer.part.16
2162 <idle>-0 3dN.2 17us : hrtimer_force_reprogram <-__remove_hrtimer
2163 <idle>-0 3dN.2 17us : tick_program_event <-hrtimer_force_reprogram
2164 <idle>-0 3dN.2 18us : clockevents_program_event <-tick_program_event
2165 <idle>-0 3dN.2 18us : ktime_get <-clockevents_program_event
2166 <idle>-0 3dN.2 18us : lapic_next_event <-clockevents_program_event
2167 <idle>-0 3dN.2 19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
2168 <idle>-0 3dN.2 19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2169 <idle>-0 3dN.1 19us : hrtimer_forward <-tick_nohz_idle_exit
2170 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
2171 <idle>-0 3dN.1 20us : ktime_add_safe <-hrtimer_forward
2172 <idle>-0 3dN.1 20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2173 <idle>-0 3dN.1 20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
2174 <idle>-0 3dN.1 21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
2175 <idle>-0 3dN.1 21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
2176 <idle>-0 3dN.1 21us : add_preempt_count <-_raw_spin_lock_irqsave
2177 <idle>-0 3dN.2 22us : ktime_add_safe <-__hrtimer_start_range_ns
2178 <idle>-0 3dN.2 22us : enqueue_hrtimer <-__hrtimer_start_range_ns
2179 <idle>-0 3dN.2 22us : tick_program_event <-__hrtimer_start_range_ns
2180 <idle>-0 3dN.2 23us : clockevents_program_event <-tick_program_event
2181 <idle>-0 3dN.2 23us : ktime_get <-clockevents_program_event
2182 <idle>-0 3dN.2 23us : lapic_next_event <-clockevents_program_event
2183 <idle>-0 3dN.2 24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
2184 <idle>-0 3dN.2 24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2185 <idle>-0 3dN.1 24us : account_idle_ticks <-tick_nohz_idle_exit
2186 <idle>-0 3dN.1 24us : account_idle_time <-account_idle_ticks
2187 <idle>-0 3.N.1 25us : sub_preempt_count <-cpu_idle
2188 <idle>-0 3.N.. 25us : schedule <-cpu_idle
2189 <idle>-0 3.N.. 25us : __schedule <-preempt_schedule
2190 <idle>-0 3.N.. 26us : add_preempt_count <-__schedule
2191 <idle>-0 3.N.1 26us : rcu_note_context_switch <-__schedule
2192 <idle>-0 3.N.1 26us : rcu_sched_qs <-rcu_note_context_switch
2193 <idle>-0 3dN.1 27us : rcu_preempt_qs <-rcu_note_context_switch
2194 <idle>-0 3.N.1 27us : _raw_spin_lock_irq <-__schedule
2195 <idle>-0 3dN.1 27us : add_preempt_count <-_raw_spin_lock_irq
2196 <idle>-0 3dN.2 28us : put_prev_task_idle <-__schedule
2197 <idle>-0 3dN.2 28us : pick_next_task_stop <-pick_next_task
2198 <idle>-0 3dN.2 28us : pick_next_task_rt <-pick_next_task
2199 <idle>-0 3dN.2 29us : dequeue_pushable_task <-pick_next_task_rt
2200 <idle>-0 3d..3 29us : __schedule <-preempt_schedule
2201 <idle>-0 3d..3 30us : 0:120:R ==> [003] 2448: 94:R sleep
2202
2203This isn't that big of a trace, even with function tracing enabled,
2204so I included the entire trace.
2205
2206The interrupt went off while when the system was idle. Somewhere
2207before task_woken_rt() was called, the NEED_RESCHED flag was set,
2208this is indicated by the first occurrence of the 'N' flag.
2209
2210Latency tracing and events
2211--------------------------
2212As function tracing can induce a much larger latency, but without
2213seeing what happens within the latency it is hard to know what
2214caused it. There is a middle ground, and that is with enabling
2215events.
2216::
2217
2218 # echo 0 > options/function-trace
2219 # echo wakeup_rt > current_tracer
2220 # echo 1 > events/enable
2221 # echo 1 > tracing_on
2222 # echo 0 > tracing_max_latency
2223 # chrt -f 5 sleep 1
2224 # echo 0 > tracing_on
2225 # cat trace
2226 # tracer: wakeup_rt
2227 #
2228 # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2229 # --------------------------------------------------------------------
2230 # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2231 # -----------------
2232 # | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
2233 # -----------------
2234 #
2235 # _------=> CPU#
2236 # / _-----=> irqs-off
2237 # | / _----=> need-resched
2238 # || / _---=> hardirq/softirq
2239 # ||| / _--=> preempt-depth
2240 # |||| / delay
2241 # cmd pid ||||| time | caller
2242 # \ / ||||| \ | /
2243 <idle>-0 2d.h4 0us : 0:120:R + [002] 5882: 94:R sleep
2244 <idle>-0 2d.h4 0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
2245 <idle>-0 2d.h4 1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
2246 <idle>-0 2dNh2 1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
2247 <idle>-0 2.N.2 2us : power_end: cpu_id=2
2248 <idle>-0 2.N.2 3us : cpu_idle: state=4294967295 cpu_id=2
2249 <idle>-0 2dN.3 4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
2250 <idle>-0 2dN.3 4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
2251 <idle>-0 2.N.2 5us : rcu_utilization: Start context switch
2252 <idle>-0 2.N.2 5us : rcu_utilization: End context switch
2253 <idle>-0 2d..3 6us : __schedule <-schedule
2254 <idle>-0 2d..3 6us : 0:120:R ==> [002] 5882: 94:R sleep
2255
2256
2257Hardware Latency Detector
2258-------------------------
2259
2260The hardware latency detector is executed by enabling the "hwlat" tracer.
2261
2262NOTE, this tracer will affect the performance of the system as it will
2263periodically make a CPU constantly busy with interrupts disabled.
2264::
2265
2266 # echo hwlat > current_tracer
2267 # sleep 100
2268 # cat trace
2269 # tracer: hwlat
2270 #
2271 # entries-in-buffer/entries-written: 13/13 #P:8
2272 #
2273 # _-----=> irqs-off
2274 # / _----=> need-resched
2275 # | / _---=> hardirq/softirq
2276 # || / _--=> preempt-depth
2277 # ||| / delay
2278 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2279 # | | | |||| | |
2280 <...>-1729 [001] d... 678.473449: #1 inner/outer(us): 11/12 ts:1581527483.343962693 count:6
2281 <...>-1729 [004] d... 689.556542: #2 inner/outer(us): 16/9 ts:1581527494.889008092 count:1
2282 <...>-1729 [005] d... 714.756290: #3 inner/outer(us): 16/16 ts:1581527519.678961629 count:5
2283 <...>-1729 [001] d... 718.788247: #4 inner/outer(us): 9/17 ts:1581527523.889012713 count:1
2284 <...>-1729 [002] d... 719.796341: #5 inner/outer(us): 13/9 ts:1581527524.912872606 count:1
2285 <...>-1729 [006] d... 844.787091: #6 inner/outer(us): 9/12 ts:1581527649.889048502 count:2
2286 <...>-1729 [003] d... 849.827033: #7 inner/outer(us): 18/9 ts:1581527654.889013793 count:1
2287 <...>-1729 [007] d... 853.859002: #8 inner/outer(us): 9/12 ts:1581527658.889065736 count:1
2288 <...>-1729 [001] d... 855.874978: #9 inner/outer(us): 9/11 ts:1581527660.861991877 count:1
2289 <...>-1729 [001] d... 863.938932: #10 inner/outer(us): 9/11 ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1
2290 <...>-1729 [007] d... 878.050780: #11 inner/outer(us): 9/12 ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1
2291 <...>-1729 [007] d... 886.114702: #12 inner/outer(us): 9/12 ts:1581527691.385001600 count:1
2292
2293
2294The above output is somewhat the same in the header. All events will have
2295interrupts disabled 'd'. Under the FUNCTION title there is:
2296
2297 #1
2298 This is the count of events recorded that were greater than the
2299 tracing_threshold (See below).
