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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Documentation for /proc/sys/kernel/* kernel version 2.2.10
2 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
Shen Feng760df932009-04-02 16:57:20 -07003 (c) 2009, Shen Feng<shen@cn.fujitsu.com>
Linus Torvalds1da177e2005-04-16 15:20:36 -07004
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:
Borislav Petkov807094c2011-07-23 10:39:29 -070020
Linus Torvalds1da177e2005-04-16 15:20:36 -070021- acct
Borislav Petkov807094c2011-07-23 10:39:29 -070022- acpi_video_flags
23- auto_msgmni
H. Peter Anvind75757a2009-12-11 14:23:44 -080024- bootloader_type [ X86 only ]
25- bootloader_version [ X86 only ]
Hans-Joachim Pichtc114728a2009-09-11 10:28:47 +020026- callhome [ S390 only ]
Dan Ballard73efc032011-10-31 17:11:20 -070027- cap_last_cap
Linus Torvalds1da177e2005-04-16 15:20:36 -070028- core_pattern
Neil Hormana2939802009-09-23 15:56:56 -070029- core_pipe_limit
Linus Torvalds1da177e2005-04-16 15:20:36 -070030- core_uses_pid
31- ctrl-alt-del
Dan Rosenbergeaf06b22010-11-11 14:05:18 -080032- dmesg_restrict
Linus Torvalds1da177e2005-04-16 15:20:36 -070033- domainname
34- hostname
35- hotplug
Jiri Kosina55537872015-11-05 18:44:41 -080036- hardlockup_all_cpu_backtrace
Aaron Tomlin270750db2014-01-20 17:34:13 +000037- hung_task_panic
38- hung_task_check_count
39- hung_task_timeout_secs
40- hung_task_warnings
Kees Cook79847542014-01-23 15:55:59 -080041- kexec_load_disabled
Dan Rosenberg455cd5a2011-01-12 16:59:41 -080042- kptr_restrict
Chuck Ebbert0741f4d2006-12-07 02:14:11 +010043- kstack_depth_to_print [ X86 only ]
Linus Torvalds1da177e2005-04-16 15:20:36 -070044- l2cr [ PPC only ]
Michael Opdenackerac76cff2008-02-13 15:03:32 -080045- modprobe ==> Documentation/debugging-modules.txt
Kees Cook3d433212009-04-02 15:49:29 -070046- modules_disabled
Stanislav Kinsbursky03f59562013-01-04 15:34:50 -080047- msg_next_id [ sysv ipc ]
Linus Torvalds1da177e2005-04-16 15:20:36 -070048- msgmax
49- msgmnb
50- msgmni
Shen Feng760df932009-04-02 16:57:20 -070051- nmi_watchdog
Linus Torvalds1da177e2005-04-16 15:20:36 -070052- osrelease
53- ostype
54- overflowgid
55- overflowuid
56- panic
Borislav Petkov807094c2011-07-23 10:39:29 -070057- panic_on_oops
Mitsuo Hayasaka55af7792011-11-29 15:08:36 +090058- panic_on_stackoverflow
Prarit Bhargava9e3961a2014-12-10 15:45:50 -080059- panic_on_unrecovered_nmi
60- panic_on_warn
Linus Torvalds1da177e2005-04-16 15:20:36 -070061- pid_max
62- powersave-nap [ PPC only ]
63- printk
Borislav Petkov807094c2011-07-23 10:39:29 -070064- printk_delay
65- printk_ratelimit
66- printk_ratelimit_burst
Jiri Kosina1ec7fd52008-02-09 23:24:08 +010067- randomize_va_space
Linus Torvalds1da177e2005-04-16 15:20:36 -070068- real-root-dev ==> Documentation/initrd.txt
69- reboot-cmd [ SPARC only ]
70- rtsig-max
71- rtsig-nr
72- sem
Stanislav Kinsbursky03f59562013-01-04 15:34:50 -080073- sem_next_id [ sysv ipc ]
Linus Torvalds1da177e2005-04-16 15:20:36 -070074- sg-big-buff [ generic SCSI device (sg) ]
Stanislav Kinsbursky03f59562013-01-04 15:34:50 -080075- shm_next_id [ sysv ipc ]
Vasiliy Kulikovb34a6b12011-07-26 16:08:48 -070076- shm_rmid_forced
Linus Torvalds1da177e2005-04-16 15:20:36 -070077- shmall
78- shmmax [ sysv ipc ]
79- shmmni
Aaron Tomlined235872014-06-23 13:22:05 -070080- softlockup_all_cpu_backtrace
Ulrich Obergfell195daf62015-04-14 15:44:13 -070081- soft_watchdog
Linus Torvalds1da177e2005-04-16 15:20:36 -070082- stop-a [ SPARC only ]
83- sysrq ==> Documentation/sysrq.txt
Kees Cookf4aacea2014-06-06 14:37:19 -070084- sysctl_writes_strict
Linus Torvalds1da177e2005-04-16 15:20:36 -070085- tainted
86- threads-max
Shen Feng760df932009-04-02 16:57:20 -070087- unknown_nmi_panic
Ulrich Obergfell195daf62015-04-14 15:44:13 -070088- watchdog
Li Zefan08825c92013-05-17 10:31:20 +080089- watchdog_thresh
Linus Torvalds1da177e2005-04-16 15:20:36 -070090- version
91
92==============================================================
93
94acct:
95
96highwater lowwater frequency
97
98If BSD-style process accounting is enabled these values control
99its behaviour. If free space on filesystem where the log lives
100goes below <lowwater>% accounting suspends. If free space gets
101above <highwater>% accounting resumes. <Frequency> determines
102how often do we check the amount of free space (value is in
103seconds). Default:
1044 2 30
105That is, suspend accounting if there left <= 2% free; resume it
106if we got >=4%; consider information about amount of free space
107valid for 30 seconds.
