1 ===================================
2 Documentation for /proc/sys/kernel/
3 ===================================
5 .. See scripts/check-sysctl-docs to keep this up to date
8 Copyright (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
10 Copyright (c) 2009, Shen Feng<shen@cn.fujitsu.com>
12 For general info and legal blurb, please look in
13 Documentation/admin-guide/sysctl/index.rst.
15 ------------------------------------------------------------------------------
17 This file contains documentation for the sysctl files in
18 ``/proc/sys/kernel/``.
20 The files in this directory can be used to tune and monitor
21 miscellaneous and general things in the operation of the Linux
22 kernel. Since some of the files *can* be used to screw up your
23 system, it is advisable to read both documentation and source
24 before actually making adjustments.
26 Currently, these files might (depending on your configuration)
27 show up in ``/proc/sys/kernel``:
37 highwater lowwater frequency
39 If BSD-style process accounting is enabled these values control
40 its behaviour. If free space on filesystem where the log lives
41 goes below ``lowwater``% accounting suspends. If free space gets
42 above ``highwater``% accounting resumes. ``frequency`` determines
43 how often do we check the amount of free space (value is in
50 That is, suspend accounting if free space drops below 2%; resume it
51 if it increases to at least 4%; consider information about amount of
52 free space valid for 30 seconds.
58 See Documentation/power/video.rst. This allows the video resume mode to be set,
59 in a similar fashion to the ``acpi_sleep`` kernel parameter, by
60 combining the following values:
72 This variable has no effect and may be removed in future kernel
73 releases. Reading it always returns 0.
74 Up to Linux 3.17, it enabled/disabled automatic recomputing of
76 upon memory add/remove or upon IPC namespace creation/removal.
77 Echoing "1" into this file enabled msgmni automatic recomputing.
78 Echoing "0" turned it off. The default value was 1.
81 bootloader_type (x86 only)
82 ==========================
84 This gives the bootloader type number as indicated by the bootloader,
85 shifted left by 4, and OR'd with the low four bits of the bootloader
86 version. The reason for this encoding is that this used to match the
87 ``type_of_loader`` field in the kernel header; the encoding is kept for
88 backwards compatibility. That is, if the full bootloader type number
89 is 0x15 and the full version number is 0x234, this file will contain
90 the value 340 = 0x154.
92 See the ``type_of_loader`` and ``ext_loader_type`` fields in
93 Documentation/x86/boot.rst for additional information.
96 bootloader_version (x86 only)
97 =============================
99 The complete bootloader version number. In the example above, this
100 file will contain the value 564 = 0x234.
102 See the ``type_of_loader`` and ``ext_loader_ver`` fields in
103 Documentation/x86/boot.rst for additional information.
109 Controls whether the kernel should collect statistics on BPF programs
110 (total time spent running, number of times run...). Enabling
111 statistics causes a slight reduction in performance on each program
112 run. The statistics can be seen using ``bpftool``.
114 = ===================================
115 0 Don't collect statistics (default).
116 1 Collect statistics.
117 = ===================================
123 This is the pid which will be signalled on reboot (notably, by
124 Ctrl-Alt-Delete). Writing a value to this file which doesn't
125 correspond to a running process will result in ``-ESRCH``.
127 See also `ctrl-alt-del`_.
133 Highest valid capability of the running kernel. Exports
134 ``CAP_LAST_CAP`` from the kernel.
140 ``core_pattern`` is used to specify a core dumpfile pattern name.
142 * max length 127 characters; default value is "core"
143 * ``core_pattern`` is used as a pattern template for the output
144 filename; certain string patterns (beginning with '%') are
145 substituted with their actual values.
146 * backward compatibility with ``core_uses_pid``:
148 If ``core_pattern`` does not include "%p" (default does not)
149 and ``core_uses_pid`` is set, then .PID will be appended to
152 * corename format specifiers
154 ======== ==========================================
155 %<NUL> '%' is dropped
158 %P global pid (init PID namespace)
160 %I global tid (init PID namespace)
161 %u uid (in initial user namespace)
162 %g gid (in initial user namespace)
163 %d dump mode, matches ``PR_SET_DUMPABLE`` and
164 ``/proc/sys/fs/suid_dumpable``
168 %e executable filename (may be shortened, could be changed by prctl etc)
169 %f executable filename
171 %c maximum size of core file by resource limit RLIMIT_CORE
172 %<OTHER> both are dropped
173 ======== ==========================================
175 * If the first character of the pattern is a '|', the kernel will treat
176 the rest of the pattern as a command to run. The core dump will be
177 written to the standard input of that program instead of to a file.