2300
2301 inner/outer(us): 11/11
2302
2303 This shows two numbers as "inner latency" and "outer latency". The test
2304 runs in a loop checking a timestamp twice. The latency detected within
2305 the two timestamps is the "inner latency" and the latency detected
2306 after the previous timestamp and the next timestamp in the loop is
2307 the "outer latency".
2308
2309 ts:1581527483.343962693
2310
2311 The absolute timestamp that the first latency was recorded in the window.
2312
2313 count:6
2314
2315 The number of times a latency was detected during the window.
2316
2317 nmi-total:7 nmi-count:1
2318
2319 On architectures that support it, if an NMI comes in during the
2320 test, the time spent in NMI is reported in "nmi-total" (in
2321 microseconds).
2322
2323 All architectures that have NMIs will show the "nmi-count" if an
2324 NMI comes in during the test.
2325
2326hwlat files:
2327
2328 tracing_threshold
2329 This gets automatically set to "10" to represent 10
2330 microseconds. This is the threshold of latency that
2331 needs to be detected before the trace will be recorded.
2332
2333 Note, when hwlat tracer is finished (another tracer is
2334 written into "current_tracer"), the original value for
2335 tracing_threshold is placed back into this file.
2336
2337 hwlat_detector/width
2338 The length of time the test runs with interrupts disabled.
2339
2340 hwlat_detector/window
2341 The length of time of the window which the test
2342 runs. That is, the test will run for "width"
2343 microseconds per "window" microseconds
2344
2345 tracing_cpumask
2346 When the test is started. A kernel thread is created that
2347 runs the test. This thread will alternate between CPUs
2348 listed in the tracing_cpumask between each period
2349 (one "window"). To limit the test to specific CPUs
2350 set the mask in this file to only the CPUs that the test
2351 should run on.
2352
2353function
2354--------
2355
2356This tracer is the function tracer. Enabling the function tracer
2357can be done from the debug file system. Make sure the
2358ftrace_enabled is set; otherwise this tracer is a nop.
2359See the "ftrace_enabled" section below.
2360::
2361
2362 # sysctl kernel.ftrace_enabled=1
2363 # echo function > current_tracer
2364 # echo 1 > tracing_on
2365 # usleep 1
2366 # echo 0 > tracing_on
2367 # cat trace
2368 # tracer: function
2369 #
2370 # entries-in-buffer/entries-written: 24799/24799 #P:4
2371 #
2372 # _-----=> irqs-off
2373 # / _----=> need-resched
2374 # | / _---=> hardirq/softirq
2375 # || / _--=> preempt-depth
2376 # ||| / delay
2377 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2378 # | | | |||| | |
2379 bash-1994 [002] .... 3082.063030: mutex_unlock <-rb_simple_write
2380 bash-1994 [002] .... 3082.063031: __mutex_unlock_slowpath <-mutex_unlock
2381 bash-1994 [002] .... 3082.063031: __fsnotify_parent <-fsnotify_modify
2382 bash-1994 [002] .... 3082.063032: fsnotify <-fsnotify_modify
2383 bash-1994 [002] .... 3082.063032: __srcu_read_lock <-fsnotify
2384 bash-1994 [002] .... 3082.063032: add_preempt_count <-__srcu_read_lock
2385 bash-1994 [002] ...1 3082.063032: sub_preempt_count <-__srcu_read_lock
2386 bash-1994 [002] .... 3082.063033: __srcu_read_unlock <-fsnotify
2387 [...]
2388
2389
2390Note: function tracer uses ring buffers to store the above
2391entries. The newest data may overwrite the oldest data.
2392Sometimes using echo to stop the trace is not sufficient because
2393the tracing could have overwritten the data that you wanted to
2394record. For this reason, it is sometimes better to disable
2395tracing directly from a program. This allows you to stop the
2396tracing at the point that you hit the part that you are
2397interested in. To disable the tracing directly from a C program,
2398something like following code snippet can be used::
2399
2400 int trace_fd;
2401 [...]
2402 int main(int argc, char *argv[]) {
2403 [...]
2404 trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
2405 [...]
2406 if (condition_hit()) {
2407 write(trace_fd, "0", 1);
2408 }
2409 [...]
2410 }
2411
2412
2413Single thread tracing
2414---------------------
2415
2416By writing into set_ftrace_pid you can trace a
2417single thread. For example::
2418
2419 # cat set_ftrace_pid
2420 no pid
2421 # echo 3111 > set_ftrace_pid
2422 # cat set_ftrace_pid
2423 3111
2424 # echo function > current_tracer
2425 # cat trace | head
2426 # tracer: function
2427 #
2428 # TASK-PID CPU# TIMESTAMP FUNCTION
2429 # | | | | |
2430 yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
2431 yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
2432 yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
2433 yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
2434 yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
2435 yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
2436 # echo > set_ftrace_pid
2437 # cat trace |head
2438 # tracer: function
2439 #
2440 # TASK-PID CPU# TIMESTAMP FUNCTION
2441 # | | | | |
2442 ##### CPU 3 buffer started ####
2443 yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
2444 yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
2445 yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
2446 yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
2447 yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
2448
2449If you want to trace a function when executing, you could use
2450something like this simple program.
2451::
2452
2453 #include <stdio.h>
2454 #include <stdlib.h>
2455 #include <sys/types.h>
2456 #include <sys/stat.h>
2457 #include <fcntl.h>
2458 #include <unistd.h>
2459 #include <string.h>
2460
2461 #define _STR(x) #x
2462 #define STR(x) _STR(x)
2463 #define MAX_PATH 256
2464
2465 const char *find_tracefs(void)
2466 {
2467 static char tracefs[MAX_PATH+1];
2468 static int tracefs_found;
2469 char type[100];
2470 FILE *fp;
2471
2472 if (tracefs_found)
2473 return tracefs;
2474
2475 if ((fp = fopen("/proc/mounts","r")) == NULL) {
2476 perror("/proc/mounts");
2477 return NULL;
2478 }
2479
2480 while (fscanf(fp, "%*s %"
2481 STR(MAX_PATH)
2482 "s %99s %*s %*d %*d\n",
2483 tracefs, type) == 2) {
2484 if (strcmp(type, "tracefs") == 0)
2485 break;
2486 }
2487 fclose(fp);
2488
2489 if (strcmp(type, "tracefs") != 0) {
2490 fprintf(stderr, "tracefs not mounted");
2491 return NULL;
2492 }
2493
2494 strcat(tracefs, "/tracing/");
2495 tracefs_found = 1;
2496
2497 return tracefs;
2498 }
2499
2500 const char *tracing_file(const char *file_name)
2501 {
2502 static char trace_file[MAX_PATH+1];
2503 snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name);
2504 return trace_file;
2505 }
2506
2507 int main (int argc, char **argv)
2508 {
2509 if (argc < 1)
2510 exit(-1);
2511
2512 if (fork() > 0) {
2513 int fd, ffd;
2514 char line[64];
2515 int s;
2516
2517 ffd = open(tracing_file("current_tracer"), O_WRONLY);
2518 if (ffd < 0)
2519 exit(-1);
2520 write(ffd, "nop", 3);
2521
2522 fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
2523 s = sprintf(line, "%d\n", getpid());
2524 write(fd, line, s);
2525
2526 write(ffd, "function", 8);
2527
2528 close(fd);
2529 close(ffd);
2530
2531 execvp(argv[1], argv+1);
2532 }
2533
2534 return 0;
2535 }
2536
2537Or this simple script!
