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1Documentation for /proc/sys/kernel/* kernel version 2.2.10
2 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
3 (c) 2009, Shen Feng<shen@cn.fujitsu.com>
4
5For general info and legal blurb, please look in README.
6
7==============================================================
8
9This file contains documentation for the sysctl files in
10/proc/sys/kernel/ and is valid for Linux kernel version 2.2.
11
12The files in this directory can be used to tune and monitor
13miscellaneous and general things in the operation of the Linux
14kernel. Since some of the files _can_ be used to screw up your
15system, it is advisable to read both documentation and source
16before actually making adjustments.
17
18Currently, these files might (depending on your configuration)
19show up in /proc/sys/kernel:
20
21- acct
22- acpi_video_flags
23- auto_msgmni
24- bootloader_type [ X86 only ]
25- bootloader_version [ X86 only ]
26- callhome [ S390 only ]
27- cap_last_cap
28- core_pattern
29- core_pipe_limit
30- core_uses_pid
31- ctrl-alt-del
32- dmesg_restrict
33- domainname
34- hostname
35- hotplug
36- kptr_restrict
37- kstack_depth_to_print [ X86 only ]
38- l2cr [ PPC only ]
39- modprobe ==> Documentation/debugging-modules.txt
40- modules_disabled
41- msgmax
42- msgmnb
43- msgmni
44- nmi_watchdog
45- osrelease
46- ostype
47- overflowgid
48- overflowuid
49- panic
50- panic_on_oops
51- panic_on_unrecovered_nmi
52- panic_on_stackoverflow
53- pid_max
54- powersave-nap [ PPC only ]
55- printk
56- printk_delay
57- printk_ratelimit
58- printk_ratelimit_burst
59- randomize_va_space
60- real-root-dev ==> Documentation/initrd.txt
61- reboot-cmd [ SPARC only ]
62- rtsig-max
63- rtsig-nr
64- sem
65- sg-big-buff [ generic SCSI device (sg) ]
66- shm_rmid_forced
67- shmall
68- shmmax [ sysv ipc ]
69- shmmni
70- softlockup_thresh
71- stop-a [ SPARC only ]
72- sysrq ==> Documentation/sysrq.txt
73- tainted
74- threads-max
75- unknown_nmi_panic
76- version
77
78==============================================================
79
80acct:
81
82highwater lowwater frequency
83
84If BSD-style process accounting is enabled these values control
85its behaviour. If free space on filesystem where the log lives
86goes below <lowwater>% accounting suspends. If free space gets
87above <highwater>% accounting resumes. <Frequency> determines
88how often do we check the amount of free space (value is in
89seconds). Default:
904 2 30
91That is, suspend accounting if there left <= 2% free; resume it
92if we got >=4%; consider information about amount of free space
93valid for 30 seconds.
94
95==============================================================
96
97acpi_video_flags:
98
99flags
100
101See Doc*/kernel/power/video.txt, it allows mode of video boot to be
102set during run time.
103
104==============================================================
105
106auto_msgmni:
107
108Enables/Disables automatic recomputing of msgmni upon memory add/remove
109or upon ipc namespace creation/removal (see the msgmni description
110above). Echoing "1" into this file enables msgmni automatic recomputing.
111Echoing "0" turns it off. auto_msgmni default value is 1.
112
113
114==============================================================
115
116bootloader_type:
117
118x86 bootloader identification
119
120This gives the bootloader type number as indicated by the bootloader,
121shifted left by 4, and OR'd with the low four bits of the bootloader
122version. The reason for this encoding is that this used to match the
123type_of_loader field in the kernel header; the encoding is kept for
124backwards compatibility. That is, if the full bootloader type number
125is 0x15 and the full version number is 0x234, this file will contain
126the value 340 = 0x154.
127
128See the type_of_loader and ext_loader_type fields in
129Documentation/x86/boot.txt for additional information.
130
131==============================================================
132
133bootloader_version:
134
135x86 bootloader version
136
137The complete bootloader version number. In the example above, this
138file will contain the value 564 = 0x234.
139
140See the type_of_loader and ext_loader_ver fields in
141Documentation/x86/boot.txt for additional information.
142
143==============================================================
144
145callhome:
146
147Controls the kernel's callhome behavior in case of a kernel panic.
148
149The s390 hardware allows an operating system to send a notification
150to a service organization (callhome) in case of an operating system panic.
151
152When the value in this file is 0 (which is the default behavior)
153nothing happens in case of a kernel panic. If this value is set to "1"
154the complete kernel oops message is send to the IBM customer service
155organization in case the mainframe the Linux operating system is running
156on has a service contract with IBM.
