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1.. SPDX-License-Identifier: GPL-2.0
2
3======================
4Histogram Design Notes
5======================
6
7:Author: Tom Zanussi <zanussi@kernel.org>
8
9This document attempts to provide a description of how the ftrace
10histograms work and how the individual pieces map to the data
11structures used to implement them in trace_events_hist.c and
12tracing_map.c.
13
14Note: All the ftrace histogram command examples assume the working
15 directory is the ftrace /tracing directory. For example::
16
17 # cd /sys/kernel/tracing
18
19Also, the histogram output displayed for those commands will be
20generally be truncated - only enough to make the point is displayed.
21
22'hist_debug' trace event files
23==============================
24
25If the kernel is compiled with CONFIG_HIST_TRIGGERS_DEBUG set, an
26event file named 'hist_debug' will appear in each event's
27subdirectory. This file can be read at any time and will display some
28of the hist trigger internals described in this document. Specific
29examples and output will be described in test cases below.
30
31Basic histograms
32================
33
34First, basic histograms. Below is pretty much the simplest thing you
35can do with histograms - create one with a single key on a single
36event and cat the output::
37
38 # echo 'hist:keys=pid' >> events/sched/sched_waking/trigger
39
40 # cat events/sched/sched_waking/hist
41
42 { pid: 18249 } hitcount: 1
43 { pid: 13399 } hitcount: 1
44 { pid: 17973 } hitcount: 1
45 { pid: 12572 } hitcount: 1
46 ...
47 { pid: 10 } hitcount: 921
48 { pid: 18255 } hitcount: 1444
49 { pid: 25526 } hitcount: 2055
50 { pid: 5257 } hitcount: 2055
51 { pid: 27367 } hitcount: 2055
52 { pid: 1728 } hitcount: 2161
53
54 Totals:
55 Hits: 21305
56 Entries: 183
57 Dropped: 0
58
59What this does is create a histogram on the sched_waking event using
60pid as a key and with a single value, hitcount, which even if not
61explicitly specified, exists for every histogram regardless.
62
63The hitcount value is a per-bucket value that's automatically
64incremented on every hit for the given key, which in this case is the
65pid.
66
67So in this histogram, there's a separate bucket for each pid, and each
68bucket contains a value for that bucket, counting the number of times
69sched_waking was called for that pid.
70
71Each histogram is represented by a hist_data struct.
72
73To keep track of each key and value field in the histogram, hist_data
74keeps an array of these fields named fields[]. The fields[] array is
75an array containing struct hist_field representations of each
76histogram val and key in the histogram (variables are also included
77here, but are discussed later). So for the above histogram we have one
78key and one value; in this case the one value is the hitcount value,
79which all histograms have, regardless of whether they define that
80value or not, which the above histogram does not.
81
82Each struct hist_field contains a pointer to the ftrace_event_field
83from the event's trace_event_file along with various bits related to
84that such as the size, offset, type, and a hist_field_fn_t function,
85which is used to grab the field's data from the ftrace event buffer
86(in most cases - some hist_fields such as hitcount don't directly map
87to an event field in the trace buffer - in these cases the function
88implementation gets its value from somewhere else). The flags field
89indicates which type of field it is - key, value, variable, variable
90reference, etc., with value being the default.
91
92The other important hist_data data structure in addition to the
93fields[] array is the tracing_map instance created for the histogram,
94which is held in the .map member. The tracing_map implements the
95lock-free hash table used to implement histograms (see
96kernel/trace/tracing_map.h for much more discussion about the
97low-level data structures implementing the tracing_map). For the
98purposes of this discussion, the tracing_map contains a number of
99buckets, each bucket corresponding to a particular tracing_map_elt
100object hashed by a given histogram key.
101
102Below is a diagram the first part of which describes the hist_data and
103associated key and value fields for the histogram described above. As
104you can see, there are two fields in the fields array, one val field
105for the hitcount and one key field for the pid key.
106
107Below that is a diagram of a run-time snapshot of what the tracing_map
108might look like for a given run. It attempts to show the
109relationships between the hist_data fields and the tracing_map
110elements for a couple hypothetical keys and values.::
111
112 +------------------+
113 | hist_data |
114 +------------------+ +----------------+
115 | .fields[] |---->| val = hitcount |----------------------------+
116 +----------------+ +----------------+ |
117 | .map | | .size | |
118 +----------------+ +--------------+ |
119 | .offset | |
120 +--------------+ |
121 | .fn() | |
122 +--------------+ |
123 . |
124 . |
125 . |
126 +----------------+ <--- n_vals |
127 | key = pid |----------------------------|--+
128 +----------------+ | |
129 | .size | | |
130 +--------------+ | |
131 | .offset | | |
132 +--------------+ | |
133 | .fn() | | |
134 +----------------+ <--- n_fields | |
135 | unused | | |
136 +----------------+ | |
137 | | | |
138 +--------------+ | |
139 | | | |
140 +--------------+ | |
141 | | | |
142 +--------------+ | |
143 n_keys = n_fields - n_vals | |
144
145The hist_data n_vals and n_fields delineate the extent of the fields[] | |
146array and separate keys from values for the rest of the code. | |
147
148Below is a run-time representation of the tracing_map part of the | |
149histogram, with pointers from various parts of the fields[] array | |
150to corresponding parts of the tracing_map. | |
151
152The tracing_map consists of an array of tracing_map_entrys and a set | |
153of preallocated tracing_map_elts (abbreviated below as map_entry and | |
154map_elt). The total number of map_entrys in the hist_data.map array = | |
155map->max_elts (actually map->map_size but only max_elts of those are | |
156used. This is a property required by the map_insert() algorithm). | |
157
158If a map_entry is unused, meaning no key has yet hashed into it, its | |
159.key value is 0 and its .val pointer is NULL. Once a map_entry has | |
160been claimed, the .key value contains the key's hash value and the | |
161.val member points to a map_elt containing the full key and an entry | |
162for each key or value in the map_elt.fields[] array. There is an | |
163entry in the map_elt.fields[] array corresponding to each hist_field | |
164in the histogram, and this is where the continually aggregated sums | |
165corresponding to each histogram value are kept. | |
166
167The diagram attempts to show the relationship between the | |
168hist_data.fields[] and the map_elt.fields[] with the links drawn | |
169between diagrams::
170
171 +-----------+ | |
172 | hist_data | | |
173 +-----------+ | |
174 | .fields | | |
175 +---------+ +-----------+ | |
176 | .map |---->| map_entry | | |
177 +---------+ +-----------+ | |
178 | .key |---> 0 | |
179 +---------+ | |
180 | .val |---> NULL | |
181 +-----------+ | |
182 | map_entry | | |
183 +-----------+ | |
184 | .key |---> pid = 999 | |
185 +---------+ +-----------+ | |
186 | .val |--->| map_elt | | |
187 +---------+ +-----------+ | |
188 . | .key |---> full key * | |
189 . +---------+ +---------------+ | |
190 . | .fields |--->| .sum (val) |<-+ |
191 +-----------+ +---------+ | 2345 | | |
192 | map_entry | +---------------+ | |
193 +-----------+ | .offset (key) |<----+
194 | .key |---> 0 | 0 | | |
195 +---------+ +---------------+ | |
196 | .val |---> NULL . | |
197 +-----------+ . | |
198 | map_entry | . | |
199 +-----------+ +---------------+ | |
200 | .key | | .sum (val) or | | |
201 +---------+ +---------+ | .offset (key) | | |
202 | .val |--->| map_elt | +---------------+ | |
203 +-----------+ +---------+ | .sum (val) or | | |
204 | map_entry | | .offset (key) | | |
205 +-----------+ +---------------+ | |
206 | .key |---> pid = 4444 | |
207 +---------+ +-----------+ | |
208 | .val | | map_elt | | |
209 +---------+ +-----------+ | |
210 | .key |---> full key * | |
211 +---------+ +---------------+ | |
212 | .fields |--->| .sum (val) |<-+ |
213 +---------+ | 65523 | |
214 +---------------+ |
215 | .offset (key) |<----+
216 | 0 |
217 +---------------+
218 .
219 .
220 .
221 +---------------+
222 | .sum (val) or |
223 | .offset (key) |
224 +---------------+
225 | .sum (val) or |
226 | .offset (key) |
227 +---------------+
228
229Abbreviations used in the diagrams::
230
231 hist_data = struct hist_trigger_data
232 hist_data.fields = struct hist_field
233 fn = hist_field_fn_t
234 map_entry = struct tracing_map_entry
235 map_elt = struct tracing_map_elt
236 map_elt.fields = struct tracing_map_field
237
238Whenever a new event occurs and it has a hist trigger associated with
239it, event_hist_trigger() is called. event_hist_trigger() first deals
240with the key: for each subkey in the key (in the above example, there
241is just one subkey corresponding to pid), the hist_field that
242represents that subkey is retrieved from hist_data.fields[] and the
243hist_field_fn_t fn() associated with that field, along with the
244field's size and offset, is used to grab that subkey's data from the
245current trace record.