183 This sysctl is only applicable when `core_pattern`_ is configured to
184 pipe core files to a user space helper (when the first character of
185 ``core_pattern`` is a '|', see above).
186 When collecting cores via a pipe to an application, it is occasionally
187 useful for the collecting application to gather data about the
188 crashing process from its ``/proc/pid`` directory.
189 In order to do this safely, the kernel must wait for the collecting
190 process to exit, so as not to remove the crashing processes proc files
192 This in turn creates the possibility that a misbehaving userspace
193 collecting process can block the reaping of a crashed process simply
195 This sysctl defends against that.
196 It defines how many concurrent crashing processes may be piped to user
197 space applications in parallel.
198 If this value is exceeded, then those crashing processes above that
199 value are noted via the kernel log and their cores are skipped.
200 0 is a special value, indicating that unlimited processes may be
201 captured in parallel, but that no waiting will take place (i.e. the
202 collecting process is not guaranteed access to ``/proc/<crashing
204 This value defaults to 0.
210 The default coredump filename is "core". By setting
211 ``core_uses_pid`` to 1, the coredump filename becomes core.PID.
212 If `core_pattern`_ does not include "%p" (default does not)
213 and ``core_uses_pid`` is set, then .PID will be appended to
220 When the value in this file is 0, ctrl-alt-del is trapped and
221 sent to the ``init(1)`` program to handle a graceful restart.
222 When, however, the value is > 0, Linux's reaction to a Vulcan
223 Nerve Pinch (tm) will be an immediate reboot, without even
224 syncing its dirty buffers.
227 when a program (like dosemu) has the keyboard in 'raw'
228 mode, the ctrl-alt-del is intercepted by the program before it
229 ever reaches the kernel tty layer, and it's up to the program
230 to decide what to do with it.
236 This toggle indicates whether unprivileged users are prevented
237 from using ``dmesg(8)`` to view messages from the kernel's log
239 When ``dmesg_restrict`` is set to 0 there are no restrictions.
240 When ``dmesg_restrict`` is set to 1, users must have
241 ``CAP_SYSLOG`` to use ``dmesg(8)``.
243 The kernel config option ``CONFIG_SECURITY_DMESG_RESTRICT`` sets the
244 default value of ``dmesg_restrict``.
247 domainname & hostname
248 =====================
250 These files can be used to set the NIS/YP domainname and the
251 hostname of your box in exactly the same way as the commands
252 domainname and hostname, i.e.::
254 # echo "darkstar" > /proc/sys/kernel/hostname
255 # echo "mydomain" > /proc/sys/kernel/domainname
257 has the same effect as::
259 # hostname "darkstar"
260 # domainname "mydomain"
262 Note, however, that the classic darkstar.frop.org has the
263 hostname "darkstar" and DNS (Internet Domain Name Server)
264 domainname "frop.org", not to be confused with the NIS (Network
265 Information Service) or YP (Yellow Pages) domainname. These two
266 domain names are in general different. For a detailed discussion
267 see the ``hostname(1)`` man page.
273 See Documentation/driver-api/firmware/fallback-mechanisms.rst.
275 The entries in this directory allow the firmware loader helper
276 fallback to be controlled:
278 * ``force_sysfs_fallback``, when set to 1, forces the use of the
280 * ``ignore_sysfs_fallback``, when set to 1, ignores any fallback.
286 Determines whether ``ftrace_dump()`` should be called on an oops (or
287 kernel panic). This will output the contents of the ftrace buffers to
288 the console. This is very useful for capturing traces that lead to
289 crashes and outputting them to a serial console.
291 = ===================================================
292 0 Disabled (default).
293 1 Dump buffers of all CPUs.
294 2 Dump the buffer of the CPU that triggered the oops.
295 = ===================================================
298 ftrace_enabled, stack_tracer_enabled
299 ====================================
301 See Documentation/trace/ftrace.rst.
304 hardlockup_all_cpu_backtrace
305 ============================
307 This value controls the hard lockup detector behavior when a hard
308 lockup condition is detected as to whether or not to gather further
309 debug information. If enabled, arch-specific all-CPU stack dumping
312 = ============================================
313 0 Do nothing. This is the default behavior.
314 1 On detection capture more debug information.