157
158==============================================================
159
160cap_last_cap
161
162Highest valid capability of the running kernel. Exports
163CAP_LAST_CAP from the kernel.
164
165==============================================================
166
167core_pattern:
168
169core_pattern is used to specify a core dumpfile pattern name.
170. max length 128 characters; default value is "core"
171. core_pattern is used as a pattern template for the output filename;
172 certain string patterns (beginning with '%') are substituted with
173 their actual values.
174. backward compatibility with core_uses_pid:
175 If core_pattern does not include "%p" (default does not)
176 and core_uses_pid is set, then .PID will be appended to
177 the filename.
178. corename format specifiers:
179 %<NUL> '%' is dropped
180 %% output one '%'
181 %p pid
182 %u uid
183 %g gid
184 %s signal number
185 %t UNIX time of dump
186 %h hostname
187 %e executable filename (may be shortened)
188 %E executable path
189 %<OTHER> both are dropped
190. If the first character of the pattern is a '|', the kernel will treat
191 the rest of the pattern as a command to run. The core dump will be
192 written to the standard input of that program instead of to a file.
193
194==============================================================
195
196core_pipe_limit:
197
198This sysctl is only applicable when core_pattern is configured to pipe
199core files to a user space helper (when the first character of
200core_pattern is a '|', see above). When collecting cores via a pipe
201to an application, it is occasionally useful for the collecting
202application to gather data about the crashing process from its
203/proc/pid directory. In order to do this safely, the kernel must wait
204for the collecting process to exit, so as not to remove the crashing
205processes proc files prematurely. This in turn creates the
206possibility that a misbehaving userspace collecting process can block
207the reaping of a crashed process simply by never exiting. This sysctl
208defends against that. It defines how many concurrent crashing
209processes may be piped to user space applications in parallel. If
210this value is exceeded, then those crashing processes above that value
211are noted via the kernel log and their cores are skipped. 0 is a
212special value, indicating that unlimited processes may be captured in
213parallel, but that no waiting will take place (i.e. the collecting
214process is not guaranteed access to /proc/<crashing pid>/). This
215value defaults to 0.
216
217==============================================================
218
219core_uses_pid:
220
221The default coredump filename is "core". By setting
222core_uses_pid to 1, the coredump filename becomes core.PID.
223If core_pattern does not include "%p" (default does not)
224and core_uses_pid is set, then .PID will be appended to
225the filename.
226
227==============================================================
228
229ctrl-alt-del:
230
231When the value in this file is 0, ctrl-alt-del is trapped and
232sent to the init(1) program to handle a graceful restart.
233When, however, the value is > 0, Linux's reaction to a Vulcan
234Nerve Pinch (tm) will be an immediate reboot, without even
235syncing its dirty buffers.
236
237Note: when a program (like dosemu) has the keyboard in 'raw'
238mode, the ctrl-alt-del is intercepted by the program before it
239ever reaches the kernel tty layer, and it's up to the program
240to decide what to do with it.
241
242==============================================================
243
244dmesg_restrict:
245
246This toggle indicates whether unprivileged users are prevented
247from using dmesg(8) to view messages from the kernel's log buffer.
248When dmesg_restrict is set to (0) there are no restrictions. When
249dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
250dmesg(8).
251
252The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
253default value of dmesg_restrict.
254
255==============================================================
256
257domainname & hostname:
258
259These files can be used to set the NIS/YP domainname and the
260hostname of your box in exactly the same way as the commands
261domainname and hostname, i.e.:
262# echo "darkstar" > /proc/sys/kernel/hostname
263# echo "mydomain" > /proc/sys/kernel/domainname
264has the same effect as
265# hostname "darkstar"
266# domainname "mydomain"
267
268Note, however, that the classic darkstar.frop.org has the
269hostname "darkstar" and DNS (Internet Domain Name Server)
270domainname "frop.org", not to be confused with the NIS (Network
271Information Service) or YP (Yellow Pages) domainname. These two
272domain names are in general different. For a detailed discussion
273see the hostname(1) man page.
274
275==============================================================
276
277hotplug:
278
279Path for the hotplug policy agent.
280Default value is "/sbin/hotplug".
281
282==============================================================
283
284kptr_restrict:
285
286This toggle indicates whether restrictions are placed on
287exposing kernel addresses via /proc and other interfaces. When
288kptr_restrict is set to (0), there are no restrictions. When
289kptr_restrict is set to (1), the default, kernel pointers
290printed using the %pK format specifier will be replaced with 0's
291unless the user has CAP_SYSLOG. When kptr_restrict is set to
292(2), kernel pointers printed using %pK will be replaced with 0's
293regardless of privileges.
294
295==============================================================
296
297kstack_depth_to_print: (X86 only)
298
299Controls the number of words to print when dumping the raw
300kernel stack.