108
109==============================================================
110
Borislav Petkov807094c2011-07-23 10:39:29 -0700111acpi_video_flags:
112
113flags
114
115See Doc*/kernel/power/video.txt, it allows mode of video boot to be
116set during run time.
117
118==============================================================
119
120auto_msgmni:
121
Manfred Spraul0050ee02014-12-12 16:58:17 -0800122This variable has no effect and may be removed in future kernel
123releases. Reading it always returns 0.
124Up to Linux 3.17, it enabled/disabled automatic recomputing of msgmni
125upon memory add/remove or upon ipc namespace creation/removal.
126Echoing "1" into this file enabled msgmni automatic recomputing.
127Echoing "0" turned it off. auto_msgmni default value was 1.
Borislav Petkov807094c2011-07-23 10:39:29 -0700128
129
130==============================================================
131
H. Peter Anvind75757a2009-12-11 14:23:44 -0800132bootloader_type:
133
134x86 bootloader identification
135
136This gives the bootloader type number as indicated by the bootloader,
137shifted left by 4, and OR'd with the low four bits of the bootloader
138version. The reason for this encoding is that this used to match the
139type_of_loader field in the kernel header; the encoding is kept for
140backwards compatibility. That is, if the full bootloader type number
141is 0x15 and the full version number is 0x234, this file will contain
142the value 340 = 0x154.
143
144See the type_of_loader and ext_loader_type fields in
145Documentation/x86/boot.txt for additional information.
146
147==============================================================
148
149bootloader_version:
150
151x86 bootloader version
152
153The complete bootloader version number. In the example above, this
154file will contain the value 564 = 0x234.
155
156See the type_of_loader and ext_loader_ver fields in
157Documentation/x86/boot.txt for additional information.
158
159==============================================================
160
Hans-Joachim Pichtc114728a2009-09-11 10:28:47 +0200161callhome:
162
163Controls the kernel's callhome behavior in case of a kernel panic.
164
165The s390 hardware allows an operating system to send a notification
166to a service organization (callhome) in case of an operating system panic.
167
168When the value in this file is 0 (which is the default behavior)
169nothing happens in case of a kernel panic. If this value is set to "1"
170the complete kernel oops message is send to the IBM customer service
171organization in case the mainframe the Linux operating system is running
172on has a service contract with IBM.
173
174==============================================================
175
Dan Ballard73efc032011-10-31 17:11:20 -0700176cap_last_cap
177
178Highest valid capability of the running kernel. Exports
179CAP_LAST_CAP from the kernel.
180
181==============================================================
182
Linus Torvalds1da177e2005-04-16 15:20:36 -0700183core_pattern:
184
185core_pattern is used to specify a core dumpfile pattern name.
Matthias Urlichscd081042006-10-11 01:21:57 -0700186. max length 128 characters; default value is "core"
Linus Torvalds1da177e2005-04-16 15:20:36 -0700187. core_pattern is used as a pattern template for the output filename;
188 certain string patterns (beginning with '%') are substituted with
189 their actual values.
190. backward compatibility with core_uses_pid:
191 If core_pattern does not include "%p" (default does not)
192 and core_uses_pid is set, then .PID will be appended to
193 the filename.
194. corename format specifiers:
195 %<NUL> '%' is dropped
196 %% output one '%'
197 %p pid
Stéphane Graber65aafb12013-09-11 14:24:32 -0700198 %P global pid (init PID namespace)
Oleg Nesterovb03023e2014-10-13 15:53:35 -0700199 %i tid
200 %I global tid (init PID namespace)
Nicolas Iooss5202efe2015-06-25 15:03:51 -0700201 %u uid (in initial user namespace)
202 %g gid (in initial user namespace)
Oleg Nesterov12a2b4b2012-10-04 17:15:25 -0700203 %d dump mode, matches PR_SET_DUMPABLE and
204 /proc/sys/fs/suid_dumpable
Linus Torvalds1da177e2005-04-16 15:20:36 -0700205 %s signal number
206 %t UNIX time of dump
207 %h hostname
Jiri Slaby57cc0832011-05-26 16:25:46 -0700208 %e executable filename (may be shortened)
209 %E executable path
Linus Torvalds1da177e2005-04-16 15:20:36 -0700210 %<OTHER> both are dropped
Matthias Urlichscd081042006-10-11 01:21:57 -0700211. If the first character of the pattern is a '|', the kernel will treat
212 the rest of the pattern as a command to run. The core dump will be
213 written to the standard input of that program instead of to a file.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700214
215==============================================================
216
Neil Hormana2939802009-09-23 15:56:56 -0700217core_pipe_limit:
218
Borislav Petkov807094c2011-07-23 10:39:29 -0700219This sysctl is only applicable when core_pattern is configured to pipe
220core files to a user space helper (when the first character of
221core_pattern is a '|', see above). When collecting cores via a pipe
222to an application, it is occasionally useful for the collecting
223application to gather data about the crashing process from its
224/proc/pid directory. In order to do this safely, the kernel must wait
225for the collecting process to exit, so as not to remove the crashing
226processes proc files prematurely. This in turn creates the
227possibility that a misbehaving userspace collecting process can block
228the reaping of a crashed process simply by never exiting. This sysctl
229defends against that. It defines how many concurrent crashing
230processes may be piped to user space applications in parallel. If
231this value is exceeded, then those crashing processes above that value
232are noted via the kernel log and their cores are skipped. 0 is a
233special value, indicating that unlimited processes may be captured in
234parallel, but that no waiting will take place (i.e. the collecting
235process is not guaranteed access to /proc/<crashing pid>/). This
236value defaults to 0.