246
247Once the complete key has been retrieved, it's used to look that key
248up in the tracing_map. If there's no tracing_map_elt associated with
249that key, an empty one is claimed and inserted in the map for the new
250key. In either case, the tracing_map_elt associated with that key is
251returned.
252
253Once a tracing_map_elt available, hist_trigger_elt_update() is called.
254As the name implies, this updates the element, which basically means
255updating the element's fields. There's a tracing_map_field associated
256with each key and value in the histogram, and each of these correspond
257to the key and value hist_fields created when the histogram was
258created. hist_trigger_elt_update() goes through each value hist_field
259and, as for the keys, uses the hist_field's fn() and size and offset
260to grab the field's value from the current trace record. Once it has
261that value, it simply adds that value to that field's
262continually-updated tracing_map_field.sum member. Some hist_field
263fn()s, such as for the hitcount, don't actually grab anything from the
264trace record (the hitcount fn() just increments the counter sum by 1),
265but the idea is the same.
266
267Once all the values have been updated, hist_trigger_elt_update() is
268done and returns. Note that there are also tracing_map_fields for
269each subkey in the key, but hist_trigger_elt_update() doesn't look at
270them or update anything - those exist only for sorting, which can
271happen later.
272
273Basic histogram test
274--------------------
275
276This is a good example to try. It produces 3 value fields and 2 key
277fields in the output::
278
279 # echo 'hist:keys=common_pid,call_site.sym:values=bytes_req,bytes_alloc,hitcount' >> events/kmem/kmalloc/trigger
280
281To see the debug data, cat the kmem/kmalloc's 'hist_debug' file. It
282will show the trigger info of the histogram it corresponds to, along
283with the address of the hist_data associated with the histogram, which
284will become useful in later examples. It then displays the number of
285total hist_fields associated with the histogram along with a count of
286how many of those correspond to keys and how many correspond to values.
287
288It then goes on to display details for each field, including the
289field's flags and the position of each field in the hist_data's
290fields[] array, which is useful information for verifying that things
291internally appear correct or not, and which again will become even
292more useful in further examples::
293
294 # cat events/kmem/kmalloc/hist_debug
295
296 # event histogram
297 #
298 # trigger info: hist:keys=common_pid,call_site.sym:vals=hitcount,bytes_req,bytes_alloc:sort=hitcount:size=2048 [active]
299 #
300
301 hist_data: 000000005e48c9a5
302
303 n_vals: 3
304 n_keys: 2
305 n_fields: 5
306
307 val fields:
308
309 hist_data->fields[0]:
310 flags:
311 VAL: HIST_FIELD_FL_HITCOUNT
312 type: u64
313 size: 8
314 is_signed: 0
315
316 hist_data->fields[1]:
317 flags:
318 VAL: normal u64 value
319 ftrace_event_field name: bytes_req
320 type: size_t
321 size: 8
322 is_signed: 0
323
324 hist_data->fields[2]:
325 flags:
326 VAL: normal u64 value
327 ftrace_event_field name: bytes_alloc
328 type: size_t
329 size: 8
330 is_signed: 0
331
332 key fields:
333
334 hist_data->fields[3]:
335 flags:
336 HIST_FIELD_FL_KEY
337 ftrace_event_field name: common_pid
338 type: int
339 size: 8
340 is_signed: 1
341
342 hist_data->fields[4]:
343 flags:
344 HIST_FIELD_FL_KEY
345 ftrace_event_field name: call_site
346 type: unsigned long
347 size: 8
348 is_signed: 0
349
350The commands below can be used to clean things up for the next test::
351
352 # echo '!hist:keys=common_pid,call_site.sym:values=bytes_req,bytes_alloc,hitcount' >> events/kmem/kmalloc/trigger
353
354Variables
355=========
356
357Variables allow data from one hist trigger to be saved by one hist
358trigger and retrieved by another hist trigger. For example, a trigger
359on the sched_waking event can capture a timestamp for a particular
360pid, and later a sched_switch event that switches to that pid event
361can grab the timestamp and use it to calculate a time delta between
362the two events::
363
364 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >>
365 events/sched/sched_waking/trigger
366
367 # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >>
368 events/sched/sched_switch/trigger
369
370In terms of the histogram data structures, variables are implemented
371as another type of hist_field and for a given hist trigger are added
372to the hist_data.fields[] array just after all the val fields. To
373distinguish them from the existing key and val fields, they're given a
374new flag type, HIST_FIELD_FL_VAR (abbreviated FL_VAR) and they also
375make use of a new .var.idx field member in struct hist_field, which
376maps them to an index in a new map_elt.vars[] array added to the
377map_elt specifically designed to store and retrieve variable values.
378The diagram below shows those new elements and adds a new variable
379entry, ts0, corresponding to the ts0 variable in the sched_waking
380trigger above.
381
382sched_waking histogram
383----------------------::
384
385 +------------------+
386 | hist_data |<-------------------------------------------------------+
387 +------------------+ +-------------------+ |
388 | .fields[] |-->| val = hitcount | |
389 +----------------+ +-------------------+ |
390 | .map | | .size | |
391 +----------------+ +-----------------+ |
392 | .offset | |
393 +-----------------+ |
394 | .fn() | |
395 +-----------------+ |
396 | .flags | |
397 +-----------------+ |
398 | .var.idx | |
399 +-------------------+ |
400 | var = ts0 | |
401 +-------------------+ |
402 | .size | |
403 +-----------------+ |
404 | .offset | |
405 +-----------------+ |
406 | .fn() | |
407 +-----------------+ |
408 | .flags & FL_VAR | |
409 +-----------------+ |
410 | .var.idx |----------------------------+-+ |
411 +-----------------+ | | |
412 . | | |
413 . | | |
414 . | | |
415 +-------------------+ <--- n_vals | | |
416 | key = pid | | | |
417 +-------------------+ | | |
418 | .size | | | |
419 +-----------------+ | | |
420 | .offset | | | |
421 +-----------------+ | | |
422 | .fn() | | | |
423 +-----------------+ | | |
424 | .flags & FL_KEY | | | |
425 +-----------------+ | | |
426 | .var.idx | | | |
427 +-------------------+ <--- n_fields | | |
428 | unused | | | |
429 +-------------------+ | | |
430 | | | | |
431 +-----------------+ | | |
432 | | | | |
433 +-----------------+ | | |
434 | | | | |
435 +-----------------+ | | |
436 | | | | |
437 +-----------------+ | | |
438 | | | | |
439 +-----------------+ | | |
440 n_keys = n_fields - n_vals | | |
441 | | |
442
443This is very similar to the basic case. In the above diagram, we can | | |
444see a new .flags member has been added to the struct hist_field | | |
445struct, and a new entry added to hist_data.fields representing the ts0 | | |
446variable. For a normal val hist_field, .flags is just 0 (modulo | | |
447modifier flags), but if the value is defined as a variable, the .flags | | |
448contains a set FL_VAR bit. | | |
449
450As you can see, the ts0 entry's .var.idx member contains the index | | |
451into the tracing_map_elts' .vars[] array containing variable values. | | |
452This idx is used whenever the value of the variable is set or read. | | |
453The map_elt.vars idx assigned to the given variable is assigned and | | |
454saved in .var.idx by create_tracing_map_fields() after it calls | | |
455tracing_map_add_var(). | | |
456
457Below is a representation of the histogram at run-time, which | | |
458populates the map, along with correspondence to the above hist_data and | | |
459hist_field data structures. | | |
460
461The diagram attempts to show the relationship between the | | |
462hist_data.fields[] and the map_elt.fields[] and map_elt.vars[] with | | |
463the links drawn between diagrams. For each of the map_elts, you can | | |
464see that the .fields[] members point to the .sum or .offset of a key | | |
465or val and the .vars[] members point to the value of a variable. The | | |
466arrows between the two diagrams show the linkages between those | | |
467tracing_map members and the field definitions in the corresponding | | |
468hist_data fields[] members.::
469
470 +-----------+ | | |
471 | hist_data | | | |
472 +-----------+ | | |
473 | .fields | | | |
474 +---------+ +-----------+ | | |
475 | .map |---->| map_entry | | | |
476 +---------+ +-----------+ | | |
477 | .key |---> 0 | | |
478 +---------+ | | |
479 | .val |---> NULL | | |
480 +-----------+ | | |
481 | map_entry | | | |