2538::
2539
2540 #!/bin/bash
2541
2542 tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
2543 echo 0 > $tracefs/tracing_on
2544 echo $$ > $tracefs/set_ftrace_pid
2545 echo function > $tracefs/current_tracer
2546 echo 1 > $tracefs/tracing_on
2547 exec "$@"
2548
2549
2550function graph tracer
2551---------------------------
2552
2553This tracer is similar to the function tracer except that it
2554probes a function on its entry and its exit. This is done by
2555using a dynamically allocated stack of return addresses in each
2556task_struct. On function entry the tracer overwrites the return
2557address of each function traced to set a custom probe. Thus the
2558original return address is stored on the stack of return address
2559in the task_struct.
2560
2561Probing on both ends of a function leads to special features
2562such as:
2563
2564- measure of a function's time execution
2565- having a reliable call stack to draw function calls graph
2566
2567This tracer is useful in several situations:
2568
2569- you want to find the reason of a strange kernel behavior and
2570 need to see what happens in detail on any areas (or specific
2571 ones).
2572
2573- you are experiencing weird latencies but it's difficult to
2574 find its origin.
2575
2576- you want to find quickly which path is taken by a specific
2577 function
2578
2579- you just want to peek inside a working kernel and want to see
2580 what happens there.
2581
2582::
2583
2584 # tracer: function_graph
2585 #
2586 # CPU DURATION FUNCTION CALLS
2587 # | | | | | | |
2588
2589 0) | sys_open() {
2590 0) | do_sys_open() {
2591 0) | getname() {
2592 0) | kmem_cache_alloc() {
2593 0) 1.382 us | __might_sleep();
2594 0) 2.478 us | }
2595 0) | strncpy_from_user() {
2596 0) | might_fault() {
2597 0) 1.389 us | __might_sleep();
2598 0) 2.553 us | }
2599 0) 3.807 us | }
2600 0) 7.876 us | }
2601 0) | alloc_fd() {
2602 0) 0.668 us | _spin_lock();
2603 0) 0.570 us | expand_files();
2604 0) 0.586 us | _spin_unlock();
2605
2606
2607There are several columns that can be dynamically
2608enabled/disabled. You can use every combination of options you
2609want, depending on your needs.
2610
2611- The cpu number on which the function executed is default
2612 enabled. It is sometimes better to only trace one cpu (see
2613 tracing_cpumask file) or you might sometimes see unordered
2614 function calls while cpu tracing switch.
2615
2616 - hide: echo nofuncgraph-cpu > trace_options
2617 - show: echo funcgraph-cpu > trace_options
2618
2619- The duration (function's time of execution) is displayed on
2620 the closing bracket line of a function or on the same line
2621 than the current function in case of a leaf one. It is default
2622 enabled.
2623
2624 - hide: echo nofuncgraph-duration > trace_options
2625 - show: echo funcgraph-duration > trace_options
2626
2627- The overhead field precedes the duration field in case of
2628 reached duration thresholds.
2629
2630 - hide: echo nofuncgraph-overhead > trace_options
2631 - show: echo funcgraph-overhead > trace_options
2632 - depends on: funcgraph-duration
2633
2634 ie::
2635
2636 3) # 1837.709 us | } /* __switch_to */
2637 3) | finish_task_switch() {
2638 3) 0.313 us | _raw_spin_unlock_irq();
2639 3) 3.177 us | }
2640 3) # 1889.063 us | } /* __schedule */
2641 3) ! 140.417 us | } /* __schedule */
2642 3) # 2034.948 us | } /* schedule */
2643 3) * 33998.59 us | } /* schedule_preempt_disabled */
2644
2645 [...]
2646
2647 1) 0.260 us | msecs_to_jiffies();
2648 1) 0.313 us | __rcu_read_unlock();
2649 1) + 61.770 us | }
2650 1) + 64.479 us | }
2651 1) 0.313 us | rcu_bh_qs();
2652 1) 0.313 us | __local_bh_enable();
2653 1) ! 217.240 us | }
2654 1) 0.365 us | idle_cpu();
2655 1) | rcu_irq_exit() {
2656 1) 0.417 us | rcu_eqs_enter_common.isra.47();
2657 1) 3.125 us | }
2658 1) ! 227.812 us | }
2659 1) ! 457.395 us | }
2660 1) @ 119760.2 us | }
2661
2662 [...]
2663
2664 2) | handle_IPI() {
2665 1) 6.979 us | }
2666 2) 0.417 us | scheduler_ipi();
2667 1) 9.791 us | }
2668 1) + 12.917 us | }
2669 2) 3.490 us | }
2670 1) + 15.729 us | }
2671 1) + 18.542 us | }
2672 2) $ 3594274 us | }
2673
2674Flags::
2675
2676 + means that the function exceeded 10 usecs.
2677 ! means that the function exceeded 100 usecs.
2678 # means that the function exceeded 1000 usecs.
2679 * means that the function exceeded 10 msecs.
2680 @ means that the function exceeded 100 msecs.
2681 $ means that the function exceeded 1 sec.
2682
2683
2684- The task/pid field displays the thread cmdline and pid which
2685 executed the function. It is default disabled.
2686
2687 - hide: echo nofuncgraph-proc > trace_options
2688 - show: echo funcgraph-proc > trace_options
2689
2690 ie::
2691
2692 # tracer: function_graph
2693 #
2694 # CPU TASK/PID DURATION FUNCTION CALLS
2695 # | | | | | | | | |
2696 0) sh-4802 | | d_free() {
2697 0) sh-4802 | | call_rcu() {
2698 0) sh-4802 | | __call_rcu() {
2699 0) sh-4802 | 0.616 us | rcu_process_gp_end();
2700 0) sh-4802 | 0.586 us | check_for_new_grace_period();
2701 0) sh-4802 | 2.899 us | }
2702 0) sh-4802 | 4.040 us | }
2703 0) sh-4802 | 5.151 us | }
2704 0) sh-4802 | + 49.370 us | }
2705
2706
2707- The absolute time field is an absolute timestamp given by the
2708 system clock since it started. A snapshot of this time is
2709 given on each entry/exit of functions
2710
2711 - hide: echo nofuncgraph-abstime > trace_options
2712 - show: echo funcgraph-abstime > trace_options
2713
2714 ie::
2715
2716 #
2717 # TIME CPU DURATION FUNCTION CALLS
2718 # | | | | | | | |
2719 360.774522 | 1) 0.541 us | }
2720 360.774522 | 1) 4.663 us | }
2721 360.774523 | 1) 0.541 us | __wake_up_bit();
2722 360.774524 | 1) 6.796 us | }
2723 360.774524 | 1) 7.952 us | }
2724 360.774525 | 1) 9.063 us | }
2725 360.774525 | 1) 0.615 us | journal_mark_dirty();
2726 360.774527 | 1) 0.578 us | __brelse();
2727 360.774528 | 1) | reiserfs_prepare_for_journal() {
2728 360.774528 | 1) | unlock_buffer() {
2729 360.774529 | 1) | wake_up_bit() {
2730 360.774529 | 1) | bit_waitqueue() {
2731 360.774530 | 1) 0.594 us | __phys_addr();
2732
2733
2734The function name is always displayed after the closing bracket
2735for a function if the start of that function is not in the
2736trace buffer.
2737
2738Display of the function name after the closing bracket may be
2739enabled for functions whose start is in the trace buffer,
2740allowing easier searching with grep for function durations.