Neil Hormana2939802009-09-23 15:56:56 -0700237
238==============================================================
239
Linus Torvalds1da177e2005-04-16 15:20:36 -0700240core_uses_pid:
241
242The default coredump filename is "core". By setting
243core_uses_pid to 1, the coredump filename becomes core.PID.
244If core_pattern does not include "%p" (default does not)
245and core_uses_pid is set, then .PID will be appended to
246the filename.
247
248==============================================================
249
250ctrl-alt-del:
251
252When the value in this file is 0, ctrl-alt-del is trapped and
253sent to the init(1) program to handle a graceful restart.
254When, however, the value is > 0, Linux's reaction to a Vulcan
255Nerve Pinch (tm) will be an immediate reboot, without even
256syncing its dirty buffers.
257
258Note: when a program (like dosemu) has the keyboard in 'raw'
259mode, the ctrl-alt-del is intercepted by the program before it
260ever reaches the kernel tty layer, and it's up to the program
261to decide what to do with it.
262
263==============================================================
264
Dan Rosenbergeaf06b22010-11-11 14:05:18 -0800265dmesg_restrict:
266
Borislav Petkov807094c2011-07-23 10:39:29 -0700267This toggle indicates whether unprivileged users are prevented
268from using dmesg(8) to view messages from the kernel's log buffer.
269When dmesg_restrict is set to (0) there are no restrictions. When
Serge E. Hallyn38ef4c22010-12-08 15:19:01 +0000270dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
Dan Rosenbergeaf06b22010-11-11 14:05:18 -0800271dmesg(8).
272
Borislav Petkov807094c2011-07-23 10:39:29 -0700273The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
274default value of dmesg_restrict.
Dan Rosenbergeaf06b22010-11-11 14:05:18 -0800275
276==============================================================
277
Linus Torvalds1da177e2005-04-16 15:20:36 -0700278domainname & hostname:
279
280These files can be used to set the NIS/YP domainname and the
281hostname of your box in exactly the same way as the commands
282domainname and hostname, i.e.:
283# echo "darkstar" > /proc/sys/kernel/hostname
284# echo "mydomain" > /proc/sys/kernel/domainname
285has the same effect as
286# hostname "darkstar"
287# domainname "mydomain"
288
289Note, however, that the classic darkstar.frop.org has the
290hostname "darkstar" and DNS (Internet Domain Name Server)
291domainname "frop.org", not to be confused with the NIS (Network
292Information Service) or YP (Yellow Pages) domainname. These two
293domain names are in general different. For a detailed discussion
294see the hostname(1) man page.
295
296==============================================================
Jiri Kosina55537872015-11-05 18:44:41 -0800297hardlockup_all_cpu_backtrace:
298
299This value controls the hard lockup detector behavior when a hard
300lockup condition is detected as to whether or not to gather further
301debug information. If enabled, arch-specific all-CPU stack dumping
302will be initiated.
303
3040: do nothing. This is the default behavior.
305
3061: on detection capture more debug information.
307==============================================================
Linus Torvalds1da177e2005-04-16 15:20:36 -0700308
309hotplug:
310
311Path for the hotplug policy agent.
312Default value is "/sbin/hotplug".
313
314==============================================================
315
Aaron Tomlin270750db2014-01-20 17:34:13 +0000316hung_task_panic:
317
318Controls the kernel's behavior when a hung task is detected.
319This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
320
3210: continue operation. This is the default behavior.
322
3231: panic immediately.
324
325==============================================================
326
327hung_task_check_count:
328
329The upper bound on the number of tasks that are checked.
330This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
331
332==============================================================
333
334hung_task_timeout_secs:
335
336Check interval. When a task in D state did not get scheduled
337for more than this value report a warning.
338This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
339
3400: means infinite timeout - no checking done.
Liu Hua80df2842014-04-07 15:38:57 -0700341Possible values to set are in range {0..LONG_MAX/HZ}.
Aaron Tomlin270750db2014-01-20 17:34:13 +0000342
343==============================================================
344
Aaron Tomlin70e0ac52014-01-27 09:00:57 +0000345hung_task_warnings:
Aaron Tomlin270750db2014-01-20 17:34:13 +0000346
347The maximum number of warnings to report. During a check interval
Aaron Tomlin70e0ac52014-01-27 09:00:57 +0000348if a hung task is detected, this value is decreased by 1.
349When this value reaches 0, no more warnings will be reported.