482 +-----------+ | | |
483 | .key |---> pid = 999 | | |
484 +---------+ +-----------+ | | |
485 | .val |--->| map_elt | | | |
486 +---------+ +-----------+ | | |
487 . | .key |---> full key * | | |
488 . +---------+ +---------------+ | | |
489 . | .fields |--->| .sum (val) | | | |
490 . +---------+ | 2345 | | | |
491 . +--| .vars | +---------------+ | | |
492 . | +---------+ | .offset (key) | | | |
493 . | | 0 | | | |
494 . | +---------------+ | | |
495 . | . | | |
496 . | . | | |
497 . | . | | |
498 . | +---------------+ | | |
499 . | | .sum (val) or | | | |
500 . | | .offset (key) | | | |
501 . | +---------------+ | | |
502 . | | .sum (val) or | | | |
503 . | | .offset (key) | | | |
504 . | +---------------+ | | |
505 . | | | |
506 . +---------------->+---------------+ | | |
507 . | ts0 |<--+ | |
508 . | 113345679876 | | | |
509 . +---------------+ | | |
510 . | unused | | | |
511 . | | | | |
512 . +---------------+ | | |
513 . . | | |
514 . . | | |
515 . . | | |
516 . +---------------+ | | |
517 . | unused | | | |
518 . | | | | |
519 . +---------------+ | | |
520 . | unused | | | |
521 . | | | | |
522 . +---------------+ | | |
523 . | | |
524 +-----------+ | | |
525 | map_entry | | | |
526 +-----------+ | | |
527 | .key |---> pid = 4444 | | |
528 +---------+ +-----------+ | | |
529 | .val |--->| map_elt | | | |
530 +---------+ +-----------+ | | |
531 . | .key |---> full key * | | |
532 . +---------+ +---------------+ | | |
533 . | .fields |--->| .sum (val) | | | |
534 +---------+ | 2345 | | | |
535 +--| .vars | +---------------+ | | |
536 | +---------+ | .offset (key) | | | |
537 | | 0 | | | |
538 | +---------------+ | | |
539 | . | | |
540 | . | | |
541 | . | | |
542 | +---------------+ | | |
543 | | .sum (val) or | | | |
544 | | .offset (key) | | | |
545 | +---------------+ | | |
546 | | .sum (val) or | | | |
547 | | .offset (key) | | | |
548 | +---------------+ | | |
549 | | | |
550 | +---------------+ | | |
551 +---------------->| ts0 |<--+ | |
552 | 213499240729 | | |
553 +---------------+ | |
554 | unused | | |
555 | | | |
556 +---------------+ | |
557 . | |
558 . | |
559 . | |
560 +---------------+ | |
561 | unused | | |
562 | | | |
563 +---------------+ | |
564 | unused | | |
565 | | | |
566 +---------------+ | |
567
568For each used map entry, there's a map_elt pointing to an array of | |
569.vars containing the current value of the variables associated with | |
570that histogram entry. So in the above, the timestamp associated with | |
571pid 999 is 113345679876, and the timestamp variable in the same | |
572.var.idx for pid 4444 is 213499240729. | |
573
574sched_switch histogram | |
575---------------------- | |
576
577The sched_switch histogram paired with the above sched_waking | |
578histogram is shown below. The most important aspect of the | |
579sched_switch histogram is that it references a variable on the | |
580sched_waking histogram above. | |
581
582The histogram diagram is very similar to the others so far displayed, | |
583but it adds variable references. You can see the normal hitcount and | |
584key fields along with a new wakeup_lat variable implemented in the | |
585same way as the sched_waking ts0 variable, but in addition there's an | |
586entry with the new FL_VAR_REF (short for HIST_FIELD_FL_VAR_REF) flag. | |
587
588Associated with the new var ref field are a couple of new hist_field | |
589members, var.hist_data and var_ref_idx. For a variable reference, the | |
590var.hist_data goes with the var.idx, which together uniquely identify | |
591a particular variable on a particular histogram. The var_ref_idx is | |
592just the index into the var_ref_vals[] array that caches the values of | |
593each variable whenever a hist trigger is updated. Those resulting | |
594values are then finally accessed by other code such as trace action | |
595code that uses the var_ref_idx values to assign param values. | |
596
597The diagram below describes the situation for the sched_switch | |
598histogram referred to before::
599
600 # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> | |
601 events/sched/sched_switch/trigger | |
602 | |
603 +------------------+ | |
604 | hist_data | | |
605 +------------------+ +-----------------------+ | |
606 | .fields[] |-->| val = hitcount | | |
607 +----------------+ +-----------------------+ | |
608 | .map | | .size | | |
609 +----------------+ +---------------------+ | |
610 +--| .var_refs[] | | .offset | | |
611 | +----------------+ +---------------------+ | |
612 | | .fn() | | |
613 | var_ref_vals[] +---------------------+ | |
614 | +-------------+ | .flags | | |
615 | | $ts0 |<---+ +---------------------+ | |
616 | +-------------+ | | .var.idx | | |
617 | | | | +---------------------+ | |
618 | +-------------+ | | .var.hist_data | | |
619 | | | | +---------------------+ | |
620 | +-------------+ | | .var_ref_idx | | |
621 | | | | +-----------------------+ | |
622 | +-------------+ | | var = wakeup_lat | | |
623 | . | +-----------------------+ | |
624 | . | | .size | | |
625 | . | +---------------------+ | |
626 | +-------------+ | | .offset | | |
627 | | | | +---------------------+ | |
628 | +-------------+ | | .fn() | | |
629 | | | | +---------------------+ | |
630 | +-------------+ | | .flags & FL_VAR | | |
631 | | +---------------------+ | |
632 | | | .var.idx | | |
633 | | +---------------------+ | |
634 | | | .var.hist_data | | |
635 | | +---------------------+ | |
636 | | | .var_ref_idx | | |
637 | | +---------------------+ | |
638 | | . | |
639 | | . | |
640 | | . | |
641 | | +-----------------------+ <--- n_vals | |
642 | | | key = pid | | |
643 | | +-----------------------+ | |
644 | | | .size | | |
645 | | +---------------------+ | |
646 | | | .offset | | |
647 | | +---------------------+ | |
648 | | | .fn() | | |
649 | | +---------------------+ | |
650 | | | .flags | | |
651 | | +---------------------+ | |
652 | | | .var.idx | | |
653 | | +-----------------------+ <--- n_fields | |
654 | | | unused | | |
655 | | +-----------------------+ | |
656 | | | | | |
657 | | +---------------------+ | |
658 | | | | | |
659 | | +---------------------+ | |
660 | | | | | |
661 | | +---------------------+ | |
662 | | | | | |
663 | | +---------------------+ | |
664 | | | | | |
665 | | +---------------------+ | |
666 | | n_keys = n_fields - n_vals | |
667 | | | |
668 | | | |
669 | | +-----------------------+ | |
670 +---------------------->| var_ref = $ts0 | | |
671 | +-----------------------+ | |
672 | | .size | | |
673 | +---------------------+ | |
674 | | .offset | | |
675 | +---------------------+ | |
676 | | .fn() | | |
677 | +---------------------+ | |
678 | | .flags & FL_VAR_REF | | |
679 | +---------------------+ | |
680 | | .var.idx |--------------------------+ |
681 | +---------------------+ |
682 | | .var.hist_data |----------------------------+
683 | +---------------------+
684 +---| .var_ref_idx |
685 +---------------------+
686
687Abbreviations used in the diagrams::
688
689 hist_data = struct hist_trigger_data
690 hist_data.fields = struct hist_field
691 fn = hist_field_fn_t
692 FL_KEY = HIST_FIELD_FL_KEY
693 FL_VAR = HIST_FIELD_FL_VAR
694 FL_VAR_REF = HIST_FIELD_FL_VAR_REF
695
696When a hist trigger makes use of a variable, a new hist_field is
697created with flag HIST_FIELD_FL_VAR_REF. For a VAR_REF field, the
698var.idx and var.hist_data take the same values as the referenced
699variable, as well as the referenced variable's size, type, and
700is_signed values. The VAR_REF field's .name is set to the name of the
701variable it references. If a variable reference was created using the
702explicit system.event.$var_ref notation, the hist_field's system and
703event_name variables are also set.
704
705So, in order to handle an event for the sched_switch histogram,
706because we have a reference to a variable on another histogram, we
707need to resolve all variable references first. This is done via the
708resolve_var_refs() calls made from event_hist_trigger(). What this
709does is grabs the var_refs[] array from the hist_data representing the
710sched_switch histogram. For each one of those, the referenced
711variable's var.hist_data along with the current key is used to look up
712the corresponding tracing_map_elt in that histogram. Once found, the
713referenced variable's var.idx is used to look up the variable's value
714using tracing_map_read_var(elt, var.idx), which yields the value of
715the variable for that element, ts0 in the case above. Note that both
716the hist_fields representing both the variable and the variable
717reference have the same var.idx, so this is straightforward.