2741It is default disabled.
2742
2743 - hide: echo nofuncgraph-tail > trace_options
2744 - show: echo funcgraph-tail > trace_options
2745
2746 Example with nofuncgraph-tail (default)::
2747
2748 0) | putname() {
2749 0) | kmem_cache_free() {
2750 0) 0.518 us | __phys_addr();
2751 0) 1.757 us | }
2752 0) 2.861 us | }
2753
2754 Example with funcgraph-tail::
2755
2756 0) | putname() {
2757 0) | kmem_cache_free() {
2758 0) 0.518 us | __phys_addr();
2759 0) 1.757 us | } /* kmem_cache_free() */
2760 0) 2.861 us | } /* putname() */
2761
2762The return value of each traced function can be displayed after
2763an equal sign "=". When encountering system call failures, it
2764can be very helpful to quickly locate the function that first
2765returns an error code.
2766
2767 - hide: echo nofuncgraph-retval > trace_options
2768 - show: echo funcgraph-retval > trace_options
2769
2770 Example with funcgraph-retval::
2771
2772 1) | cgroup_migrate() {
2773 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
2774 1) | cgroup_migrate_execute() {
2775 1) | cpu_cgroup_can_attach() {
2776 1) | cgroup_taskset_first() {
2777 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
2778 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
2779 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */
2780 1) 2.335 us | } /* cpu_cgroup_can_attach = -22 */
2781 1) 4.369 us | } /* cgroup_migrate_execute = -22 */
2782 1) 7.143 us | } /* cgroup_migrate = -22 */
2783
2784The above example shows that the function cpu_cgroup_can_attach
2785returned the error code -22 firstly, then we can read the code
2786of this function to get the root cause.
2787
2788When the option funcgraph-retval-hex is not set, the return value can
2789be displayed in a smart way. Specifically, if it is an error code,
2790it will be printed in signed decimal format, otherwise it will
2791printed in hexadecimal format.
2792
2793 - smart: echo nofuncgraph-retval-hex > trace_options
2794 - hexadecimal: echo funcgraph-retval-hex > trace_options
2795
2796 Example with funcgraph-retval-hex::
2797
2798 1) | cgroup_migrate() {
2799 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
2800 1) | cgroup_migrate_execute() {
2801 1) | cpu_cgroup_can_attach() {
2802 1) | cgroup_taskset_first() {
2803 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
2804 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
2805 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */
2806 1) 2.335 us | } /* cpu_cgroup_can_attach = 0xffffffea */
2807 1) 4.369 us | } /* cgroup_migrate_execute = 0xffffffea */
2808 1) 7.143 us | } /* cgroup_migrate = 0xffffffea */
2809
2810At present, there are some limitations when using the funcgraph-retval
2811option, and these limitations will be eliminated in the future:
2812
2813- Even if the function return type is void, a return value will still
2814 be printed, and you can just ignore it.
2815
2816- Even if return values are stored in multiple registers, only the
2817 value contained in the first register will be recorded and printed.
2818 To illustrate, in the x86 architecture, eax and edx are used to store
2819 a 64-bit return value, with the lower 32 bits saved in eax and the
2820 upper 32 bits saved in edx. However, only the value stored in eax
2821 will be recorded and printed.
2822
2823- In certain procedure call standards, such as arm64's AAPCS64, when a
2824 type is smaller than a GPR, it is the responsibility of the consumer
2825 to perform the narrowing, and the upper bits may contain UNKNOWN values.
2826 Therefore, it is advisable to check the code for such cases. For instance,
2827 when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values,
2828 especially when larger types are truncated, whether explicitly or implicitly.
2829 Here are some specific cases to illustrate this point:
2830
2831 **Case One**:
2832
2833 The function narrow_to_u8 is defined as follows::
2834
2835 u8 narrow_to_u8(u64 val)
2836 {
2837 // implicitly truncated
2838 return val;
2839 }
2840
2841 It may be compiled to::
2842
2843 narrow_to_u8:
2844 < ... ftrace instrumentation ... >
2845 RET
2846
2847 If you pass 0x123456789abcdef to this function and want to narrow it,
2848 it may be recorded as 0x123456789abcdef instead of 0xef.
2849
2850 **Case Two**:
2851
2852 The function error_if_not_4g_aligned is defined as follows::
2853
2854 int error_if_not_4g_aligned(u64 val)
2855 {
2856 if (val & GENMASK(31, 0))
2857 return -EINVAL;
2858
2859 return 0;
2860 }
2861
2862 It could be compiled to::
2863
2864 error_if_not_4g_aligned:
2865 CBNZ w0, .Lnot_aligned
2866 RET // bits [31:0] are zero, bits
2867 // [63:32] are UNKNOWN
2868 .Lnot_aligned:
2869 MOV x0, #-EINVAL
2870 RET
2871
2872 When passing 0x2_0000_0000 to it, the return value may be recorded as
2873 0x2_0000_0000 instead of 0.
2874
2875You can put some comments on specific functions by using
2876trace_printk() For example, if you want to put a comment inside
2877the __might_sleep() function, you just have to include
2878<linux/ftrace.h> and call trace_printk() inside __might_sleep()::
2879
2880 trace_printk("I'm a comment!\n")
2881
2882will produce::
2883
2884 1) | __might_sleep() {
2885 1) | /* I'm a comment! */
2886 1) 1.449 us | }
2887
2888
2889You might find other useful features for this tracer in the
2890following "dynamic ftrace" section such as tracing only specific
2891functions or tasks.
2892
2893dynamic ftrace
2894--------------
2895
2896If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2897virtually no overhead when function tracing is disabled. The way
2898this works is the mcount function call (placed at the start of
2899every kernel function, produced by the -pg switch in gcc),
2900starts of pointing to a simple return. (Enabling FTRACE will
2901include the -pg switch in the compiling of the kernel.)
2902
2903At compile time every C file object is run through the
2904recordmcount program (located in the scripts directory). This
2905program will parse the ELF headers in the C object to find all
2906the locations in the .text section that call mcount. Starting
2907with gcc version 4.6, the -mfentry has been added for x86, which
2908calls "__fentry__" instead of "mcount". Which is called before
2909the creation of the stack frame.
2910
2911Note, not all sections are traced. They may be prevented by either
2912a notrace, or blocked another way and all inline functions are not
2913traced. Check the "available_filter_functions" file to see what functions
2914can be traced.
2915
2916A section called "__mcount_loc" is created that holds
2917references to all the mcount/fentry call sites in the .text section.
2918The recordmcount program re-links this section back into the
2919original object. The final linking stage of the kernel will add all these
2920references into a single table.
2921
2922On boot up, before SMP is initialized, the dynamic ftrace code
2923scans this table and updates all the locations into nops. It
2924also records the locations, which are added to the
2925available_filter_functions list. Modules are processed as they
2926are loaded and before they are executed. When a module is
2927unloaded, it also removes its functions from the ftrace function
2928list. This is automatic in the module unload code, and the
2929module author does not need to worry about it.
2930
2931When tracing is enabled, the process of modifying the function
2932tracepoints is dependent on architecture. The old method is to use
2933kstop_machine to prevent races with the CPUs executing code being
2934modified (which can cause the CPU to do undesirable things, especially
2935if the modified code crosses cache (or page) boundaries), and the nops are
2936patched back to calls. But this time, they do not call mcount
2937(which is just a function stub). They now call into the ftrace
2938infrastructure.
2939
2940The new method of modifying the function tracepoints is to place
2941a breakpoint at the location to be modified, sync all CPUs, modify
2942the rest of the instruction not covered by the breakpoint. Sync
2943all CPUs again, and then remove the breakpoint with the finished
2944version to the ftrace call site.