Aaron Tomlin270750db2014-01-20 17:34:13 +0000350This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
351
352-1: report an infinite number of warnings.
353
354==============================================================
355
Kees Cook79847542014-01-23 15:55:59 -0800356kexec_load_disabled:
357
358A toggle indicating if the kexec_load syscall has been disabled. This
359value defaults to 0 (false: kexec_load enabled), but can be set to 1
360(true: kexec_load disabled). Once true, kexec can no longer be used, and
361the toggle cannot be set back to false. This allows a kexec image to be
362loaded before disabling the syscall, allowing a system to set up (and
363later use) an image without it being altered. Generally used together
364with the "modules_disabled" sysctl.
365
366==============================================================
367
Dan Rosenberg455cd5a2011-01-12 16:59:41 -0800368kptr_restrict:
369
370This toggle indicates whether restrictions are placed on
Ryan Mallon312b4e22013-11-12 15:08:51 -0800371exposing kernel addresses via /proc and other interfaces.
372
373When kptr_restrict is set to (0), the default, there are no restrictions.
374
375When kptr_restrict is set to (1), kernel pointers printed using the %pK
376format specifier will be replaced with 0's unless the user has CAP_SYSLOG
377and effective user and group ids are equal to the real ids. This is
378because %pK checks are done at read() time rather than open() time, so
379if permissions are elevated between the open() and the read() (e.g via
380a setuid binary) then %pK will not leak kernel pointers to unprivileged
381users. Note, this is a temporary solution only. The correct long-term
382solution is to do the permission checks at open() time. Consider removing
383world read permissions from files that use %pK, and using dmesg_restrict
384to protect against uses of %pK in dmesg(8) if leaking kernel pointer
385values to unprivileged users is a concern.
386
387When kptr_restrict is set to (2), kernel pointers printed using
388%pK will be replaced with 0's regardless of privileges.
Dan Rosenberg455cd5a2011-01-12 16:59:41 -0800389
390==============================================================
391
Chuck Ebbert0741f4d2006-12-07 02:14:11 +0100392kstack_depth_to_print: (X86 only)
393
394Controls the number of words to print when dumping the raw
395kernel stack.
396
397==============================================================
398
Borislav Petkov807094c2011-07-23 10:39:29 -0700399l2cr: (PPC only)
400
401This flag controls the L2 cache of G3 processor boards. If
4020, the cache is disabled. Enabled if nonzero.
403
404==============================================================
405
Kees Cook3d433212009-04-02 15:49:29 -0700406modules_disabled:
407
408A toggle value indicating if modules are allowed to be loaded
409in an otherwise modular kernel. This toggle defaults to off
410(0), but can be set true (1). Once true, modules can be
411neither loaded nor unloaded, and the toggle cannot be set back
Kees Cook79847542014-01-23 15:55:59 -0800412to false. Generally used with the "kexec_load_disabled" toggle.
Kees Cook3d433212009-04-02 15:49:29 -0700413
414==============================================================
415
Stanislav Kinsbursky03f59562013-01-04 15:34:50 -0800416msg_next_id, sem_next_id, and shm_next_id:
417
418These three toggles allows to specify desired id for next allocated IPC
419object: message, semaphore or shared memory respectively.
420
421By default they are equal to -1, which means generic allocation logic.
422Possible values to set are in range {0..INT_MAX}.
423
424Notes:
4251) kernel doesn't guarantee, that new object will have desired id. So,
426it's up to userspace, how to handle an object with "wrong" id.
4272) Toggle with non-default value will be set back to -1 by kernel after
428successful IPC object allocation.
429
430==============================================================
431
Borislav Petkov807094c2011-07-23 10:39:29 -0700432nmi_watchdog:
433
Ulrich Obergfell195daf62015-04-14 15:44:13 -0700434This parameter can be used to control the NMI watchdog
435(i.e. the hard lockup detector) on x86 systems.
Borislav Petkov807094c2011-07-23 10:39:29 -0700436
Ulrich Obergfell195daf62015-04-14 15:44:13 -0700437 0 - disable the hard lockup detector
438 1 - enable the hard lockup detector
439
440The hard lockup detector monitors each CPU for its ability to respond to
441timer interrupts. The mechanism utilizes CPU performance counter registers
442that are programmed to generate Non-Maskable Interrupts (NMIs) periodically
443while a CPU is busy. Hence, the alternative name 'NMI watchdog'.
444
445The NMI watchdog is disabled by default if the kernel is running as a guest
446in a KVM virtual machine. This default can be overridden by adding
447
448 nmi_watchdog=1
449
450to the guest kernel command line (see Documentation/kernel-parameters.txt).
Borislav Petkov807094c2011-07-23 10:39:29 -0700451
452==============================================================
453
Mel Gorman10fc05d2013-10-07 11:28:40 +0100454numa_balancing
455
456Enables/disables automatic page fault based NUMA memory
457balancing. Memory is moved automatically to nodes
458that access it often.
459
460Enables/disables automatic NUMA memory balancing. On NUMA machines, there
461is a performance penalty if remote memory is accessed by a CPU. When this
462feature is enabled the kernel samples what task thread is accessing memory
463by periodically unmapping pages and later trapping a page fault. At the
464time of the page fault, it is determined if the data being accessed should
465be migrated to a local memory node.