301
302==============================================================
303
304l2cr: (PPC only)
305
306This flag controls the L2 cache of G3 processor boards. If
3070, the cache is disabled. Enabled if nonzero.
308
309==============================================================
310
311modules_disabled:
312
313A toggle value indicating if modules are allowed to be loaded
314in an otherwise modular kernel. This toggle defaults to off
315(0), but can be set true (1). Once true, modules can be
316neither loaded nor unloaded, and the toggle cannot be set back
317to false.
318
319==============================================================
320
321nmi_watchdog:
322
323Enables/Disables the NMI watchdog on x86 systems. When the value is
324non-zero the NMI watchdog is enabled and will continuously test all
325online cpus to determine whether or not they are still functioning
326properly. Currently, passing "nmi_watchdog=" parameter at boot time is
327required for this function to work.
328
329If LAPIC NMI watchdog method is in use (nmi_watchdog=2 kernel
330parameter), the NMI watchdog shares registers with oprofile. By
331disabling the NMI watchdog, oprofile may have more registers to
332utilize.
333
334==============================================================
335
336osrelease, ostype & version:
337
338# cat osrelease
3392.1.88
340# cat ostype
341Linux
342# cat version
343#5 Wed Feb 25 21:49:24 MET 1998
344
345The files osrelease and ostype should be clear enough. Version
346needs a little more clarification however. The '#5' means that
347this is the fifth kernel built from this source base and the
348date behind it indicates the time the kernel was built.
349The only way to tune these values is to rebuild the kernel :-)
350
351==============================================================
352
353overflowgid & overflowuid:
354
355if your architecture did not always support 32-bit UIDs (i.e. arm,
356i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
357applications that use the old 16-bit UID/GID system calls, if the
358actual UID or GID would exceed 65535.
359
360These sysctls allow you to change the value of the fixed UID and GID.
361The default is 65534.
362
363==============================================================
364
365panic:
366
367The value in this file represents the number of seconds the kernel
368waits before rebooting on a panic. When you use the software watchdog,
369the recommended setting is 60.
370
371==============================================================
372
373panic_on_unrecovered_nmi:
374
375The default Linux behaviour on an NMI of either memory or unknown is
376to continue operation. For many environments such as scientific
377computing it is preferable that the box is taken out and the error
378dealt with than an uncorrected parity/ECC error get propagated.
379
380A small number of systems do generate NMI's for bizarre random reasons
381such as power management so the default is off. That sysctl works like
382the existing panic controls already in that directory.
383
384==============================================================
385
386panic_on_oops:
387
388Controls the kernel's behaviour when an oops or BUG is encountered.
389
3900: try to continue operation
391
3921: panic immediately. If the `panic' sysctl is also non-zero then the
393 machine will be rebooted.
394
395==============================================================
396
397panic_on_stackoverflow:
398
399Controls the kernel's behavior when detecting the overflows of
400kernel, IRQ and exception stacks except a user stack.
401This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.
402
4030: try to continue operation.
404
4051: panic immediately.
406
407==============================================================
408
409
410pid_max:
411
412PID allocation wrap value. When the kernel's next PID value
413reaches this value, it wraps back to a minimum PID value.
414PIDs of value pid_max or larger are not allocated.
415
416==============================================================
417
418ns_last_pid:
419
420The last pid allocated in the current (the one task using this sysctl
421lives in) pid namespace. When selecting a pid for a next task on fork
422kernel tries to allocate a number starting from this one.
423
424==============================================================
425
426powersave-nap: (PPC only)
427
428If set, Linux-PPC will use the 'nap' mode of powersaving,
429otherwise the 'doze' mode will be used.
430
431==============================================================
432
433printk:
434
435The four values in printk denote: console_loglevel,
436default_message_loglevel, minimum_console_loglevel and
437default_console_loglevel respectively.
438
439These values influence printk() behavior when printing or
440logging error messages. See 'man 2 syslog' for more info on
441the different loglevels.
442
443- console_loglevel: messages with a higher priority than
444 this will be printed to the console
445- default_message_loglevel: messages without an explicit priority
446 will be printed with this priority
447- minimum_console_loglevel: minimum (highest) value to which
448 console_loglevel can be set
449- default_console_loglevel: default value for console_loglevel
450
451==============================================================
452
453printk_delay:
454
455Delay each printk message in printk_delay milliseconds
456
457Value from 0 - 10000 is allowed.
458
459==============================================================
460
461printk_ratelimit:
462
463Some warning messages are rate limited. printk_ratelimit specifies
464the minimum length of time between these messages (in jiffies), by
465default we allow one every 5 seconds.
466
467A value of 0 will disable rate limiting.