2945
2946Some archs do not even need to monkey around with the synchronization,
2947and can just slap the new code on top of the old without any
2948problems with other CPUs executing it at the same time.
2949
2950One special side-effect to the recording of the functions being
2951traced is that we can now selectively choose which functions we
2952wish to trace and which ones we want the mcount calls to remain
2953as nops.
2954
2955Two files are used, one for enabling and one for disabling the
2956tracing of specified functions. They are:
2957
2958 set_ftrace_filter
2959
2960and
2961
2962 set_ftrace_notrace
2963
2964A list of available functions that you can add to these files is
2965listed in:
2966
2967 available_filter_functions
2968
2969::
2970
2971 # cat available_filter_functions
2972 put_prev_task_idle
2973 kmem_cache_create
2974 pick_next_task_rt
2975 cpus_read_lock
2976 pick_next_task_fair
2977 mutex_lock
2978 [...]
2979
2980If I am only interested in sys_nanosleep and hrtimer_interrupt::
2981
2982 # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2983 # echo function > current_tracer
2984 # echo 1 > tracing_on
2985 # usleep 1
2986 # echo 0 > tracing_on
2987 # cat trace
2988 # tracer: function
2989 #
2990 # entries-in-buffer/entries-written: 5/5 #P:4
2991 #
2992 # _-----=> irqs-off
2993 # / _----=> need-resched
2994 # | / _---=> hardirq/softirq
2995 # || / _--=> preempt-depth
2996 # ||| / delay
2997 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
2998 # | | | |||| | |
2999 usleep-2665 [001] .... 4186.475355: sys_nanosleep <-system_call_fastpath
3000 <idle>-0 [001] d.h1 4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
3001 usleep-2665 [001] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
3002 <idle>-0 [003] d.h1 4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
3003 <idle>-0 [002] d.h1 4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
3004
3005To see which functions are being traced, you can cat the file:
3006::
3007
3008 # cat set_ftrace_filter
3009 hrtimer_interrupt
3010 sys_nanosleep
3011
3012
3013Perhaps this is not enough. The filters also allow glob(7) matching.
3014
3015 ``<match>*``
3016 will match functions that begin with <match>
3017 ``*<match>``
3018 will match functions that end with <match>
3019 ``*<match>*``
3020 will match functions that have <match> in it
3021 ``<match1>*<match2>``
3022 will match functions that begin with <match1> and end with <match2>
3023
3024.. note::
3025 It is better to use quotes to enclose the wild cards,
3026 otherwise the shell may expand the parameters into names
3027 of files in the local directory.
3028
3029::
3030
3031 # echo 'hrtimer_*' > set_ftrace_filter
3032
3033Produces::
3034
3035 # tracer: function
3036 #
3037 # entries-in-buffer/entries-written: 897/897 #P:4
3038 #
3039 # _-----=> irqs-off
3040 # / _----=> need-resched
3041 # | / _---=> hardirq/softirq
3042 # || / _--=> preempt-depth
3043 # ||| / delay
3044 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3045 # | | | |||| | |
3046 <idle>-0 [003] dN.1 4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
3047 <idle>-0 [003] dN.1 4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
3048 <idle>-0 [003] dN.2 4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
3049 <idle>-0 [003] dN.1 4228.547805: hrtimer_forward <-tick_nohz_idle_exit
3050 <idle>-0 [003] dN.1 4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
3051 <idle>-0 [003] d..1 4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
3052 <idle>-0 [003] d..1 4228.547859: hrtimer_start <-__tick_nohz_idle_enter
3053 <idle>-0 [003] d..2 4228.547860: hrtimer_force_reprogram <-__rem
3054
3055Notice that we lost the sys_nanosleep.
3056::
3057
3058 # cat set_ftrace_filter
3059 hrtimer_run_queues
3060 hrtimer_run_pending
3061 hrtimer_init
3062 hrtimer_cancel
3063 hrtimer_try_to_cancel
3064 hrtimer_forward
3065 hrtimer_start
3066 hrtimer_reprogram
3067 hrtimer_force_reprogram
3068 hrtimer_get_next_event
3069 hrtimer_interrupt
3070 hrtimer_nanosleep
3071 hrtimer_wakeup
3072 hrtimer_get_remaining
3073 hrtimer_get_res
3074 hrtimer_init_sleeper
3075
3076
3077This is because the '>' and '>>' act just like they do in bash.
3078To rewrite the filters, use '>'
3079To append to the filters, use '>>'
3080
3081To clear out a filter so that all functions will be recorded
3082again::
3083
3084 # echo > set_ftrace_filter
3085 # cat set_ftrace_filter
3086 #
3087
3088Again, now we want to append.
3089
3090::
3091
3092 # echo sys_nanosleep > set_ftrace_filter
3093 # cat set_ftrace_filter
3094 sys_nanosleep
3095 # echo 'hrtimer_*' >> set_ftrace_filter
3096 # cat set_ftrace_filter
3097 hrtimer_run_queues
3098 hrtimer_run_pending
3099 hrtimer_init
3100 hrtimer_cancel
3101 hrtimer_try_to_cancel
3102 hrtimer_forward
3103 hrtimer_start
3104 hrtimer_reprogram
3105 hrtimer_force_reprogram
3106 hrtimer_get_next_event
3107 hrtimer_interrupt
3108 sys_nanosleep
3109 hrtimer_nanosleep
3110 hrtimer_wakeup
3111 hrtimer_get_remaining
3112 hrtimer_get_res
3113 hrtimer_init_sleeper
3114
3115
3116The set_ftrace_notrace prevents those functions from being
3117traced.
3118::
3119
3120 # echo '*preempt*' '*lock*' > set_ftrace_notrace
3121
3122Produces::
3123
3124 # tracer: function
3125 #
3126 # entries-in-buffer/entries-written: 39608/39608 #P:4
3127 #
3128 # _-----=> irqs-off
3129 # / _----=> need-resched
3130 # | / _---=> hardirq/softirq
3131 # || / _--=> preempt-depth
3132 # ||| / delay
3133 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3134 # | | | |||| | |
3135 bash-1994 [000] .... 4342.324896: file_ra_state_init <-do_dentry_open
3136 bash-1994 [000] .... 4342.324897: open_check_o_direct <-do_last
3137 bash-1994 [000] .... 4342.324897: ima_file_check <-do_last
3138 bash-1994 [000] .... 4342.324898: process_measurement <-ima_file_check
3139 bash-1994 [000] .... 4342.324898: ima_get_action <-process_measurement
3140 bash-1994 [000] .... 4342.324898: ima_match_policy <-ima_get_action
3141 bash-1994 [000] .... 4342.324899: do_truncate <-do_last
3142 bash-1994 [000] .... 4342.324899: setattr_should_drop_suidgid <-do_truncate
3143 bash-1994 [000] .... 4342.324899: notify_change <-do_truncate
3144 bash-1994 [000] .... 4342.324900: current_fs_time <-notify_change
3145 bash-1994 [000] .... 4342.324900: current_kernel_time <-current_fs_time
3146 bash-1994 [000] .... 4342.324900: timespec_trunc <-current_fs_time
3147
3148We can see that there's no more lock or preempt tracing.
3149
3150Selecting function filters via index
3151------------------------------------
3152
3153Because processing of strings is expensive (the address of the function
3154needs to be looked up before comparing to the string being passed in),
3155an index can be used as well to enable functions. This is useful in the
3156case of setting thousands of specific functions at a time. By passing
3157in a list of numbers, no string processing will occur. Instead, the function
3158at the specific location in the internal array (which corresponds to the
3159functions in the "available_filter_functions" file), is selected.