466
467The unmapping of pages and trapping faults incur additional overhead that
468ideally is offset by improved memory locality but there is no universal
469guarantee. If the target workload is already bound to NUMA nodes then this
470feature should be disabled. Otherwise, if the system overhead from the
471feature is too high then the rate the kernel samples for NUMA hinting
472faults may be controlled by the numa_balancing_scan_period_min_ms,
Mel Gorman930aa172013-10-07 11:29:37 +0100473numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms,
Rik van Riel52bf84a2014-01-27 17:03:40 -0500474numa_balancing_scan_size_mb, and numa_balancing_settle_count sysctls.
Mel Gorman10fc05d2013-10-07 11:28:40 +0100475
476==============================================================
477
478numa_balancing_scan_period_min_ms, numa_balancing_scan_delay_ms,
Mel Gorman930aa172013-10-07 11:29:37 +0100479numa_balancing_scan_period_max_ms, numa_balancing_scan_size_mb
Mel Gorman10fc05d2013-10-07 11:28:40 +0100480
481Automatic NUMA balancing scans tasks address space and unmaps pages to
482detect if pages are properly placed or if the data should be migrated to a
483memory node local to where the task is running. Every "scan delay" the task
484scans the next "scan size" number of pages in its address space. When the
485end of the address space is reached the scanner restarts from the beginning.
486
487In combination, the "scan delay" and "scan size" determine the scan rate.
488When "scan delay" decreases, the scan rate increases. The scan delay and
489hence the scan rate of every task is adaptive and depends on historical
490behaviour. If pages are properly placed then the scan delay increases,
491otherwise the scan delay decreases. The "scan size" is not adaptive but
492the higher the "scan size", the higher the scan rate.
493
494Higher scan rates incur higher system overhead as page faults must be
495trapped and potentially data must be migrated. However, the higher the scan
496rate, the more quickly a tasks memory is migrated to a local node if the
497workload pattern changes and minimises performance impact due to remote
498memory accesses. These sysctls control the thresholds for scan delays and
499the number of pages scanned.
500
Mel Gorman598f0ec2013-10-07 11:28:55 +0100501numa_balancing_scan_period_min_ms is the minimum time in milliseconds to
502scan a tasks virtual memory. It effectively controls the maximum scanning
503rate for each task.
Mel Gorman10fc05d2013-10-07 11:28:40 +0100504
505numa_balancing_scan_delay_ms is the starting "scan delay" used for a task
506when it initially forks.
507
Mel Gorman598f0ec2013-10-07 11:28:55 +0100508numa_balancing_scan_period_max_ms is the maximum time in milliseconds to
509scan a tasks virtual memory. It effectively controls the minimum scanning
510rate for each task.
Mel Gorman10fc05d2013-10-07 11:28:40 +0100511
512numa_balancing_scan_size_mb is how many megabytes worth of pages are
513scanned for a given scan.
514
Mel Gorman10fc05d2013-10-07 11:28:40 +0100515==============================================================
516
Linus Torvalds1da177e2005-04-16 15:20:36 -0700517osrelease, ostype & version:
518
519# cat osrelease
5202.1.88
521# cat ostype
522Linux
523# cat version
524#5 Wed Feb 25 21:49:24 MET 1998
525
526The files osrelease and ostype should be clear enough. Version
527needs a little more clarification however. The '#5' means that
528this is the fifth kernel built from this source base and the
529date behind it indicates the time the kernel was built.
530The only way to tune these values is to rebuild the kernel :-)
531
532==============================================================
533
534overflowgid & overflowuid:
535
Borislav Petkov807094c2011-07-23 10:39:29 -0700536if your architecture did not always support 32-bit UIDs (i.e. arm,
537i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
538applications that use the old 16-bit UID/GID system calls, if the
539actual UID or GID would exceed 65535.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700540
541These sysctls allow you to change the value of the fixed UID and GID.
542The default is 65534.
543
544==============================================================
545
546panic:
547
Borislav Petkov807094c2011-07-23 10:39:29 -0700548The value in this file represents the number of seconds the kernel
549waits before rebooting on a panic. When you use the software watchdog,
550the recommended setting is 60.
551
552==============================================================
553
Linus Torvalds1da177e2005-04-16 15:20:36 -0700554panic_on_oops:
555
556Controls the kernel's behaviour when an oops or BUG is encountered.
557
5580: try to continue operation
559
Matt LaPlantea982ac02007-05-09 07:35:06 +02005601: panic immediately. If the `panic' sysctl is also non-zero then the
Maxime Bizon8b23d04d2006-08-05 12:14:32 -0700561 machine will be rebooted.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700562
563==============================================================
564
Mitsuo Hayasaka55af7792011-11-29 15:08:36 +0900565panic_on_stackoverflow:
566
567Controls the kernel's behavior when detecting the overflows of
568kernel, IRQ and exception stacks except a user stack.
569This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.
570
5710: try to continue operation.
572
5731: panic immediately.
574
575==============================================================
576
Prarit Bhargava9e3961a2014-12-10 15:45:50 -0800577panic_on_unrecovered_nmi:
578
579The default Linux behaviour on an NMI of either memory or unknown is
580to continue operation. For many environments such as scientific
581computing it is preferable that the box is taken out and the error
582dealt with than an uncorrected parity/ECC error get propagated.