718
719Variable and variable reference test
720------------------------------------
721
722This example creates a variable on the sched_waking event, ts0, and
723uses it in the sched_switch trigger. The sched_switch trigger also
724creates its own variable, wakeup_lat, but nothing yet uses it::
725
726 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
727
728 # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> events/sched/sched_switch/trigger
729
730Looking at the sched_waking 'hist_debug' output, in addition to the
731normal key and value hist_fields, in the val fields section we see a
732field with the HIST_FIELD_FL_VAR flag, which indicates that that field
733represents a variable. Note that in addition to the variable name,
734contained in the var.name field, it includes the var.idx, which is the
735index into the tracing_map_elt.vars[] array of the actual variable
736location. Note also that the output shows that variables live in the
737same part of the hist_data->fields[] array as normal values::
738
739 # cat events/sched/sched_waking/hist_debug
740
741 # event histogram
742 #
743 # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
744 #
745
746 hist_data: 000000009536f554
747
748 n_vals: 2
749 n_keys: 1
750 n_fields: 3
751
752 val fields:
753
754 hist_data->fields[0]:
755 flags:
756 VAL: HIST_FIELD_FL_HITCOUNT
757 type: u64
758 size: 8
759 is_signed: 0
760
761 hist_data->fields[1]:
762 flags:
763 HIST_FIELD_FL_VAR
764 var.name: ts0
765 var.idx (into tracing_map_elt.vars[]): 0
766 type: u64
767 size: 8
768 is_signed: 0
769
770 key fields:
771
772 hist_data->fields[2]:
773 flags:
774 HIST_FIELD_FL_KEY
775 ftrace_event_field name: pid
776 type: pid_t
777 size: 8
778 is_signed: 1
779
780Moving on to the sched_switch trigger hist_debug output, in addition
781to the unused wakeup_lat variable, we see a new section displaying
782variable references. Variable references are displayed in a separate
783section because in addition to being logically separate from
784variables and values, they actually live in a separate hist_data
785array, var_refs[].
786
787In this example, the sched_switch trigger has a reference to a
788variable on the sched_waking trigger, $ts0. Looking at the details,
789we can see that the var.hist_data value of the referenced variable
790matches the previously displayed sched_waking trigger, and the var.idx
791value matches the previously displayed var.idx value for that
792variable. Also displayed is the var_ref_idx value for that variable
793reference, which is where the value for that variable is cached for
794use when the trigger is invoked::
795
796 # cat events/sched/sched_switch/hist_debug
797
798 # event histogram
799 #
800 # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global [active]
801 #
802
803 hist_data: 00000000f4ee8006
804
805 n_vals: 2
806 n_keys: 1
807 n_fields: 3
808
809 val fields:
810
811 hist_data->fields[0]:
812 flags:
813 VAL: HIST_FIELD_FL_HITCOUNT
814 type: u64
815 size: 8
816 is_signed: 0
817
818 hist_data->fields[1]:
819 flags:
820 HIST_FIELD_FL_VAR
821 var.name: wakeup_lat
822 var.idx (into tracing_map_elt.vars[]): 0
823 type: u64
824 size: 0
825 is_signed: 0
826
827 key fields:
828
829 hist_data->fields[2]:
830 flags:
831 HIST_FIELD_FL_KEY
832 ftrace_event_field name: next_pid
833 type: pid_t
834 size: 8
835 is_signed: 1
836
837 variable reference fields:
838
839 hist_data->var_refs[0]:
840 flags:
841 HIST_FIELD_FL_VAR_REF
842 name: ts0
843 var.idx (into tracing_map_elt.vars[]): 0
844 var.hist_data: 000000009536f554
845 var_ref_idx (into hist_data->var_refs[]): 0
846 type: u64
847 size: 8
848 is_signed: 0
849
850The commands below can be used to clean things up for the next test::
851
852 # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> events/sched/sched_switch/trigger
853
854 # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
855
856Actions and Handlers
857====================
858
859Adding onto the previous example, we will now do something with that
860wakeup_lat variable, namely send it and another field as a synthetic
861event.
862
863The onmatch() action below basically says that whenever we have a
864sched_switch event, if we have a matching sched_waking event, in this
865case if we have a pid in the sched_waking histogram that matches the
866next_pid field on this sched_switch event, we retrieve the
867variables specified in the wakeup_latency() trace action, and use
868them to generate a new wakeup_latency event into the trace stream.
869
870Note that the way the trace handlers such as wakeup_latency() (which
871could equivalently be written trace(wakeup_latency,$wakeup_lat,next_pid)
872are implemented, the parameters specified to the trace handler must be
873variables. In this case, $wakeup_lat is obviously a variable, but
874next_pid isn't, since it's just naming a field in the sched_switch
875trace event. Since this is something that almost every trace() and
876save() action does, a special shortcut is implemented to allow field
877names to be used directly in those cases. How it works is that under
878the covers, a temporary variable is created for the named field, and
879this variable is what is actually passed to the trace handler. In the
880code and documentation, this type of variable is called a 'field
881variable'.
882
883Fields on other trace event's histograms can be used as well. In that
884case we have to generate a new histogram and an unfortunately named
885'synthetic_field' (the use of synthetic here has nothing to do with
886synthetic events) and use that special histogram field as a variable.
887
888The diagram below illustrates the new elements described above in the
889context of the sched_switch histogram using the onmatch() handler and
890the trace() action.
891
892First, we define the wakeup_latency synthetic event::
893
894 # echo 'wakeup_latency u64 lat; pid_t pid' >> synthetic_events
895
896Next, the sched_waking hist trigger as before::
897
898 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >>
899 events/sched/sched_waking/trigger
900
901Finally, we create a hist trigger on the sched_switch event that
902generates a wakeup_latency() trace event. In this case we pass
903next_pid into the wakeup_latency synthetic event invocation, which
904means it will be automatically converted into a field variable::
905
906 # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
907 onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid)' >>
908 /sys/kernel/tracing/events/sched/sched_switch/trigger
909
910The diagram for the sched_switch event is similar to previous examples
911but shows the additional field_vars[] array for hist_data and shows
912the linkages between the field_vars and the variables and references
913created to implement the field variables. The details are discussed
914below::
915
916 +------------------+
917 | hist_data |
918 +------------------+ +-----------------------+
919 | .fields[] |-->| val = hitcount |
920 +----------------+ +-----------------------+
921 | .map | | .size |
922 +----------------+ +---------------------+
923 +---| .field_vars[] | | .offset |
924 | +----------------+ +---------------------+
925 |+--| .var_refs[] | | .offset |
926 || +----------------+ +---------------------+
927 || | .fn() |
928 || var_ref_vals[] +---------------------+
929 || +-------------+ | .flags |
930 || | $ts0 |<---+ +---------------------+
931 || +-------------+ | | .var.idx |
932 || | $next_pid |<-+ | +---------------------+
933 || +-------------+ | | | .var.hist_data |
934 ||+>| $wakeup_lat | | | +---------------------+
935 ||| +-------------+ | | | .var_ref_idx |
936 ||| | | | | +-----------------------+
937 ||| +-------------+ | | | var = wakeup_lat |
938 ||| . | | +-----------------------+
939 ||| . | | | .size |
940 ||| . | | +---------------------+
941 ||| +-------------+ | | | .offset |
942 ||| | | | | +---------------------+
943 ||| +-------------+ | | | .fn() |
944 ||| | | | | +---------------------+
945 ||| +-------------+ | | | .flags & FL_VAR |
946 ||| | | +---------------------+
947 ||| | | | .var.idx |
948 ||| | | +---------------------+
949 ||| | | | .var.hist_data |
950 ||| | | +---------------------+
951 ||| | | | .var_ref_idx |
952 ||| | | +---------------------+
953 ||| | | .
954 ||| | | .
955 ||| | | .
956 ||| | | .
957 ||| +--------------+ | | .
958 +-->| field_var | | | .
959 || +--------------+ | | .
960 || | var | | | .
961 || +------------+ | | .
962 || | val | | | .
963 || +--------------+ | | .
964 || | field_var | | | .
965 || +--------------+ | | .
966 || | var | | | .
967 || +------------+ | | .
968 || | val | | | .
969 || +------------+ | | .
970 || . | | .
971 || . | | .
972 || . | | +-----------------------+ <--- n_vals
973 || +--------------+ | | | key = pid |
974 || | field_var | | | +-----------------------+
975 || +--------------+ | | | .size |
976 || | var |--+| +---------------------+
977 || +------------+ ||| | .offset |
978 || | val |-+|| +---------------------+
979 || +------------+ ||| | .fn() |
980 || ||| +---------------------+
981 || ||| | .flags |
982 || ||| +---------------------+
983 || ||| | .var.idx |
984 || ||| +---------------------+ <--- n_fields
985 || |||
986 || ||| n_keys = n_fields - n_vals
987 || ||| +-----------------------+
988 || |+->| var = next_pid |
989 || | | +-----------------------+
990 || | | | .size |
991 || | | +---------------------+
992 || | | | .offset |
993 || | | +---------------------+
994 || | | | .flags & FL_VAR |
995 || | | +---------------------+
996 || | | | .var.idx |
997 || | | +---------------------+
998 || | | | .var.hist_data |
999 || | | +-----------------------+
1000 || +-->| val for next_pid |
1001 || | | +-----------------------+
1002 || | | | .size |
1003 || | | +---------------------+
1004 || | | | .offset |
1005 || | | +---------------------+
1006 || | | | .fn() |
1007 || | | +---------------------+
1008 || | | | .flags |
1009 || | | +---------------------+
1010 || | | | |
1011 || | | +---------------------+
1012 || | |
1013 || | |
1014 || | | +-----------------------+
1015 +|------------------|-|>| var_ref = $ts0 |
1016 | | | +-----------------------+
1017 | | | | .size |
1018 | | | +---------------------+
1019 | | | | .offset |
1020 | | | +---------------------+
1021 | | | | .fn() |
1022 | | | +---------------------+
1023 | | | | .flags & FL_VAR_REF |
1024 | | | +---------------------+
1025 | | +---| .var_ref_idx |
1026 | | +-----------------------+
1027 | | | var_ref = $next_pid |
1028 | | +-----------------------+
1029 | | | .size |
1030 | | +---------------------+
1031 | | | .offset |
1032 | | +---------------------+
1033 | | | .fn() |
1034 | | +---------------------+
1035 | | | .flags & FL_VAR_REF |
1036 | | +---------------------+
1037 | +-----| .var_ref_idx |
1038 | +-----------------------+
1039 | | var_ref = $wakeup_lat |
1040 | +-----------------------+
1041 | | .size |
1042 | +---------------------+
1043 | | .offset |
1044 | +---------------------+
1045 | | .fn() |
1046 | +---------------------+
1047 | | .flags & FL_VAR_REF |
1048 | +---------------------+
1049 +------------------------| .var_ref_idx |
1050 +---------------------+
1051
1052As you can see, for a field variable, two hist_fields are created: one
1053representing the variable, in this case next_pid, and one to actually
1054get the value of the field from the trace stream, like a normal val
1055field does. These are created separately from normal variable
1056creation and are saved in the hist_data->field_vars[] array. See
1057below for how these are used. In addition, a reference hist_field is
1058also created, which is needed to reference the field variables such as
1059$next_pid variable in the trace() action.