3160
3161::
3162
3163 # echo 1 > set_ftrace_filter
3164
3165Will select the first function listed in "available_filter_functions"
3166
3167::
3168
3169 # head -1 available_filter_functions
3170 trace_initcall_finish_cb
3171
3172 # cat set_ftrace_filter
3173 trace_initcall_finish_cb
3174
3175 # head -50 available_filter_functions | tail -1
3176 x86_pmu_commit_txn
3177
3178 # echo 1 50 > set_ftrace_filter
3179 # cat set_ftrace_filter
3180 trace_initcall_finish_cb
3181 x86_pmu_commit_txn
3182
3183Dynamic ftrace with the function graph tracer
3184---------------------------------------------
3185
3186Although what has been explained above concerns both the
3187function tracer and the function-graph-tracer, there are some
3188special features only available in the function-graph tracer.
3189
3190If you want to trace only one function and all of its children,
3191you just have to echo its name into set_graph_function::
3192
3193 echo __do_fault > set_graph_function
3194
3195will produce the following "expanded" trace of the __do_fault()
3196function::
3197
3198 0) | __do_fault() {
3199 0) | filemap_fault() {
3200 0) | find_lock_page() {
3201 0) 0.804 us | find_get_page();
3202 0) | __might_sleep() {
3203 0) 1.329 us | }
3204 0) 3.904 us | }
3205 0) 4.979 us | }
3206 0) 0.653 us | _spin_lock();
3207 0) 0.578 us | page_add_file_rmap();
3208 0) 0.525 us | native_set_pte_at();
3209 0) 0.585 us | _spin_unlock();
3210 0) | unlock_page() {
3211 0) 0.541 us | page_waitqueue();
3212 0) 0.639 us | __wake_up_bit();
3213 0) 2.786 us | }
3214 0) + 14.237 us | }
3215 0) | __do_fault() {
3216 0) | filemap_fault() {
3217 0) | find_lock_page() {
3218 0) 0.698 us | find_get_page();
3219 0) | __might_sleep() {
3220 0) 1.412 us | }
3221 0) 3.950 us | }
3222 0) 5.098 us | }
3223 0) 0.631 us | _spin_lock();
3224 0) 0.571 us | page_add_file_rmap();
3225 0) 0.526 us | native_set_pte_at();
3226 0) 0.586 us | _spin_unlock();
3227 0) | unlock_page() {
3228 0) 0.533 us | page_waitqueue();
3229 0) 0.638 us | __wake_up_bit();
3230 0) 2.793 us | }
3231 0) + 14.012 us | }
3232
3233You can also expand several functions at once::
3234
3235 echo sys_open > set_graph_function
3236 echo sys_close >> set_graph_function
3237
3238Now if you want to go back to trace all functions you can clear
3239this special filter via::
3240
3241 echo > set_graph_function
3242
3243
3244ftrace_enabled
3245--------------
3246
3247Note, the proc sysctl ftrace_enable is a big on/off switch for the
3248function tracer. By default it is enabled (when function tracing is
3249enabled in the kernel). If it is disabled, all function tracing is
3250disabled. This includes not only the function tracers for ftrace, but
3251also for any other uses (perf, kprobes, stack tracing, profiling, etc). It
3252cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set
3253registered.
3254
3255Please disable this with care.
3256
3257This can be disable (and enabled) with::
3258
3259 sysctl kernel.ftrace_enabled=0
3260 sysctl kernel.ftrace_enabled=1
3261
3262 or
3263
3264 echo 0 > /proc/sys/kernel/ftrace_enabled
3265 echo 1 > /proc/sys/kernel/ftrace_enabled
3266
3267
3268Filter commands
3269---------------
3270
3271A few commands are supported by the set_ftrace_filter interface.
3272Trace commands have the following format::
3273
3274 <function>:<command>:<parameter>
3275
3276The following commands are supported:
3277
3278- mod:
3279 This command enables function filtering per module. The
3280 parameter defines the module. For example, if only the write*
3281 functions in the ext3 module are desired, run:
3282
3283 echo 'write*:mod:ext3' > set_ftrace_filter
3284
3285 This command interacts with the filter in the same way as
3286 filtering based on function names. Thus, adding more functions
3287 in a different module is accomplished by appending (>>) to the
3288 filter file. Remove specific module functions by prepending
3289 '!'::
3290
3291 echo '!writeback*:mod:ext3' >> set_ftrace_filter
3292
3293 Mod command supports module globbing. Disable tracing for all
3294 functions except a specific module::
3295
3296 echo '!*:mod:!ext3' >> set_ftrace_filter
3297
3298 Disable tracing for all modules, but still trace kernel::
3299
3300 echo '!*:mod:*' >> set_ftrace_filter
3301
3302 Enable filter only for kernel::
3303
3304 echo '*write*:mod:!*' >> set_ftrace_filter
3305
3306 Enable filter for module globbing::
3307
3308 echo '*write*:mod:*snd*' >> set_ftrace_filter
3309
3310- traceon/traceoff:
3311 These commands turn tracing on and off when the specified
3312 functions are hit. The parameter determines how many times the
3313 tracing system is turned on and off. If unspecified, there is
3314 no limit. For example, to disable tracing when a schedule bug
3315 is hit the first 5 times, run::
3316
3317 echo '__schedule_bug:traceoff:5' > set_ftrace_filter
3318
3319 To always disable tracing when __schedule_bug is hit::
3320
3321 echo '__schedule_bug:traceoff' > set_ftrace_filter
3322
3323 These commands are cumulative whether or not they are appended
3324 to set_ftrace_filter. To remove a command, prepend it by '!'
3325 and drop the parameter::
3326
3327 echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
3328
3329 The above removes the traceoff command for __schedule_bug
3330 that have a counter. To remove commands without counters::
3331
3332 echo '!__schedule_bug:traceoff' > set_ftrace_filter
3333
3334- snapshot:
3335 Will cause a snapshot to be triggered when the function is hit.
3336 ::
3337
3338 echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
3339
3340 To only snapshot once:
3341 ::
3342
3343 echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
3344
3345 To remove the above commands::
3346
3347 echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
3348 echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
3349
3350- enable_event/disable_event:
3351 These commands can enable or disable a trace event. Note, because
3352 function tracing callbacks are very sensitive, when these commands
3353 are registered, the trace point is activated, but disabled in
3354 a "soft" mode. That is, the tracepoint will be called, but
3355 just will not be traced. The event tracepoint stays in this mode
3356 as long as there's a command that triggers it.
3357 ::
3358
3359 echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
3360 set_ftrace_filter
3361
3362 The format is::
3363
3364 <function>:enable_event:<system>:<event>[:count]
3365 <function>:disable_event:<system>:<event>[:count]
3366
3367 To remove the events commands::
3368
3369 echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
3370 set_ftrace_filter
3371 echo '!schedule:disable_event:sched:sched_switch' > \
3372 set_ftrace_filter
3373
3374- dump:
3375 When the function is hit, it will dump the contents of the ftrace
3376 ring buffer to the console. This is useful if you need to debug
3377 something, and want to dump the trace when a certain function
3378 is hit. Perhaps it's a function that is called before a triple
3379 fault happens and does not allow you to get a regular dump.
3380
3381- cpudump:
3382 When the function is hit, it will dump the contents of the ftrace
3383 ring buffer for the current CPU to the console. Unlike the "dump"
3384 command, it only prints out the contents of the ring buffer for the
3385 CPU that executed the function that triggered the dump.
3386
3387- stacktrace:
3388 When the function is hit, a stack trace is recorded.
3389
3390trace_pipe
3391----------
3392
3393The trace_pipe outputs the same content as the trace file, but
3394the effect on the tracing is different. Every read from
3395trace_pipe is consumed. This means that subsequent reads will be
3396different. The trace is live.