583
584A small number of systems do generate NMI's for bizarre random reasons
585such as power management so the default is off. That sysctl works like
586the existing panic controls already in that directory.
587
588==============================================================
589
590panic_on_warn:
591
592Calls panic() in the WARN() path when set to 1. This is useful to avoid
593a kernel rebuild when attempting to kdump at the location of a WARN().
594
5950: only WARN(), default behaviour.
596
5971: call panic() after printing out WARN() location.
598
599==============================================================
600
Dave Hansen14c63f12013-06-21 08:51:36 -0700601perf_cpu_time_max_percent:
602
603Hints to the kernel how much CPU time it should be allowed to
604use to handle perf sampling events. If the perf subsystem
605is informed that its samples are exceeding this limit, it
606will drop its sampling frequency to attempt to reduce its CPU
607usage.
608
609Some perf sampling happens in NMIs. If these samples
610unexpectedly take too long to execute, the NMIs can become
611stacked up next to each other so much that nothing else is
612allowed to execute.
613
6140: disable the mechanism. Do not monitor or correct perf's
615 sampling rate no matter how CPU time it takes.
616
6171-100: attempt to throttle perf's sample rate to this
618 percentage of CPU. Note: the kernel calculates an
619 "expected" length of each sample event. 100 here means
620 100% of that expected length. Even if this is set to
621 100, you may still see sample throttling if this
622 length is exceeded. Set to 0 if you truly do not care
623 how much CPU is consumed.
624
625==============================================================
626
Mitsuo Hayasaka55af7792011-11-29 15:08:36 +0900627
Linus Torvalds1da177e2005-04-16 15:20:36 -0700628pid_max:
629
Robert P. J. Daybeb7dd82007-05-09 07:14:03 +0200630PID allocation wrap value. When the kernel's next PID value
Linus Torvalds1da177e2005-04-16 15:20:36 -0700631reaches this value, it wraps back to a minimum PID value.
632PIDs of value pid_max or larger are not allocated.
633
634==============================================================
635
Pavel Emelyanovb8f566b2012-01-12 17:20:27 -0800636ns_last_pid:
637
638The last pid allocated in the current (the one task using this sysctl
639lives in) pid namespace. When selecting a pid for a next task on fork
640kernel tries to allocate a number starting from this one.
641
642==============================================================
643
Linus Torvalds1da177e2005-04-16 15:20:36 -0700644powersave-nap: (PPC only)
645
646If set, Linux-PPC will use the 'nap' mode of powersaving,
647otherwise the 'doze' mode will be used.
648
649==============================================================
650
651printk:
652
653The four values in printk denote: console_loglevel,
654default_message_loglevel, minimum_console_loglevel and
655default_console_loglevel respectively.
656
657These values influence printk() behavior when printing or
658logging error messages. See 'man 2 syslog' for more info on
659the different loglevels.
660
661- console_loglevel: messages with a higher priority than
662 this will be printed to the console
Paul Bolle87889e12011-02-06 21:00:41 +0100663- default_message_loglevel: messages without an explicit priority
Linus Torvalds1da177e2005-04-16 15:20:36 -0700664 will be printed with this priority
665- minimum_console_loglevel: minimum (highest) value to which
666 console_loglevel can be set
667- default_console_loglevel: default value for console_loglevel
668
669==============================================================
670
Borislav Petkov807094c2011-07-23 10:39:29 -0700671printk_delay:
672
673Delay each printk message in printk_delay milliseconds
674
675Value from 0 - 10000 is allowed.
676
677==============================================================
678
Linus Torvalds1da177e2005-04-16 15:20:36 -0700679printk_ratelimit:
680
681Some warning messages are rate limited. printk_ratelimit specifies
682the minimum length of time between these messages (in jiffies), by
683default we allow one every 5 seconds.
684
685A value of 0 will disable rate limiting.
686
687==============================================================
688
689printk_ratelimit_burst:
690
691While long term we enforce one message per printk_ratelimit
692seconds, we do allow a burst of messages to pass through.
693printk_ratelimit_burst specifies the number of messages we can
694send before ratelimiting kicks in.
695
696==============================================================
697
Borislav Petkov807094c2011-07-23 10:39:29 -0700698randomize_va_space:
Jiri Kosina1ec7fd52008-02-09 23:24:08 +0100699
700This option can be used to select the type of process address
701space randomization that is used in the system, for architectures
702that support this feature.
703
Horst Schirmeierb7f5ab62009-07-03 14:20:17 +02007040 - Turn the process address space randomization off. This is the
705 default for architectures that do not support this feature anyways,
706 and kernels that are booted with the "norandmaps" parameter.
Jiri Kosina1ec7fd52008-02-09 23:24:08 +0100707
7081 - Make the addresses of mmap base, stack and VDSO page randomized.
709 This, among other things, implies that shared libraries will be
Horst Schirmeierb7f5ab62009-07-03 14:20:17 +0200710 loaded to random addresses. Also for PIE-linked binaries, the
711 location of code start is randomized. This is the default if the
712 CONFIG_COMPAT_BRK option is enabled.
Jiri Kosina1ec7fd52008-02-09 23:24:08 +0100713
Horst Schirmeierb7f5ab62009-07-03 14:20:17 +02007142 - Additionally enable heap randomization. This is the default if
715 CONFIG_COMPAT_BRK is disabled.
716
717 There are a few legacy applications out there (such as some ancient
Jiri Kosina1ec7fd52008-02-09 23:24:08 +0100718 versions of libc.so.5 from 1996) that assume that brk area starts
Horst Schirmeierb7f5ab62009-07-03 14:20:17 +0200719 just after the end of the code+bss. These applications break when
720 start of the brk area is randomized. There are however no known
Jiri Kosina1ec7fd52008-02-09 23:24:08 +0100721 non-legacy applications that would be broken this way, so for most
Horst Schirmeierb7f5ab62009-07-03 14:20:17 +0200722 systems it is safe to choose full randomization.