1060
1061Note that $wakeup_lat is also a variable reference, referencing the
1062value of the expression common_timestamp-$ts0, and so also needs to
1063have a hist field entry representing that reference created.
1064
1065When hist_trigger_elt_update() is called to get the normal key and
1066value fields, it also calls update_field_vars(), which goes through
1067each field_var created for the histogram, and available from
1068hist_data->field_vars and calls val->fn() to get the data from the
1069current trace record, and then uses the var's var.idx to set the
1070variable at the var.idx offset in the appropriate tracing_map_elt's
1071variable at elt->vars[var.idx].
1072
1073Once all the variables have been updated, resolve_var_refs() can be
1074called from event_hist_trigger(), and not only can our $ts0 and
1075$next_pid references be resolved but the $wakeup_lat reference as
1076well. At this point, the trace() action can simply access the values
1077assembled in the var_ref_vals[] array and generate the trace event.
1078
1079The same process occurs for the field variables associated with the
1080save() action.
1081
1082Abbreviations used in the diagram::
1083
1084 hist_data = struct hist_trigger_data
1085 hist_data.fields = struct hist_field
1086 field_var = struct field_var
1087 fn = hist_field_fn_t
1088 FL_KEY = HIST_FIELD_FL_KEY
1089 FL_VAR = HIST_FIELD_FL_VAR
1090 FL_VAR_REF = HIST_FIELD_FL_VAR_REF
1091
1092trace() action field variable test
1093----------------------------------
1094
1095This example adds to the previous test example by finally making use
1096of the wakeup_lat variable, but in addition also creates a couple of
1097field variables that then are all passed to the wakeup_latency() trace
1098action via the onmatch() handler.
1099
1100First, we create the wakeup_latency synthetic event::
1101
1102 # echo 'wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
1103
1104Next, the sched_waking trigger from previous examples::
1105
1106 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
1107
1108Finally, as in the previous test example, we calculate and assign the
1109wakeup latency using the $ts0 reference from the sched_waking trigger
1110to the wakeup_lat variable, and finally use it along with a couple
1111sched_switch event fields, next_pid and next_comm, to generate a
1112wakeup_latency trace event. The next_pid and next_comm event fields
1113are automatically converted into field variables for this purpose::
1114
1115 # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm)' >> /sys/kernel/tracing/events/sched/sched_switch/trigger
1116
1117The sched_waking hist_debug output shows the same data as in the
1118previous test example::
1119
1120 # cat events/sched/sched_waking/hist_debug
1121
1122 # event histogram
1123 #
1124 # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
1125 #
1126
1127 hist_data: 00000000d60ff61f
1128
1129 n_vals: 2
1130 n_keys: 1
1131 n_fields: 3
1132
1133 val fields:
1134
1135 hist_data->fields[0]:
1136 flags:
1137 VAL: HIST_FIELD_FL_HITCOUNT
1138 type: u64
1139 size: 8
1140 is_signed: 0
1141
1142 hist_data->fields[1]:
1143 flags:
1144 HIST_FIELD_FL_VAR
1145 var.name: ts0
1146 var.idx (into tracing_map_elt.vars[]): 0
1147 type: u64
1148 size: 8
1149 is_signed: 0
1150
1151 key fields:
1152
1153 hist_data->fields[2]:
1154 flags:
1155 HIST_FIELD_FL_KEY
1156 ftrace_event_field name: pid
1157 type: pid_t
1158 size: 8
1159 is_signed: 1
1160
1161The sched_switch hist_debug output shows the same key and value fields
1162as in the previous test example - note that wakeup_lat is still in the
1163val fields section, but that the new field variables are not there -
1164although the field variables are variables, they're held separately in
1165the hist_data's field_vars[] array. Although the field variables and
1166the normal variables are located in separate places, you can see that
1167the actual variable locations for those variables in the
1168tracing_map_elt.vars[] do have increasing indices as expected:
1169wakeup_lat takes the var.idx = 0 slot, while the field variables for
1170next_pid and next_comm have values var.idx = 1, and var.idx = 2. Note
1171also that those are the same values displayed for the variable
1172references corresponding to those variables in the variable reference
1173fields section. Since there are two triggers and thus two hist_data
1174addresses, those addresses also need to be accounted for when doing
1175the matching - you can see that the first variable refers to the 0
1176var.idx on the previous hist trigger (see the hist_data address
1177associated with that trigger), while the second variable refers to the
11780 var.idx on the sched_switch hist trigger, as do all the remaining
1179variable references.
1180
1181Finally, the action tracking variables section just shows the system
1182and event name for the onmatch() handler::
1183
1184 # cat events/sched/sched_switch/hist_debug
1185
1186 # event histogram
1187 #
1188 # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm) [active]
1189 #
1190
1191 hist_data: 0000000008f551b7
1192
1193 n_vals: 2
1194 n_keys: 1
1195 n_fields: 3
1196
1197 val fields:
1198
1199 hist_data->fields[0]:
1200 flags:
1201 VAL: HIST_FIELD_FL_HITCOUNT
1202 type: u64
1203 size: 8
1204 is_signed: 0
1205
1206 hist_data->fields[1]:
1207 flags:
1208 HIST_FIELD_FL_VAR
1209 var.name: wakeup_lat
1210 var.idx (into tracing_map_elt.vars[]): 0
1211 type: u64
1212 size: 0
1213 is_signed: 0
1214
1215 key fields:
1216
1217 hist_data->fields[2]:
1218 flags:
1219 HIST_FIELD_FL_KEY
1220 ftrace_event_field name: next_pid
1221 type: pid_t
1222 size: 8
1223 is_signed: 1
1224
1225 variable reference fields:
1226
1227 hist_data->var_refs[0]:
1228 flags:
1229 HIST_FIELD_FL_VAR_REF
1230 name: ts0
1231 var.idx (into tracing_map_elt.vars[]): 0
1232 var.hist_data: 00000000d60ff61f
1233 var_ref_idx (into hist_data->var_refs[]): 0
1234 type: u64
1235 size: 8
1236 is_signed: 0
1237
1238 hist_data->var_refs[1]:
1239 flags:
1240 HIST_FIELD_FL_VAR_REF
1241 name: wakeup_lat
1242 var.idx (into tracing_map_elt.vars[]): 0
1243 var.hist_data: 0000000008f551b7
1244 var_ref_idx (into hist_data->var_refs[]): 1
1245 type: u64
1246 size: 0
1247 is_signed: 0
1248
1249 hist_data->var_refs[2]:
1250 flags:
1251 HIST_FIELD_FL_VAR_REF
1252 name: next_pid
1253 var.idx (into tracing_map_elt.vars[]): 1
1254 var.hist_data: 0000000008f551b7
1255 var_ref_idx (into hist_data->var_refs[]): 2
1256 type: pid_t
1257 size: 4
1258 is_signed: 0
1259
1260 hist_data->var_refs[3]:
1261 flags:
1262 HIST_FIELD_FL_VAR_REF
1263 name: next_comm
1264 var.idx (into tracing_map_elt.vars[]): 2
1265 var.hist_data: 0000000008f551b7
1266 var_ref_idx (into hist_data->var_refs[]): 3
1267 type: char[16]
1268 size: 256
1269 is_signed: 0
1270
1271 field variables:
1272
1273 hist_data->field_vars[0]:
1274
1275 field_vars[0].var:
1276 flags:
1277 HIST_FIELD_FL_VAR
1278 var.name: next_pid
1279 var.idx (into tracing_map_elt.vars[]): 1
1280
1281 field_vars[0].val:
1282 ftrace_event_field name: next_pid
1283 type: pid_t
1284 size: 4
1285 is_signed: 1
1286
1287 hist_data->field_vars[1]:
1288
1289 field_vars[1].var:
1290 flags:
1291 HIST_FIELD_FL_VAR
1292 var.name: next_comm
1293 var.idx (into tracing_map_elt.vars[]): 2
1294
1295 field_vars[1].val:
1296 ftrace_event_field name: next_comm
1297 type: char[16]
1298 size: 256
1299 is_signed: 0
1300
1301 action tracking variables (for onmax()/onchange()/onmatch()):
1302
1303 hist_data->actions[0].match_data.event_system: sched
1304 hist_data->actions[0].match_data.event: sched_waking
1305
1306The commands below can be used to clean things up for the next test::
1307
1308 # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm)' >> /sys/kernel/tracing/events/sched/sched_switch/trigger
1309
1310 # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
1311
1312 # echo '!wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
1313
1314action_data and the trace() action
1315----------------------------------
1316
1317As mentioned above, when the trace() action generates a synthetic
1318event, all the parameters to the synthetic event either already are
1319variables or are converted into variables (via field variables), and
1320finally all those variable values are collected via references to them
1321into a var_ref_vals[] array.