468
469==============================================================
470
471printk_ratelimit_burst:
472
473While long term we enforce one message per printk_ratelimit
474seconds, we do allow a burst of messages to pass through.
475printk_ratelimit_burst specifies the number of messages we can
476send before ratelimiting kicks in.
477
478==============================================================
479
480randomize_va_space:
481
482This option can be used to select the type of process address
483space randomization that is used in the system, for architectures
484that support this feature.
485
4860 - Turn the process address space randomization off. This is the
487 default for architectures that do not support this feature anyways,
488 and kernels that are booted with the "norandmaps" parameter.
489
4901 - Make the addresses of mmap base, stack and VDSO page randomized.
491 This, among other things, implies that shared libraries will be
492 loaded to random addresses. Also for PIE-linked binaries, the
493 location of code start is randomized. This is the default if the
494 CONFIG_COMPAT_BRK option is enabled.
495
4962 - Additionally enable heap randomization. This is the default if
497 CONFIG_COMPAT_BRK is disabled.
498
499 There are a few legacy applications out there (such as some ancient
500 versions of libc.so.5 from 1996) that assume that brk area starts
501 just after the end of the code+bss. These applications break when
502 start of the brk area is randomized. There are however no known
503 non-legacy applications that would be broken this way, so for most
504 systems it is safe to choose full randomization.
505
506 Systems with ancient and/or broken binaries should be configured
507 with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
508 address space randomization.
509
510==============================================================
511
512reboot-cmd: (Sparc only)
513
514??? This seems to be a way to give an argument to the Sparc
515ROM/Flash boot loader. Maybe to tell it what to do after
516rebooting. ???
517
518==============================================================
519
520rtsig-max & rtsig-nr:
521
522The file rtsig-max can be used to tune the maximum number
523of POSIX realtime (queued) signals that can be outstanding
524in the system.
525
526rtsig-nr shows the number of RT signals currently queued.
527
528==============================================================
529
530sg-big-buff:
531
532This file shows the size of the generic SCSI (sg) buffer.
533You can't tune it just yet, but you could change it on
534compile time by editing include/scsi/sg.h and changing
535the value of SG_BIG_BUFF.
536
537There shouldn't be any reason to change this value. If
538you can come up with one, you probably know what you
539are doing anyway :)
540
541==============================================================
542
543shmmax:
544
545This value can be used to query and set the run time limit
546on the maximum shared memory segment size that can be created.
547Shared memory segments up to 1Gb are now supported in the
548kernel. This value defaults to SHMMAX.
549
550==============================================================
551
552shm_rmid_forced:
553
554Linux lets you set resource limits, including how much memory one
555process can consume, via setrlimit(2). Unfortunately, shared memory
556segments are allowed to exist without association with any process, and
557thus might not be counted against any resource limits. If enabled,
558shared memory segments are automatically destroyed when their attach
559count becomes zero after a detach or a process termination. It will
560also destroy segments that were created, but never attached to, on exit
561from the process. The only use left for IPC_RMID is to immediately
562destroy an unattached segment. Of course, this breaks the way things are
563defined, so some applications might stop working. Note that this
564feature will do you no good unless you also configure your resource
565limits (in particular, RLIMIT_AS and RLIMIT_NPROC). Most systems don't
566need this.
567
568Note that if you change this from 0 to 1, already created segments
569without users and with a dead originative process will be destroyed.
570
571==============================================================
572
573softlockup_thresh:
574
575This value can be used to lower the softlockup tolerance threshold. The
576default threshold is 60 seconds. If a cpu is locked up for 60 seconds,
577the kernel complains. Valid values are 1-60 seconds. Setting this
578tunable to zero will disable the softlockup detection altogether.
579
580==============================================================
581
582tainted:
583
584Non-zero if the kernel has been tainted. Numeric values, which
585can be ORed together:
586
587 1 - A module with a non-GPL license has been loaded, this
588 includes modules with no license.
589 Set by modutils >= 2.4.9 and module-init-tools.
590 2 - A module was force loaded by insmod -f.
591 Set by modutils >= 2.4.9 and module-init-tools.
592 4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
593 8 - A module was forcibly unloaded from the system by rmmod -f.
594 16 - A hardware machine check error occurred on the system.
595 32 - A bad page was discovered on the system.
596 64 - The user has asked that the system be marked "tainted". This
597 could be because they are running software that directly modifies
598 the hardware, or for other reasons.
599 128 - The system has died.
600 256 - The ACPI DSDT has been overridden with one supplied by the user
601 instead of using the one provided by the hardware.
602 512 - A kernel warning has occurred.
6031024 - A module from drivers/staging was loaded.
6042048 - The system is working around a severe firmware bug.