723
724 Systems with ancient and/or broken binaries should be configured
725 with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
726 address space randomization.
Jiri Kosina1ec7fd52008-02-09 23:24:08 +0100727
728==============================================================
729
Linus Torvalds1da177e2005-04-16 15:20:36 -0700730reboot-cmd: (Sparc only)
731
732??? This seems to be a way to give an argument to the Sparc
733ROM/Flash boot loader. Maybe to tell it what to do after
734rebooting. ???
735
736==============================================================
737
738rtsig-max & rtsig-nr:
739
740The file rtsig-max can be used to tune the maximum number
741of POSIX realtime (queued) signals that can be outstanding
742in the system.
743
744rtsig-nr shows the number of RT signals currently queued.
745
746==============================================================
747
748sg-big-buff:
749
750This file shows the size of the generic SCSI (sg) buffer.
751You can't tune it just yet, but you could change it on
752compile time by editing include/scsi/sg.h and changing
753the value of SG_BIG_BUFF.
754
755There shouldn't be any reason to change this value. If
756you can come up with one, you probably know what you
757are doing anyway :)
758
759==============================================================
760
Carlos Alberto Lopez Perez358e4192013-01-04 15:35:05 -0800761shmall:
762
763This parameter sets the total amount of shared memory pages that
764can be used system wide. Hence, SHMALL should always be at least
765ceil(shmmax/PAGE_SIZE).
766
767If you are not sure what the default PAGE_SIZE is on your Linux
768system, you can run the following command:
769
770# getconf PAGE_SIZE
771
772==============================================================
773
Borislav Petkov807094c2011-07-23 10:39:29 -0700774shmmax:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700775
776This value can be used to query and set the run time limit
777on the maximum shared memory segment size that can be created.
Borislav Petkov807094c2011-07-23 10:39:29 -0700778Shared memory segments up to 1Gb are now supported in the
Linus Torvalds1da177e2005-04-16 15:20:36 -0700779kernel. This value defaults to SHMMAX.
780
781==============================================================
782
Vasiliy Kulikovb34a6b12011-07-26 16:08:48 -0700783shm_rmid_forced:
784
785Linux lets you set resource limits, including how much memory one
786process can consume, via setrlimit(2). Unfortunately, shared memory
787segments are allowed to exist without association with any process, and
788thus might not be counted against any resource limits. If enabled,
789shared memory segments are automatically destroyed when their attach
790count becomes zero after a detach or a process termination. It will
791also destroy segments that were created, but never attached to, on exit
792from the process. The only use left for IPC_RMID is to immediately
793destroy an unattached segment. Of course, this breaks the way things are
794defined, so some applications might stop working. Note that this
795feature will do you no good unless you also configure your resource
796limits (in particular, RLIMIT_AS and RLIMIT_NPROC). Most systems don't
797need this.
798
799Note that if you change this from 0 to 1, already created segments
800without users and with a dead originative process will be destroyed.
801
802==============================================================
803
Kees Cookf4aacea2014-06-06 14:37:19 -0700804sysctl_writes_strict:
805
806Control how file position affects the behavior of updating sysctl values
807via the /proc/sys interface:
808
809 -1 - Legacy per-write sysctl value handling, with no printk warnings.
810 Each write syscall must fully contain the sysctl value to be
811 written, and multiple writes on the same sysctl file descriptor
812 will rewrite the sysctl value, regardless of file position.
813 0 - (default) Same behavior as above, but warn about processes that
814 perform writes to a sysctl file descriptor when the file position
815 is not 0.
816 1 - Respect file position when writing sysctl strings. Multiple writes
817 will append to the sysctl value buffer. Anything past the max length
818 of the sysctl value buffer will be ignored. Writes to numeric sysctl
819 entries must always be at file position 0 and the value must be
820 fully contained in the buffer sent in the write syscall.
821
822==============================================================
823
Aaron Tomlined235872014-06-23 13:22:05 -0700824softlockup_all_cpu_backtrace:
825
826This value controls the soft lockup detector thread's behavior
827when a soft lockup condition is detected as to whether or not
828to gather further debug information. If enabled, each cpu will
829be issued an NMI and instructed to capture stack trace.
830
831This feature is only applicable for architectures which support
832NMI.
833
8340: do nothing. This is the default behavior.
835
8361: on detection capture more debug information.
837
838==============================================================
839
Ulrich Obergfell195daf62015-04-14 15:44:13 -0700840soft_watchdog
841
842This parameter can be used to control the soft lockup detector.
843
844 0 - disable the soft lockup detector
845 1 - enable the soft lockup detector
846
847The soft lockup detector monitors CPUs for threads that are hogging the CPUs
848without rescheduling voluntarily, and thus prevent the 'watchdog/N' threads
849from running. The mechanism depends on the CPUs ability to respond to timer
850interrupts which are needed for the 'watchdog/N' threads to be woken up by
851the watchdog timer function, otherwise the NMI watchdog - if enabled - can
852detect a hard lockup condition.