1322
1323The values in the var_ref_vals[] array, however, don't necessarily
1324follow the same ordering as the synthetic event params. To address
1325that, struct action_data contains another array, var_ref_idx[] that
1326maps the trace action params to the var_ref_vals[] values. Below is a
1327diagram illustrating that for the wakeup_latency() synthetic event::
1328
1329 +------------------+ wakeup_latency()
1330 | action_data | event params var_ref_vals[]
1331 +------------------+ +-----------------+ +-----------------+
1332 | .var_ref_idx[] |--->| $wakeup_lat idx |---+ | |
1333 +----------------+ +-----------------+ | +-----------------+
1334 | .synth_event | | $next_pid idx |---|-+ | $wakeup_lat val |
1335 +----------------+ +-----------------+ | | +-----------------+
1336 . | +->| $next_pid val |
1337 . | +-----------------+
1338 . | .
1339 +-----------------+ | .
1340 | | | .
1341 +-----------------+ | +-----------------+
1342 +--->| $wakeup_lat val |
1343 +-----------------+
1344
1345Basically, how this ends up getting used in the synthetic event probe
1346function, trace_event_raw_event_synth(), is as follows::
1347
1348 for each field i in .synth_event
1349 val_idx = .var_ref_idx[i]
1350 val = var_ref_vals[val_idx]
1351
1352action_data and the onXXX() handlers
1353------------------------------------
1354
1355The hist trigger onXXX() actions other than onmatch(), such as onmax()
1356and onchange(), also make use of and internally create hidden
1357variables. This information is contained in the
1358action_data.track_data struct, and is also visible in the hist_debug
1359output as will be described in the example below.
1360
1361Typically, the onmax() or onchange() handlers are used in conjunction
1362with the save() and snapshot() actions. For example::
1363
1364 # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
1365 onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >>
1366 /sys/kernel/tracing/events/sched/sched_switch/trigger
1367
1368or::
1369
1370 # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
1371 onmax($wakeup_lat).snapshot()' >>
1372 /sys/kernel/tracing/events/sched/sched_switch/trigger
1373
1374save() action field variable test
1375---------------------------------
1376
1377For this example, instead of generating a synthetic event, the save()
1378action is used to save field values whenever an onmax() handler
1379detects that a new max latency has been hit. As in the previous
1380example, the values being saved are also field values, but in this
1381case, are kept in a separate hist_data array named save_vars[].
1382
1383As in previous test examples, we set up the sched_waking trigger::
1384
1385 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
1386
1387In this case, however, we set up the sched_switch trigger to save some
1388sched_switch field values whenever we hit a new maximum latency. For
1389both the onmax() handler and save() action, variables will be created,
1390which we can use the hist_debug files to examine::
1391
1392 # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >> events/sched/sched_switch/trigger
1393
1394The sched_waking hist_debug output shows the same data as in the
1395previous test examples::
1396
1397 # cat events/sched/sched_waking/hist_debug
1398
1399 #
1400 # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
1401 #
1402
1403 hist_data: 00000000e6290f48
1404
1405 n_vals: 2
1406 n_keys: 1
1407 n_fields: 3
1408
1409 val fields:
1410
1411 hist_data->fields[0]:
1412 flags:
1413 VAL: HIST_FIELD_FL_HITCOUNT
1414 type: u64
1415 size: 8
1416 is_signed: 0
1417
1418 hist_data->fields[1]:
1419 flags:
1420 HIST_FIELD_FL_VAR
1421 var.name: ts0
1422 var.idx (into tracing_map_elt.vars[]): 0
1423 type: u64
1424 size: 8
1425 is_signed: 0
1426
1427 key fields:
1428
1429 hist_data->fields[2]:
1430 flags:
1431 HIST_FIELD_FL_KEY
1432 ftrace_event_field name: pid
1433 type: pid_t
1434 size: 8
1435 is_signed: 1
1436
1437The output of the sched_switch trigger shows the same val and key
1438values as before, but also shows a couple new sections.
1439
1440First, the action tracking variables section now shows the
1441actions[].track_data information describing the special tracking
1442variables and references used to track, in this case, the running
1443maximum value. The actions[].track_data.var_ref member contains the
1444reference to the variable being tracked, in this case the $wakeup_lat
1445variable. In order to perform the onmax() handler function, there
1446also needs to be a variable that tracks the current maximum by getting
1447updated whenever a new maximum is hit. In this case, we can see that
1448an auto-generated variable named ' __max' has been created and is
1449visible in the actions[].track_data.track_var variable.
1450
1451Finally, in the new 'save action variables' section, we can see that
1452the 4 params to the save() function have resulted in 4 field variables
1453being created for the purposes of saving the values of the named
1454fields when the max is hit. These variables are kept in a separate
1455save_vars[] array off of hist_data, so are displayed in a separate
1456section::
1457
1458 # cat events/sched/sched_switch/hist_debug
1459
1460 # event histogram
1461 #
1462 # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm) [active]
1463 #
1464
1465 hist_data: 0000000057bcd28d
1466
1467 n_vals: 2
1468 n_keys: 1
1469 n_fields: 3
1470
1471 val fields:
1472
1473 hist_data->fields[0]:
1474 flags:
1475 VAL: HIST_FIELD_FL_HITCOUNT
1476 type: u64
1477 size: 8
1478 is_signed: 0
1479
1480 hist_data->fields[1]:
1481 flags:
1482 HIST_FIELD_FL_VAR
1483 var.name: wakeup_lat
1484 var.idx (into tracing_map_elt.vars[]): 0
1485 type: u64
1486 size: 0
1487 is_signed: 0
1488
1489 key fields:
1490
1491 hist_data->fields[2]:
1492 flags:
1493 HIST_FIELD_FL_KEY
1494 ftrace_event_field name: next_pid
1495 type: pid_t
1496 size: 8
1497 is_signed: 1
1498
1499 variable reference fields:
1500
1501 hist_data->var_refs[0]:
1502 flags:
1503 HIST_FIELD_FL_VAR_REF
1504 name: ts0
1505 var.idx (into tracing_map_elt.vars[]): 0
1506 var.hist_data: 00000000e6290f48
1507 var_ref_idx (into hist_data->var_refs[]): 0
1508 type: u64
1509 size: 8
1510 is_signed: 0
1511
1512 hist_data->var_refs[1]:
1513 flags:
1514 HIST_FIELD_FL_VAR_REF
1515 name: wakeup_lat
1516 var.idx (into tracing_map_elt.vars[]): 0
1517 var.hist_data: 0000000057bcd28d
1518 var_ref_idx (into hist_data->var_refs[]): 1
1519 type: u64
1520 size: 0
1521 is_signed: 0
1522
1523 action tracking variables (for onmax()/onchange()/onmatch()):
1524
1525 hist_data->actions[0].track_data.var_ref:
1526 flags:
1527 HIST_FIELD_FL_VAR_REF
1528 name: wakeup_lat
1529 var.idx (into tracing_map_elt.vars[]): 0
1530 var.hist_data: 0000000057bcd28d
1531 var_ref_idx (into hist_data->var_refs[]): 1
1532 type: u64
1533 size: 0
1534 is_signed: 0
1535
1536 hist_data->actions[0].track_data.track_var:
1537 flags:
1538 HIST_FIELD_FL_VAR
1539 var.name: __max
1540 var.idx (into tracing_map_elt.vars[]): 1
1541 type: u64
1542 size: 8
1543 is_signed: 0
1544
1545 save action variables (save() params):
1546
1547 hist_data->save_vars[0]:
1548
1549 save_vars[0].var:
1550 flags:
1551 HIST_FIELD_FL_VAR
1552 var.name: next_comm
1553 var.idx (into tracing_map_elt.vars[]): 2
1554
1555 save_vars[0].val:
1556 ftrace_event_field name: next_comm
1557 type: char[16]
1558 size: 256
1559 is_signed: 0
1560
1561 hist_data->save_vars[1]:
1562
1563 save_vars[1].var:
1564 flags:
1565 HIST_FIELD_FL_VAR
1566 var.name: prev_pid
1567 var.idx (into tracing_map_elt.vars[]): 3
1568
1569 save_vars[1].val:
1570 ftrace_event_field name: prev_pid
1571 type: pid_t
1572 size: 4
1573 is_signed: 1
1574
1575 hist_data->save_vars[2]:
1576
1577 save_vars[2].var:
1578 flags:
1579 HIST_FIELD_FL_VAR
1580 var.name: prev_prio
1581 var.idx (into tracing_map_elt.vars[]): 4
1582
1583 save_vars[2].val:
1584 ftrace_event_field name: prev_prio
1585 type: int
1586 size: 4
1587 is_signed: 1
1588
1589 hist_data->save_vars[3]:
1590
1591 save_vars[3].var:
1592 flags:
1593 HIST_FIELD_FL_VAR
1594 var.name: prev_comm
1595 var.idx (into tracing_map_elt.vars[]): 5
1596
1597 save_vars[3].val:
1598 ftrace_event_field name: prev_comm
1599 type: char[16]
1600 size: 256
1601 is_signed: 0
1602
1603The commands below can be used to clean things up for the next test::
1604
1605 # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >> events/sched/sched_switch/trigger
1606
1607 # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
1608
1609A couple special cases
1610======================
1611
1612While the above covers the basics of the histogram internals, there
1613are a couple of special cases that should be discussed, since they
1614tend to create even more confusion. Those are field variables on other
1615histograms, and aliases, both described below through example tests
1616using the hist_debug files.