6054096 - An out-of-tree module has been loaded.
606
607==============================================================
608
609unknown_nmi_panic:
610
611The value in this file affects behavior of handling NMI. When the
612value is non-zero, unknown NMI is trapped and then panic occurs. At
613that time, kernel debugging information is displayed on console.
614
615NMI switch that most IA32 servers have fires unknown NMI up, for
616example. If a system hangs up, try pressing the NMI switch.
1Documentation for /proc/sys/kernel/* kernel version 2.2.10
2 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
3 (c) 2009, Shen Feng<shen@cn.fujitsu.com>
4
5For general info and legal blurb, please look in README.
6
7==============================================================
8
9This file contains documentation for the sysctl files in
10/proc/sys/kernel/ and is valid for Linux kernel version 2.2.
11
12The files in this directory can be used to tune and monitor
13miscellaneous and general things in the operation of the Linux
14kernel. Since some of the files _can_ be used to screw up your
15system, it is advisable to read both documentation and source
16before actually making adjustments.
17
18Currently, these files might (depending on your configuration)
19show up in /proc/sys/kernel:
20
21- acct
22- acpi_video_flags
23- auto_msgmni
24- bootloader_type [ X86 only ]
25- bootloader_version [ X86 only ]
26- callhome [ S390 only ]
27- core_pattern
28- core_pipe_limit
29- core_uses_pid
30- ctrl-alt-del
31- dmesg_restrict
32- domainname
33- hostname
34- hotplug
35- kptr_restrict
36- kstack_depth_to_print [ X86 only ]
37- l2cr [ PPC only ]
38- modprobe ==> Documentation/debugging-modules.txt
39- modules_disabled
40- msgmax
41- msgmnb
42- msgmni
43- nmi_watchdog
44- osrelease
45- ostype
46- overflowgid
47- overflowuid
48- panic
49- panic_on_oops
50- panic_on_unrecovered_nmi
51- pid_max
52- powersave-nap [ PPC only ]
53- printk
54- printk_delay
55- printk_ratelimit
56- printk_ratelimit_burst
57- randomize_va_space
58- real-root-dev ==> Documentation/initrd.txt
59- reboot-cmd [ SPARC only ]
60- rtsig-max
61- rtsig-nr
62- sem
63- sg-big-buff [ generic SCSI device (sg) ]
64- shm_rmid_forced
65- shmall
66- shmmax [ sysv ipc ]
67- shmmni
68- softlockup_thresh
69- stop-a [ SPARC only ]
70- sysrq ==> Documentation/sysrq.txt
71- tainted
72- threads-max
73- unknown_nmi_panic
74- version
75
76==============================================================
77
78acct:
79
80highwater lowwater frequency
81
82If BSD-style process accounting is enabled these values control
83its behaviour. If free space on filesystem where the log lives
84goes below <lowwater>% accounting suspends. If free space gets
85above <highwater>% accounting resumes. <Frequency> determines
86how often do we check the amount of free space (value is in
87seconds). Default:
884 2 30
89That is, suspend accounting if there left <= 2% free; resume it
90if we got >=4%; consider information about amount of free space
91valid for 30 seconds.
92
93==============================================================
94
95acpi_video_flags:
96
97flags
98
99See Doc*/kernel/power/video.txt, it allows mode of video boot to be
100set during run time.
101
102==============================================================
103
104auto_msgmni:
105
106Enables/Disables automatic recomputing of msgmni upon memory add/remove
107or upon ipc namespace creation/removal (see the msgmni description
108above). Echoing "1" into this file enables msgmni automatic recomputing.
109Echoing "0" turns it off. auto_msgmni default value is 1.
110
111
112==============================================================
113
114bootloader_type:
115
116x86 bootloader identification
117
118This gives the bootloader type number as indicated by the bootloader,
119shifted left by 4, and OR'd with the low four bits of the bootloader
120version. The reason for this encoding is that this used to match the
121type_of_loader field in the kernel header; the encoding is kept for
122backwards compatibility. That is, if the full bootloader type number
123is 0x15 and the full version number is 0x234, this file will contain
124the value 340 = 0x154.
125
126See the type_of_loader and ext_loader_type fields in
127Documentation/x86/boot.txt for additional information.
128
129==============================================================
130
131bootloader_version:
132
133x86 bootloader version
134
135The complete bootloader version number. In the example above, this
136file will contain the value 564 = 0x234.
137
138See the type_of_loader and ext_loader_ver fields in
139Documentation/x86/boot.txt for additional information.
140
141==============================================================
142
143callhome:
144
145Controls the kernel's callhome behavior in case of a kernel panic.
146
147The s390 hardware allows an operating system to send a notification
148to a service organization (callhome) in case of an operating system panic.