853
854==============================================================
855
Borislav Petkov807094c2011-07-23 10:39:29 -0700856tainted:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700857
858Non-zero if the kernel has been tainted. Numeric values, which
859can be ORed together:
860
Greg Kroah-Hartmanbb206982008-10-17 15:01:07 -0700861 1 - A module with a non-GPL license has been loaded, this
862 includes modules with no license.
863 Set by modutils >= 2.4.9 and module-init-tools.
864 2 - A module was force loaded by insmod -f.
865 Set by modutils >= 2.4.9 and module-init-tools.
866 4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
867 8 - A module was forcibly unloaded from the system by rmmod -f.
868 16 - A hardware machine check error occurred on the system.
869 32 - A bad page was discovered on the system.
870 64 - The user has asked that the system be marked "tainted". This
871 could be because they are running software that directly modifies
872 the hardware, or for other reasons.
873 128 - The system has died.
874 256 - The ACPI DSDT has been overridden with one supplied by the user
875 instead of using the one provided by the hardware.
876 512 - A kernel warning has occurred.
8771024 - A module from drivers/staging was loaded.
Larry Fingerf5fe1842012-02-06 09:49:50 -08008782048 - The system is working around a severe firmware bug.
8794096 - An out-of-tree module has been loaded.
Mathieu Desnoyers66cc69e2014-03-13 12:11:30 +10308808192 - An unsigned module has been loaded in a kernel supporting module
881 signature.
Josh Hunt69361ee2014-08-08 14:22:31 -070088216384 - A soft lockup has previously occurred on the system.
Seth Jenningsc5f45462014-12-16 11:58:18 -060088332768 - The kernel has been live patched.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700884
Shen Feng760df932009-04-02 16:57:20 -0700885==============================================================
886
Heinrich Schuchardt0ec62af2015-04-16 12:47:53 -0700887threads-max
888
889This value controls the maximum number of threads that can be created
890using fork().
891
892During initialization the kernel sets this value such that even if the
893maximum number of threads is created, the thread structures occupy only
894a part (1/8th) of the available RAM pages.
895
896The minimum value that can be written to threads-max is 20.
897The maximum value that can be written to threads-max is given by the
898constant FUTEX_TID_MASK (0x3fffffff).
899If a value outside of this range is written to threads-max an error
900EINVAL occurs.
901
902The value written is checked against the available RAM pages. If the
903thread structures would occupy too much (more than 1/8th) of the
904available RAM pages threads-max is reduced accordingly.
905
906==============================================================
907
Shen Feng760df932009-04-02 16:57:20 -0700908unknown_nmi_panic:
909
Borislav Petkov807094c2011-07-23 10:39:29 -0700910The value in this file affects behavior of handling NMI. When the
911value is non-zero, unknown NMI is trapped and then panic occurs. At
912that time, kernel debugging information is displayed on console.
Shen Feng760df932009-04-02 16:57:20 -0700913
Borislav Petkov807094c2011-07-23 10:39:29 -0700914NMI switch that most IA32 servers have fires unknown NMI up, for
915example. If a system hangs up, try pressing the NMI switch.
Li Zefan08825c92013-05-17 10:31:20 +0800916
917==============================================================
918
Ulrich Obergfell195daf62015-04-14 15:44:13 -0700919watchdog:
920
921This parameter can be used to disable or enable the soft lockup detector
922_and_ the NMI watchdog (i.e. the hard lockup detector) at the same time.
923
924 0 - disable both lockup detectors
925 1 - enable both lockup detectors
926
927The soft lockup detector and the NMI watchdog can also be disabled or
928enabled individually, using the soft_watchdog and nmi_watchdog parameters.
929If the watchdog parameter is read, for example by executing
930
931 cat /proc/sys/kernel/watchdog
932
933the output of this command (0 or 1) shows the logical OR of soft_watchdog
934and nmi_watchdog.
935
936==============================================================
937
Chris Metcalffe4ba3c2015-06-24 16:55:45 -0700938watchdog_cpumask:
939
940This value can be used to control on which cpus the watchdog may run.
941The default cpumask is all possible cores, but if NO_HZ_FULL is
942enabled in the kernel config, and cores are specified with the
943nohz_full= boot argument, those cores are excluded by default.
944Offline cores can be included in this mask, and if the core is later
945brought online, the watchdog will be started based on the mask value.
946
947Typically this value would only be touched in the nohz_full case
948to re-enable cores that by default were not running the watchdog,
949if a kernel lockup was suspected on those cores.
950
951The argument value is the standard cpulist format for cpumasks,
952so for example to enable the watchdog on cores 0, 2, 3, and 4 you
953might say:
954
955 echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask
956
957==============================================================
958
Li Zefan08825c92013-05-17 10:31:20 +0800959watchdog_thresh:
960
961This value can be used to control the frequency of hrtimer and NMI
962events and the soft and hard lockup thresholds. The default threshold
963is 10 seconds.
964
965The softlockup threshold is (2 * watchdog_thresh). Setting this
966tunable to zero will disable lockup detection altogether.
967
968==============================================================