1617
1618Test of field variables on other histograms
1619-------------------------------------------
1620
1621This example is similar to the previous examples, but in this case,
1622the sched_switch trigger references a hist trigger field on another
1623event, namely the sched_waking event. In order to accomplish this, a
1624field variable is created for the other event, but since an existing
1625histogram can't be used, as existing histograms are immutable, a new
1626histogram with a matching variable is created and used, and we'll see
1627that reflected in the hist_debug output shown below.
1628
1629First, we create the wakeup_latency synthetic event. Note the
1630addition of the prio field::
1631
1632 # echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> synthetic_events
1633
1634As in previous test examples, we set up the sched_waking trigger::
1635
1636 # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
1637
1638Here we set up a hist trigger on sched_switch to send a wakeup_latency
1639event using an onmatch handler naming the sched_waking event. Note
1640that the third param being passed to the wakeup_latency() is prio,
1641which is a field name that needs to have a field variable created for
1642it. There isn't however any prio field on the sched_switch event so
1643it would seem that it wouldn't be possible to create a field variable
1644for it. The matching sched_waking event does have a prio field, so it
1645should be possible to make use of it for this purpose. The problem
1646with that is that it's not currently possible to define a new variable
1647on an existing histogram, so it's not possible to add a new prio field
1648variable to the existing sched_waking histogram. It is however
1649possible to create an additional new 'matching' sched_waking histogram
1650for the same event, meaning that it uses the same key and filters, and
1651define the new prio field variable on that.
1652
1653Here's the sched_switch trigger::
1654
1655 # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio)' >> events/sched/sched_switch/trigger
1656
1657And here's the output of the hist_debug information for the
1658sched_waking hist trigger. Note that there are two histograms
1659displayed in the output: the first is the normal sched_waking
1660histogram we've seen in the previous examples, and the second is the
1661special histogram we created to provide the prio field variable.
1662
1663Looking at the second histogram below, we see a variable with the name
1664synthetic_prio. This is the field variable created for the prio field
1665on that sched_waking histogram::
1666
1667 # cat events/sched/sched_waking/hist_debug
1668
1669 # event histogram
1670 #
1671 # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
1672 #
1673
1674 hist_data: 00000000349570e4
1675
1676 n_vals: 2
1677 n_keys: 1
1678 n_fields: 3
1679
1680 val fields:
1681
1682 hist_data->fields[0]:
1683 flags:
1684 VAL: HIST_FIELD_FL_HITCOUNT
1685 type: u64
1686 size: 8
1687 is_signed: 0
1688
1689 hist_data->fields[1]:
1690 flags:
1691 HIST_FIELD_FL_VAR
1692 var.name: ts0
1693 var.idx (into tracing_map_elt.vars[]): 0
1694 type: u64
1695 size: 8
1696 is_signed: 0
1697
1698 key fields:
1699
1700 hist_data->fields[2]:
1701 flags:
1702 HIST_FIELD_FL_KEY
1703 ftrace_event_field name: pid
1704 type: pid_t
1705 size: 8
1706 is_signed: 1
1707
1708
1709 # event histogram
1710 #
1711 # trigger info: hist:keys=pid:vals=hitcount:synthetic_prio=prio:sort=hitcount:size=2048 [active]
1712 #
1713
1714 hist_data: 000000006920cf38
1715
1716 n_vals: 2
1717 n_keys: 1
1718 n_fields: 3
1719
1720 val fields:
1721
1722 hist_data->fields[0]:
1723 flags:
1724 VAL: HIST_FIELD_FL_HITCOUNT
1725 type: u64
1726 size: 8
1727 is_signed: 0
1728
1729 hist_data->fields[1]:
1730 flags:
1731 HIST_FIELD_FL_VAR
1732 ftrace_event_field name: prio
1733 var.name: synthetic_prio
1734 var.idx (into tracing_map_elt.vars[]): 0
1735 type: int
1736 size: 4
1737 is_signed: 1
1738
1739 key fields:
1740
1741 hist_data->fields[2]:
1742 flags:
1743 HIST_FIELD_FL_KEY
1744 ftrace_event_field name: pid
1745 type: pid_t
1746 size: 8
1747 is_signed: 1
1748
1749Looking at the sched_switch histogram below, we can see a reference to
1750the synthetic_prio variable on sched_waking, and looking at the
1751associated hist_data address we see that it is indeed associated with
1752the new histogram. Note also that the other references are to a
1753normal variable, wakeup_lat, and to a normal field variable, next_pid,
1754the details of which are in the field variables section::
1755
1756 # cat events/sched/sched_switch/hist_debug
1757
1758 # event histogram
1759 #
1760 # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio) [active]
1761 #
1762
1763 hist_data: 00000000a73b67df
1764
1765 n_vals: 2
1766 n_keys: 1
1767 n_fields: 3
1768
1769 val fields:
1770
1771 hist_data->fields[0]:
1772 flags:
1773 VAL: HIST_FIELD_FL_HITCOUNT
1774 type: u64
1775 size: 8
1776 is_signed: 0
1777
1778 hist_data->fields[1]:
1779 flags:
1780 HIST_FIELD_FL_VAR
1781 var.name: wakeup_lat
1782 var.idx (into tracing_map_elt.vars[]): 0
1783 type: u64
1784 size: 0
1785 is_signed: 0
1786
1787 key fields:
1788
1789 hist_data->fields[2]:
1790 flags:
1791 HIST_FIELD_FL_KEY
1792 ftrace_event_field name: next_pid
1793 type: pid_t
1794 size: 8
1795 is_signed: 1
1796
1797 variable reference fields:
1798
1799 hist_data->var_refs[0]:
1800 flags:
1801 HIST_FIELD_FL_VAR_REF
1802 name: ts0
1803 var.idx (into tracing_map_elt.vars[]): 0
1804 var.hist_data: 00000000349570e4
1805 var_ref_idx (into hist_data->var_refs[]): 0
1806 type: u64
1807 size: 8
1808 is_signed: 0
1809
1810 hist_data->var_refs[1]:
1811 flags:
1812 HIST_FIELD_FL_VAR_REF
1813 name: wakeup_lat
1814 var.idx (into tracing_map_elt.vars[]): 0
1815 var.hist_data: 00000000a73b67df
1816 var_ref_idx (into hist_data->var_refs[]): 1
1817 type: u64
1818 size: 0
1819 is_signed: 0
1820
1821 hist_data->var_refs[2]:
1822 flags:
1823 HIST_FIELD_FL_VAR_REF
1824 name: next_pid
1825 var.idx (into tracing_map_elt.vars[]): 1
1826 var.hist_data: 00000000a73b67df
1827 var_ref_idx (into hist_data->var_refs[]): 2
1828 type: pid_t
1829 size: 4
1830 is_signed: 0
1831
1832 hist_data->var_refs[3]:
1833 flags:
1834 HIST_FIELD_FL_VAR_REF
1835 name: synthetic_prio
1836 var.idx (into tracing_map_elt.vars[]): 0
1837 var.hist_data: 000000006920cf38
1838 var_ref_idx (into hist_data->var_refs[]): 3
1839 type: int
1840 size: 4
1841 is_signed: 1
1842
1843 field variables:
1844
1845 hist_data->field_vars[0]:
1846
1847 field_vars[0].var:
1848 flags:
1849 HIST_FIELD_FL_VAR
1850 var.name: next_pid
1851 var.idx (into tracing_map_elt.vars[]): 1
1852
1853 field_vars[0].val:
1854 ftrace_event_field name: next_pid
1855 type: pid_t
1856 size: 4
1857 is_signed: 1
1858
1859 action tracking variables (for onmax()/onchange()/onmatch()):
1860
1861 hist_data->actions[0].match_data.event_system: sched
1862 hist_data->actions[0].match_data.event: sched_waking
1863
1864The commands below can be used to clean things up for the next test::
1865
1866 # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio)' >> events/sched/sched_switch/trigger
1867
1868 # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
1869
1870 # echo '!wakeup_latency u64 lat; pid_t pid; int prio' >> synthetic_events
1871
1872Alias test
1873----------
1874
1875This example is very similar to previous examples, but demonstrates
1876the alias flag.