149
150When the value in this file is 0 (which is the default behavior)
151nothing happens in case of a kernel panic. If this value is set to "1"
152the complete kernel oops message is send to the IBM customer service
153organization in case the mainframe the Linux operating system is running
154on has a service contract with IBM.
155
156==============================================================
157
158core_pattern:
159
160core_pattern is used to specify a core dumpfile pattern name.
161. max length 128 characters; default value is "core"
162. core_pattern is used as a pattern template for the output filename;
163 certain string patterns (beginning with '%') are substituted with
164 their actual values.
165. backward compatibility with core_uses_pid:
166 If core_pattern does not include "%p" (default does not)
167 and core_uses_pid is set, then .PID will be appended to
168 the filename.
169. corename format specifiers:
170 %<NUL> '%' is dropped
171 %% output one '%'
172 %p pid
173 %u uid
174 %g gid
175 %s signal number
176 %t UNIX time of dump
177 %h hostname
178 %e executable filename (may be shortened)
179 %E executable path
180 %<OTHER> both are dropped
181. If the first character of the pattern is a '|', the kernel will treat
182 the rest of the pattern as a command to run. The core dump will be
183 written to the standard input of that program instead of to a file.
184
185==============================================================
186
187core_pipe_limit:
188
189This sysctl is only applicable when core_pattern is configured to pipe
190core files to a user space helper (when the first character of
191core_pattern is a '|', see above). When collecting cores via a pipe
192to an application, it is occasionally useful for the collecting
193application to gather data about the crashing process from its
194/proc/pid directory. In order to do this safely, the kernel must wait
195for the collecting process to exit, so as not to remove the crashing
196processes proc files prematurely. This in turn creates the
197possibility that a misbehaving userspace collecting process can block
198the reaping of a crashed process simply by never exiting. This sysctl
199defends against that. It defines how many concurrent crashing
200processes may be piped to user space applications in parallel. If
201this value is exceeded, then those crashing processes above that value
202are noted via the kernel log and their cores are skipped. 0 is a
203special value, indicating that unlimited processes may be captured in
204parallel, but that no waiting will take place (i.e. the collecting
205process is not guaranteed access to /proc/<crashing pid>/). This
206value defaults to 0.
207
208==============================================================
209
210core_uses_pid:
211
212The default coredump filename is "core". By setting
213core_uses_pid to 1, the coredump filename becomes core.PID.
214If core_pattern does not include "%p" (default does not)
215and core_uses_pid is set, then .PID will be appended to
216the filename.
217
218==============================================================
219
220ctrl-alt-del:
221
222When the value in this file is 0, ctrl-alt-del is trapped and
223sent to the init(1) program to handle a graceful restart.
224When, however, the value is > 0, Linux's reaction to a Vulcan
225Nerve Pinch (tm) will be an immediate reboot, without even
226syncing its dirty buffers.
227
228Note: when a program (like dosemu) has the keyboard in 'raw'
229mode, the ctrl-alt-del is intercepted by the program before it
230ever reaches the kernel tty layer, and it's up to the program
231to decide what to do with it.
232
233==============================================================
234
235dmesg_restrict:
236
237This toggle indicates whether unprivileged users are prevented
238from using dmesg(8) to view messages from the kernel's log buffer.
239When dmesg_restrict is set to (0) there are no restrictions. When
240dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
241dmesg(8).
242
243The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
244default value of dmesg_restrict.
245
246==============================================================
247
248domainname & hostname:
249
250These files can be used to set the NIS/YP domainname and the
251hostname of your box in exactly the same way as the commands
252domainname and hostname, i.e.:
253# echo "darkstar" > /proc/sys/kernel/hostname
254# echo "mydomain" > /proc/sys/kernel/domainname
255has the same effect as
256# hostname "darkstar"
257# domainname "mydomain"
258
259Note, however, that the classic darkstar.frop.org has the
260hostname "darkstar" and DNS (Internet Domain Name Server)
261domainname "frop.org", not to be confused with the NIS (Network
262Information Service) or YP (Yellow Pages) domainname. These two
263domain names are in general different. For a detailed discussion
264see the hostname(1) man page.
265
266==============================================================
267
268hotplug:
269
270Path for the hotplug policy agent.
271Default value is "/sbin/hotplug".
272
273==============================================================
274
275kptr_restrict:
276
277This toggle indicates whether restrictions are placed on
278exposing kernel addresses via /proc and other interfaces. When
279kptr_restrict is set to (0), there are no restrictions. When
280kptr_restrict is set to (1), the default, kernel pointers
281printed using the %pK format specifier will be replaced with 0's
282unless the user has CAP_SYSLOG. When kptr_restrict is set to
283(2), kernel pointers printed using %pK will be replaced with 0's
284regardless of privileges.