3397::
3398
3399 # echo function > current_tracer
3400 # cat trace_pipe > /tmp/trace.out &
3401 [1] 4153
3402 # echo 1 > tracing_on
3403 # usleep 1
3404 # echo 0 > tracing_on
3405 # cat trace
3406 # tracer: function
3407 #
3408 # entries-in-buffer/entries-written: 0/0 #P:4
3409 #
3410 # _-----=> irqs-off
3411 # / _----=> need-resched
3412 # | / _---=> hardirq/softirq
3413 # || / _--=> preempt-depth
3414 # ||| / delay
3415 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3416 # | | | |||| | |
3417
3418 #
3419 # cat /tmp/trace.out
3420 bash-1994 [000] .... 5281.568961: mutex_unlock <-rb_simple_write
3421 bash-1994 [000] .... 5281.568963: __mutex_unlock_slowpath <-mutex_unlock
3422 bash-1994 [000] .... 5281.568963: __fsnotify_parent <-fsnotify_modify
3423 bash-1994 [000] .... 5281.568964: fsnotify <-fsnotify_modify
3424 bash-1994 [000] .... 5281.568964: __srcu_read_lock <-fsnotify
3425 bash-1994 [000] .... 5281.568964: add_preempt_count <-__srcu_read_lock
3426 bash-1994 [000] ...1 5281.568965: sub_preempt_count <-__srcu_read_lock
3427 bash-1994 [000] .... 5281.568965: __srcu_read_unlock <-fsnotify
3428 bash-1994 [000] .... 5281.568967: sys_dup2 <-system_call_fastpath
3429
3430
3431Note, reading the trace_pipe file will block until more input is
3432added. This is contrary to the trace file. If any process opened
3433the trace file for reading, it will actually disable tracing and
3434prevent new entries from being added. The trace_pipe file does
3435not have this limitation.
3436
3437trace entries
3438-------------
3439
3440Having too much or not enough data can be troublesome in
3441diagnosing an issue in the kernel. The file buffer_size_kb is
3442used to modify the size of the internal trace buffers. The
3443number listed is the number of entries that can be recorded per
3444CPU. To know the full size, multiply the number of possible CPUs
3445with the number of entries.
3446::
3447
3448 # cat buffer_size_kb
3449 1408 (units kilobytes)
3450
3451Or simply read buffer_total_size_kb
3452::
3453
3454 # cat buffer_total_size_kb
3455 5632
3456
3457To modify the buffer, simple echo in a number (in 1024 byte segments).
3458::
3459
3460 # echo 10000 > buffer_size_kb
3461 # cat buffer_size_kb
3462 10000 (units kilobytes)
3463
3464It will try to allocate as much as possible. If you allocate too
3465much, it can cause Out-Of-Memory to trigger.
3466::
3467
3468 # echo 1000000000000 > buffer_size_kb
3469 -bash: echo: write error: Cannot allocate memory
3470 # cat buffer_size_kb
3471 85
3472
3473The per_cpu buffers can be changed individually as well:
3474::
3475
3476 # echo 10000 > per_cpu/cpu0/buffer_size_kb
3477 # echo 100 > per_cpu/cpu1/buffer_size_kb
3478
3479When the per_cpu buffers are not the same, the buffer_size_kb
3480at the top level will just show an X
3481::
3482
3483 # cat buffer_size_kb
3484 X
3485
3486This is where the buffer_total_size_kb is useful:
3487::
3488
3489 # cat buffer_total_size_kb
3490 12916
3491
3492Writing to the top level buffer_size_kb will reset all the buffers
3493to be the same again.
3494
3495Snapshot
3496--------
3497CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
3498available to all non latency tracers. (Latency tracers which
3499record max latency, such as "irqsoff" or "wakeup", can't use
3500this feature, since those are already using the snapshot
3501mechanism internally.)
3502
3503Snapshot preserves a current trace buffer at a particular point
3504in time without stopping tracing. Ftrace swaps the current
3505buffer with a spare buffer, and tracing continues in the new
3506current (=previous spare) buffer.
3507
3508The following tracefs files in "tracing" are related to this
3509feature:
3510
3511 snapshot:
3512
3513 This is used to take a snapshot and to read the output
3514 of the snapshot. Echo 1 into this file to allocate a
3515 spare buffer and to take a snapshot (swap), then read
3516 the snapshot from this file in the same format as
3517 "trace" (described above in the section "The File
3518 System"). Both reads snapshot and tracing are executable
3519 in parallel. When the spare buffer is allocated, echoing
3520 0 frees it, and echoing else (positive) values clear the
3521 snapshot contents.
3522 More details are shown in the table below.
3523
3524 +--------------+------------+------------+------------+
3525 |status\\input | 0 | 1 | else |
3526 +==============+============+============+============+
3527 |not allocated |(do nothing)| alloc+swap |(do nothing)|
3528 +--------------+------------+------------+------------+
3529 |allocated | free | swap | clear |
3530 +--------------+------------+------------+------------+
3531
3532Here is an example of using the snapshot feature.
3533::
3534
3535 # echo 1 > events/sched/enable
3536 # echo 1 > snapshot
3537 # cat snapshot
3538 # tracer: nop
3539 #
3540 # entries-in-buffer/entries-written: 71/71 #P:8
3541 #
3542 # _-----=> irqs-off
3543 # / _----=> need-resched
3544 # | / _---=> hardirq/softirq
3545 # || / _--=> preempt-depth
3546 # ||| / delay
3547 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3548 # | | | |||| | |
3549 <idle>-0 [005] d... 2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
3550 sleep-2242 [005] d... 2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120 prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
3551 [...]
3552 <idle>-0 [002] d... 2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120
3553
3554 # cat trace
3555 # tracer: nop
3556 #
3557 # entries-in-buffer/entries-written: 77/77 #P:8
3558 #
3559 # _-----=> irqs-off
3560 # / _----=> need-resched
3561 # | / _---=> hardirq/softirq
3562 # || / _--=> preempt-depth
3563 # ||| / delay
3564 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3565 # | | | |||| | |
3566 <idle>-0 [007] d... 2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
3567 snapshot-test-2-2229 [002] d... 2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
3568 [...]
3569
3570
3571If you try to use this snapshot feature when current tracer is
3572one of the latency tracers, you will get the following results.
3573::
3574
3575 # echo wakeup > current_tracer
3576 # echo 1 > snapshot
3577 bash: echo: write error: Device or resource busy
3578 # cat snapshot
3579 cat: snapshot: Device or resource busy
3580
3581
3582Instances
3583---------
3584In the tracefs tracing directory, there is a directory called "instances".
3585This directory can have new directories created inside of it using
3586mkdir, and removing directories with rmdir. The directory created
3587with mkdir in this directory will already contain files and other
3588directories after it is created.
3589::
3590
3591 # mkdir instances/foo
3592 # ls instances/foo
3593 buffer_size_kb buffer_total_size_kb events free_buffer per_cpu
3594 set_event snapshot trace trace_clock trace_marker trace_options
3595 trace_pipe tracing_on
3596
3597As you can see, the new directory looks similar to the tracing directory
3598itself. In fact, it is very similar, except that the buffer and
3599events are agnostic from the main directory, or from any other
3600instances that are created.
3601
3602The files in the new directory work just like the files with the
3603same name in the tracing directory except the buffer that is used
3604is a separate and new buffer. The files affect that buffer but do not
3605affect the main buffer with the exception of trace_options. Currently,
3606the trace_options affect all instances and the top level buffer
3607the same, but this may change in future releases. That is, options
3608may become specific to the instance they reside in.