1877
1878First, we create the wakeup_latency synthetic event::
1879
1880 # echo 'wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events
1881
1882Next, we create a sched_waking trigger similar to previous examples,
1883but in this case we save the pid in the waking_pid variable::
1884
1885 # echo 'hist:keys=pid:waking_pid=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
1886
1887For the sched_switch trigger, instead of using $waking_pid directly in
1888the wakeup_latency synthetic event invocation, we create an alias of
1889$waking_pid named $woken_pid, and use that in the synthetic event
1890invocation instead::
1891
1892 # echo 'hist:keys=next_pid:woken_pid=$waking_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm)' >> events/sched/sched_switch/trigger
1893
1894Looking at the sched_waking hist_debug output, in addition to the
1895normal fields, we can see the waking_pid variable::
1896
1897 # cat events/sched/sched_waking/hist_debug
1898
1899 # event histogram
1900 #
1901 # trigger info: hist:keys=pid:vals=hitcount:waking_pid=pid,ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active]
1902 #
1903
1904 hist_data: 00000000a250528c
1905
1906 n_vals: 3
1907 n_keys: 1
1908 n_fields: 4
1909
1910 val fields:
1911
1912 hist_data->fields[0]:
1913 flags:
1914 VAL: HIST_FIELD_FL_HITCOUNT
1915 type: u64
1916 size: 8
1917 is_signed: 0
1918
1919 hist_data->fields[1]:
1920 flags:
1921 HIST_FIELD_FL_VAR
1922 ftrace_event_field name: pid
1923 var.name: waking_pid
1924 var.idx (into tracing_map_elt.vars[]): 0
1925 type: pid_t
1926 size: 4
1927 is_signed: 1
1928
1929 hist_data->fields[2]:
1930 flags:
1931 HIST_FIELD_FL_VAR
1932 var.name: ts0
1933 var.idx (into tracing_map_elt.vars[]): 1
1934 type: u64
1935 size: 8
1936 is_signed: 0
1937
1938 key fields:
1939
1940 hist_data->fields[3]:
1941 flags:
1942 HIST_FIELD_FL_KEY
1943 ftrace_event_field name: pid
1944 type: pid_t
1945 size: 8
1946 is_signed: 1
1947
1948The sched_switch hist_debug output shows that a variable named
1949woken_pid has been created but that it also has the
1950HIST_FIELD_FL_ALIAS flag set. It also has the HIST_FIELD_FL_VAR flag
1951set, which is why it appears in the val field section.
1952
1953Despite that implementation detail, an alias variable is actually more
1954like a variable reference; in fact it can be thought of as a reference
1955to a reference. The implementation copies the var_ref->fn() from the
1956variable reference being referenced, in this case, the waking_pid
1957fn(), which is hist_field_var_ref() and makes that the fn() of the
1958alias. The hist_field_var_ref() fn() requires the var_ref_idx of the
1959variable reference it's using, so waking_pid's var_ref_idx is also
1960copied to the alias. The end result is that when the value of alias
1961is retrieved, in the end it just does the same thing the original
1962reference would have done and retrieves the same value from the
1963var_ref_vals[] array. You can verify this in the output by noting
1964that the var_ref_idx of the alias, in this case woken_pid, is the same
1965as the var_ref_idx of the reference, waking_pid, in the variable
1966reference fields section.
1967
1968Additionally, once it gets that value, since it is also a variable, it
1969then saves that value into its var.idx. So the var.idx of the
1970woken_pid alias is 0, which it fills with the value from var_ref_idx 0
1971when its fn() is called to update itself. You'll also notice that
1972there's a woken_pid var_ref in the variable refs section. That is the
1973reference to the woken_pid alias variable, and you can see that it
1974retrieves the value from the same var.idx as the woken_pid alias, 0,
1975and then in turn saves that value in its own var_ref_idx slot, 3, and
1976the value at this position is finally what gets assigned to the
1977$woken_pid slot in the trace event invocation::
1978
1979 # cat events/sched/sched_switch/hist_debug
1980
1981 # event histogram
1982 #
1983 # trigger info: hist:keys=next_pid:vals=hitcount:woken_pid=$waking_pid,wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm) [active]
1984 #
1985
1986 hist_data: 0000000055d65ed0
1987
1988 n_vals: 3
1989 n_keys: 1
1990 n_fields: 4
1991
1992 val fields:
1993
1994 hist_data->fields[0]:
1995 flags:
1996 VAL: HIST_FIELD_FL_HITCOUNT
1997 type: u64
1998 size: 8
1999 is_signed: 0
2000
2001 hist_data->fields[1]:
2002 flags:
2003 HIST_FIELD_FL_VAR
2004 HIST_FIELD_FL_ALIAS
2005 var.name: woken_pid
2006 var.idx (into tracing_map_elt.vars[]): 0
2007 var_ref_idx (into hist_data->var_refs[]): 0
2008 type: pid_t
2009 size: 4
2010 is_signed: 1
2011
2012 hist_data->fields[2]:
2013 flags:
2014 HIST_FIELD_FL_VAR
2015 var.name: wakeup_lat
2016 var.idx (into tracing_map_elt.vars[]): 1
2017 type: u64
2018 size: 0
2019 is_signed: 0
2020
2021 key fields:
2022
2023 hist_data->fields[3]:
2024 flags:
2025 HIST_FIELD_FL_KEY
2026 ftrace_event_field name: next_pid
2027 type: pid_t
2028 size: 8
2029 is_signed: 1
2030
2031 variable reference fields:
2032
2033 hist_data->var_refs[0]:
2034 flags:
2035 HIST_FIELD_FL_VAR_REF
2036 name: waking_pid
2037 var.idx (into tracing_map_elt.vars[]): 0
2038 var.hist_data: 00000000a250528c
2039 var_ref_idx (into hist_data->var_refs[]): 0
2040 type: pid_t
2041 size: 4
2042 is_signed: 1
2043
2044 hist_data->var_refs[1]:
2045 flags:
2046 HIST_FIELD_FL_VAR_REF
2047 name: ts0
2048 var.idx (into tracing_map_elt.vars[]): 1
2049 var.hist_data: 00000000a250528c
2050 var_ref_idx (into hist_data->var_refs[]): 1
2051 type: u64
2052 size: 8
2053 is_signed: 0
2054
2055 hist_data->var_refs[2]:
2056 flags:
2057 HIST_FIELD_FL_VAR_REF
2058 name: wakeup_lat
2059 var.idx (into tracing_map_elt.vars[]): 1
2060 var.hist_data: 0000000055d65ed0
2061 var_ref_idx (into hist_data->var_refs[]): 2
2062 type: u64
2063 size: 0
2064 is_signed: 0
2065
2066 hist_data->var_refs[3]:
2067 flags:
2068 HIST_FIELD_FL_VAR_REF
2069 name: woken_pid
2070 var.idx (into tracing_map_elt.vars[]): 0
2071 var.hist_data: 0000000055d65ed0
2072 var_ref_idx (into hist_data->var_refs[]): 3
2073 type: pid_t
2074 size: 4
2075 is_signed: 1
2076
2077 hist_data->var_refs[4]:
2078 flags:
2079 HIST_FIELD_FL_VAR_REF
2080 name: next_comm
2081 var.idx (into tracing_map_elt.vars[]): 2
2082 var.hist_data: 0000000055d65ed0
2083 var_ref_idx (into hist_data->var_refs[]): 4
2084 type: char[16]
2085 size: 256
2086 is_signed: 0
2087
2088 field variables:
2089
2090 hist_data->field_vars[0]:
2091
2092 field_vars[0].var:
2093 flags:
2094 HIST_FIELD_FL_VAR
2095 var.name: next_comm
2096 var.idx (into tracing_map_elt.vars[]): 2
2097
2098 field_vars[0].val:
2099 ftrace_event_field name: next_comm
2100 type: char[16]
2101 size: 256
2102 is_signed: 0
2103
2104 action tracking variables (for onmax()/onchange()/onmatch()):
2105
2106 hist_data->actions[0].match_data.event_system: sched
2107 hist_data->actions[0].match_data.event: sched_waking
2108
2109The commands below can be used to clean things up for the next test::
2110
2111 # echo '!hist:keys=next_pid:woken_pid=$waking_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm)' >> events/sched/sched_switch/trigger
2112
2113 # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger
2114
2115 # echo '!wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events