285
286==============================================================
287
288kstack_depth_to_print: (X86 only)
289
290Controls the number of words to print when dumping the raw
291kernel stack.
292
293==============================================================
294
295l2cr: (PPC only)
296
297This flag controls the L2 cache of G3 processor boards. If
2980, the cache is disabled. Enabled if nonzero.
299
300==============================================================
301
302modules_disabled:
303
304A toggle value indicating if modules are allowed to be loaded
305in an otherwise modular kernel. This toggle defaults to off
306(0), but can be set true (1). Once true, modules can be
307neither loaded nor unloaded, and the toggle cannot be set back
308to false.
309
310==============================================================
311
312nmi_watchdog:
313
314Enables/Disables the NMI watchdog on x86 systems. When the value is
315non-zero the NMI watchdog is enabled and will continuously test all
316online cpus to determine whether or not they are still functioning
317properly. Currently, passing "nmi_watchdog=" parameter at boot time is
318required for this function to work.
319
320If LAPIC NMI watchdog method is in use (nmi_watchdog=2 kernel
321parameter), the NMI watchdog shares registers with oprofile. By
322disabling the NMI watchdog, oprofile may have more registers to
323utilize.
324
325==============================================================
326
327osrelease, ostype & version:
328
329# cat osrelease
3302.1.88
331# cat ostype
332Linux
333# cat version
334#5 Wed Feb 25 21:49:24 MET 1998
335
336The files osrelease and ostype should be clear enough. Version
337needs a little more clarification however. The '#5' means that
338this is the fifth kernel built from this source base and the
339date behind it indicates the time the kernel was built.
340The only way to tune these values is to rebuild the kernel :-)
341
342==============================================================
343
344overflowgid & overflowuid:
345
346if your architecture did not always support 32-bit UIDs (i.e. arm,
347i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
348applications that use the old 16-bit UID/GID system calls, if the
349actual UID or GID would exceed 65535.
350
351These sysctls allow you to change the value of the fixed UID and GID.
352The default is 65534.
353
354==============================================================
355
356panic:
357
358The value in this file represents the number of seconds the kernel
359waits before rebooting on a panic. When you use the software watchdog,
360the recommended setting is 60.
361
362==============================================================
363
364panic_on_unrecovered_nmi:
365
366The default Linux behaviour on an NMI of either memory or unknown is
367to continue operation. For many environments such as scientific
368computing it is preferable that the box is taken out and the error
369dealt with than an uncorrected parity/ECC error get propagated.
370
371A small number of systems do generate NMI's for bizarre random reasons
372such as power management so the default is off. That sysctl works like
373the existing panic controls already in that directory.
374
375==============================================================
376
377panic_on_oops:
378
379Controls the kernel's behaviour when an oops or BUG is encountered.
380
3810: try to continue operation
382
3831: panic immediately. If the `panic' sysctl is also non-zero then the
384 machine will be rebooted.
385
386==============================================================
387
388pid_max:
389
390PID allocation wrap value. When the kernel's next PID value
391reaches this value, it wraps back to a minimum PID value.
392PIDs of value pid_max or larger are not allocated.
393
394==============================================================
395
396powersave-nap: (PPC only)
397
398If set, Linux-PPC will use the 'nap' mode of powersaving,
399otherwise the 'doze' mode will be used.
400
401==============================================================
402
403printk:
404
405The four values in printk denote: console_loglevel,
406default_message_loglevel, minimum_console_loglevel and
407default_console_loglevel respectively.
408
409These values influence printk() behavior when printing or
410logging error messages. See 'man 2 syslog' for more info on
411the different loglevels.
412
413- console_loglevel: messages with a higher priority than
414 this will be printed to the console
415- default_message_loglevel: messages without an explicit priority
416 will be printed with this priority
417- minimum_console_loglevel: minimum (highest) value to which
418 console_loglevel can be set
419- default_console_loglevel: default value for console_loglevel
420
421==============================================================
422
423printk_delay:
424
425Delay each printk message in printk_delay milliseconds
426
427Value from 0 - 10000 is allowed.
428
429==============================================================
430
431printk_ratelimit:
432
433Some warning messages are rate limited. printk_ratelimit specifies
434the minimum length of time between these messages (in jiffies), by
435default we allow one every 5 seconds.
436
437A value of 0 will disable rate limiting.
438
439==============================================================
440
441printk_ratelimit_burst:
442
443While long term we enforce one message per printk_ratelimit
444seconds, we do allow a burst of messages to pass through.
445printk_ratelimit_burst specifies the number of messages we can
446send before ratelimiting kicks in.