3609
3610Notice that none of the function tracer files are there, nor is
3611current_tracer and available_tracers. This is because the buffers
3612can currently only have events enabled for them.
3613::
3614
3615 # mkdir instances/foo
3616 # mkdir instances/bar
3617 # mkdir instances/zoot
3618 # echo 100000 > buffer_size_kb
3619 # echo 1000 > instances/foo/buffer_size_kb
3620 # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
3621 # echo function > current_trace
3622 # echo 1 > instances/foo/events/sched/sched_wakeup/enable
3623 # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
3624 # echo 1 > instances/foo/events/sched/sched_switch/enable
3625 # echo 1 > instances/bar/events/irq/enable
3626 # echo 1 > instances/zoot/events/syscalls/enable
3627 # cat trace_pipe
3628 CPU:2 [LOST 11745 EVENTS]
3629 bash-2044 [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
3630 bash-2044 [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
3631 bash-2044 [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
3632 bash-2044 [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
3633 bash-2044 [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
3634 bash-2044 [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
3635 bash-2044 [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
3636 bash-2044 [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
3637 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
3638 bash-2044 [002] d... 10594.481034: __inc_zone_state <-zone_statistics
3639 bash-2044 [002] .... 10594.481035: arch_dup_task_struct <-copy_process
3640 [...]
3641
3642 # cat instances/foo/trace_pipe
3643 bash-1998 [000] d..4 136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
3644 bash-1998 [000] dN.4 136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
3645 <idle>-0 [003] d.h3 136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
3646 <idle>-0 [003] d..3 136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
3647 rcu_preempt-9 [003] d..3 136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
3648 bash-1998 [000] d..4 136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
3649 bash-1998 [000] dN.4 136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
3650 bash-1998 [000] d..3 136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
3651 kworker/0:1-59 [000] d..4 136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
3652 kworker/0:1-59 [000] d..3 136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120
3653 [...]
3654
3655 # cat instances/bar/trace_pipe
3656 migration/1-14 [001] d.h3 138.732674: softirq_raise: vec=3 [action=NET_RX]
3657 <idle>-0 [001] dNh3 138.732725: softirq_raise: vec=3 [action=NET_RX]
3658 bash-1998 [000] d.h1 138.733101: softirq_raise: vec=1 [action=TIMER]
3659 bash-1998 [000] d.h1 138.733102: softirq_raise: vec=9 [action=RCU]
3660 bash-1998 [000] ..s2 138.733105: softirq_entry: vec=1 [action=TIMER]
3661 bash-1998 [000] ..s2 138.733106: softirq_exit: vec=1 [action=TIMER]
3662 bash-1998 [000] ..s2 138.733106: softirq_entry: vec=9 [action=RCU]
3663 bash-1998 [000] ..s2 138.733109: softirq_exit: vec=9 [action=RCU]
3664 sshd-1995 [001] d.h1 138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
3665 sshd-1995 [001] d.h1 138.733280: irq_handler_exit: irq=21 ret=unhandled
3666 sshd-1995 [001] d.h1 138.733281: irq_handler_entry: irq=21 name=eth0
3667 sshd-1995 [001] d.h1 138.733283: irq_handler_exit: irq=21 ret=handled
3668 [...]
3669
3670 # cat instances/zoot/trace
3671 # tracer: nop
3672 #
3673 # entries-in-buffer/entries-written: 18996/18996 #P:4
3674 #
3675 # _-----=> irqs-off
3676 # / _----=> need-resched
3677 # | / _---=> hardirq/softirq
3678 # || / _--=> preempt-depth
3679 # ||| / delay
3680 # TASK-PID CPU# |||| TIMESTAMP FUNCTION
3681 # | | | |||| | |
3682 bash-1998 [000] d... 140.733501: sys_write -> 0x2
3683 bash-1998 [000] d... 140.733504: sys_dup2(oldfd: a, newfd: 1)
3684 bash-1998 [000] d... 140.733506: sys_dup2 -> 0x1
3685 bash-1998 [000] d... 140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
3686 bash-1998 [000] d... 140.733509: sys_fcntl -> 0x1
3687 bash-1998 [000] d... 140.733510: sys_close(fd: a)
3688 bash-1998 [000] d... 140.733510: sys_close -> 0x0
3689 bash-1998 [000] d... 140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
3690 bash-1998 [000] d... 140.733515: sys_rt_sigprocmask -> 0x0
3691 bash-1998 [000] d... 140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
3692 bash-1998 [000] d... 140.733516: sys_rt_sigaction -> 0x0
3693
3694You can see that the trace of the top most trace buffer shows only
3695the function tracing. The foo instance displays wakeups and task
3696switches.
3697
3698To remove the instances, simply delete their directories:
3699::
3700
3701 # rmdir instances/foo
3702 # rmdir instances/bar
3703 # rmdir instances/zoot
3704
3705Note, if a process has a trace file open in one of the instance
3706directories, the rmdir will fail with EBUSY.
3707
3708
3709Stack trace
3710-----------
3711Since the kernel has a fixed sized stack, it is important not to
3712waste it in functions. A kernel developer must be conscious of
3713what they allocate on the stack. If they add too much, the system
3714can be in danger of a stack overflow, and corruption will occur,
3715usually leading to a system panic.
3716
3717There are some tools that check this, usually with interrupts
3718periodically checking usage. But if you can perform a check
3719at every function call that will become very useful. As ftrace provides
3720a function tracer, it makes it convenient to check the stack size
3721at every function call. This is enabled via the stack tracer.
3722
3723CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
3724To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
3725::
3726
3727 # echo 1 > /proc/sys/kernel/stack_tracer_enabled
3728
3729You can also enable it from the kernel command line to trace
3730the stack size of the kernel during boot up, by adding "stacktrace"
3731to the kernel command line parameter.
3732
3733After running it for a few minutes, the output looks like:
3734::
3735
3736 # cat stack_max_size
3737 2928
3738
3739 # cat stack_trace
3740 Depth Size Location (18 entries)
3741 ----- ---- --------
3742 0) 2928 224 update_sd_lb_stats+0xbc/0x4ac
3743 1) 2704 160 find_busiest_group+0x31/0x1f1
3744 2) 2544 256 load_balance+0xd9/0x662
3745 3) 2288 80 idle_balance+0xbb/0x130
3746 4) 2208 128 __schedule+0x26e/0x5b9
3747 5) 2080 16 schedule+0x64/0x66
3748 6) 2064 128 schedule_timeout+0x34/0xe0
3749 7) 1936 112 wait_for_common+0x97/0xf1
3750 8) 1824 16 wait_for_completion+0x1d/0x1f
3751 9) 1808 128 flush_work+0xfe/0x119
3752 10) 1680 16 tty_flush_to_ldisc+0x1e/0x20
3753 11) 1664 48 input_available_p+0x1d/0x5c
3754 12) 1616 48 n_tty_poll+0x6d/0x134
3755 13) 1568 64 tty_poll+0x64/0x7f
3756 14) 1504 880 do_select+0x31e/0x511
3757 15) 624 400 core_sys_select+0x177/0x216
3758 16) 224 96 sys_select+0x91/0xb9
3759 17) 128 128 system_call_fastpath+0x16/0x1b
3760
3761Note, if -mfentry is being used by gcc, functions get traced before
3762they set up the stack frame. This means that leaf level functions
3763are not tested by the stack tracer when -mfentry is used.
3764
3765Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
3766
3767More
3768----
3769More details can be found in the source code, in the `kernel/trace/*.c` files.