447
448==============================================================
449
450randomize_va_space:
451
452This option can be used to select the type of process address
453space randomization that is used in the system, for architectures
454that support this feature.
455
4560 - Turn the process address space randomization off. This is the
457 default for architectures that do not support this feature anyways,
458 and kernels that are booted with the "norandmaps" parameter.
459
4601 - Make the addresses of mmap base, stack and VDSO page randomized.
461 This, among other things, implies that shared libraries will be
462 loaded to random addresses. Also for PIE-linked binaries, the
463 location of code start is randomized. This is the default if the
464 CONFIG_COMPAT_BRK option is enabled.
465
4662 - Additionally enable heap randomization. This is the default if
467 CONFIG_COMPAT_BRK is disabled.
468
469 There are a few legacy applications out there (such as some ancient
470 versions of libc.so.5 from 1996) that assume that brk area starts
471 just after the end of the code+bss. These applications break when
472 start of the brk area is randomized. There are however no known
473 non-legacy applications that would be broken this way, so for most
474 systems it is safe to choose full randomization.
475
476 Systems with ancient and/or broken binaries should be configured
477 with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
478 address space randomization.
479
480==============================================================
481
482reboot-cmd: (Sparc only)
483
484??? This seems to be a way to give an argument to the Sparc
485ROM/Flash boot loader. Maybe to tell it what to do after
486rebooting. ???
487
488==============================================================
489
490rtsig-max & rtsig-nr:
491
492The file rtsig-max can be used to tune the maximum number
493of POSIX realtime (queued) signals that can be outstanding
494in the system.
495
496rtsig-nr shows the number of RT signals currently queued.
497
498==============================================================
499
500sg-big-buff:
501
502This file shows the size of the generic SCSI (sg) buffer.
503You can't tune it just yet, but you could change it on
504compile time by editing include/scsi/sg.h and changing
505the value of SG_BIG_BUFF.
506
507There shouldn't be any reason to change this value. If
508you can come up with one, you probably know what you
509are doing anyway :)
510
511==============================================================
512
513shmmax:
514
515This value can be used to query and set the run time limit
516on the maximum shared memory segment size that can be created.
517Shared memory segments up to 1Gb are now supported in the
518kernel. This value defaults to SHMMAX.
519
520==============================================================
521
522shm_rmid_forced:
523
524Linux lets you set resource limits, including how much memory one
525process can consume, via setrlimit(2). Unfortunately, shared memory
526segments are allowed to exist without association with any process, and
527thus might not be counted against any resource limits. If enabled,
528shared memory segments are automatically destroyed when their attach
529count becomes zero after a detach or a process termination. It will
530also destroy segments that were created, but never attached to, on exit
531from the process. The only use left for IPC_RMID is to immediately
532destroy an unattached segment. Of course, this breaks the way things are
533defined, so some applications might stop working. Note that this
534feature will do you no good unless you also configure your resource
535limits (in particular, RLIMIT_AS and RLIMIT_NPROC). Most systems don't
536need this.
537
538Note that if you change this from 0 to 1, already created segments
539without users and with a dead originative process will be destroyed.
540
541==============================================================
542
543softlockup_thresh:
544
545This value can be used to lower the softlockup tolerance threshold. The
546default threshold is 60 seconds. If a cpu is locked up for 60 seconds,
547the kernel complains. Valid values are 1-60 seconds. Setting this
548tunable to zero will disable the softlockup detection altogether.
549
550==============================================================
551
552tainted:
553
554Non-zero if the kernel has been tainted. Numeric values, which
555can be ORed together:
556
557 1 - A module with a non-GPL license has been loaded, this
558 includes modules with no license.
559 Set by modutils >= 2.4.9 and module-init-tools.
560 2 - A module was force loaded by insmod -f.
561 Set by modutils >= 2.4.9 and module-init-tools.
562 4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
563 8 - A module was forcibly unloaded from the system by rmmod -f.
564 16 - A hardware machine check error occurred on the system.
565 32 - A bad page was discovered on the system.
566 64 - The user has asked that the system be marked "tainted". This
567 could be because they are running software that directly modifies
568 the hardware, or for other reasons.
569 128 - The system has died.
570 256 - The ACPI DSDT has been overridden with one supplied by the user
571 instead of using the one provided by the hardware.
572 512 - A kernel warning has occurred.
5731024 - A module from drivers/staging was loaded.
574
575==============================================================
576
577unknown_nmi_panic:
578
579The value in this file affects behavior of handling NMI. When the
580value is non-zero, unknown NMI is trapped and then panic occurs. At
581that time, kernel debugging information is displayed on console.
582
583NMI switch that most IA32 servers have fires unknown NMI up, for
584example. If a system hangs up, try pressing the NMI switch.