315 = ============================================
321 This parameter can be used to control whether the kernel panics
322 when a hard lockup is detected.
324 = ===========================
325 0 Don't panic on hard lockup.
326 1 Panic on hard lockup.
327 = ===========================
329 See Documentation/admin-guide/lockup-watchdogs.rst for more information.
330 This can also be set using the nmi_watchdog kernel parameter.
336 Path for the hotplug policy agent.
337 Default value is ``CONFIG_UEVENT_HELPER_PATH``, which in turn defaults
340 This file only exists when ``CONFIG_UEVENT_HELPER`` is enabled. Most
341 modern systems rely exclusively on the netlink-based uevent source and
345 hung_task_all_cpu_backtrace
346 ===========================
348 If this option is set, the kernel will send an NMI to all CPUs to dump
349 their backtraces when a hung task is detected. This file shows up if
350 CONFIG_DETECT_HUNG_TASK and CONFIG_SMP are enabled.
352 0: Won't show all CPUs backtraces when a hung task is detected.
353 This is the default behavior.
355 1: Will non-maskably interrupt all CPUs and dump their backtraces when
356 a hung task is detected.
362 Controls the kernel's behavior when a hung task is detected.
363 This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
365 = =================================================
366 0 Continue operation. This is the default behavior.
368 = =================================================
371 hung_task_check_count
372 =====================
374 The upper bound on the number of tasks that are checked.
375 This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
378 hung_task_timeout_secs
379 ======================
381 When a task in D state did not get scheduled
382 for more than this value report a warning.
383 This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
385 0 means infinite timeout, no checking is done.
387 Possible values to set are in range {0:``LONG_MAX``/``HZ``}.
390 hung_task_check_interval_secs
391 =============================
393 Hung task check interval. If hung task checking is enabled
394 (see `hung_task_timeout_secs`_), the check is done every
395 ``hung_task_check_interval_secs`` seconds.
396 This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
398 0 (default) means use ``hung_task_timeout_secs`` as checking
401 Possible values to set are in range {0:``LONG_MAX``/``HZ``}.
407 The maximum number of warnings to report. During a check interval
408 if a hung task is detected, this value is decreased by 1.
409 When this value reaches 0, no more warnings will be reported.
410 This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
412 -1: report an infinite number of warnings.
415 hyperv_record_panic_msg
416 =======================
418 Controls whether the panic kmsg data should be reported to Hyper-V.
420 = =========================================================
421 0 Do not report panic kmsg data.
422 1 Report the panic kmsg data. This is the default behavior.
423 = =========================================================
426 ignore-unaligned-usertrap
427 =========================
429 On architectures where unaligned accesses cause traps, and where this
430 feature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_NO_WARN``;
431 currently, ``arc`` and ``ia64``), controls whether all unaligned traps
434 = =============================================================
435 0 Log all unaligned accesses.
436 1 Only warn the first time a process traps. This is the default
438 = =============================================================
440 See also `unaligned-trap`_ and `unaligned-dump-stack`_. On ``ia64``,
441 this allows system administrators to override the
442 ``IA64_THREAD_UAC_NOPRINT`` ``prctl`` and avoid logs being flooded.
448 A toggle indicating if the ``kexec_load`` syscall has been disabled.
449 This value defaults to 0 (false: ``kexec_load`` enabled), but can be
450 set to 1 (true: ``kexec_load`` disabled).
451 Once true, kexec can no longer be used, and the toggle cannot be set
453 This allows a kexec image to be loaded before disabling the syscall,
454 allowing a system to set up (and later use) an image without it being
456 Generally used together with the `modules_disabled`_ sysctl.
462 This toggle indicates whether restrictions are placed on
463 exposing kernel addresses via ``/proc`` and other interfaces.
465 When ``kptr_restrict`` is set to 0 (the default) the address is hashed
467 (This is the equivalent to %p.)
469 When ``kptr_restrict`` is set to 1, kernel pointers printed using the
470 %pK format specifier will be replaced with 0s unless the user has
471 ``CAP_SYSLOG`` and effective user and group ids are equal to the real
473 This is because %pK checks are done at read() time rather than open()
474 time, so if permissions are elevated between the open() and the read()
475 (e.g via a setuid binary) then %pK will not leak kernel pointers to
477 Note, this is a temporary solution only.
478 The correct long-term solution is to do the permission checks at
480 Consider removing world read permissions from files that use %pK, and
481 using `dmesg_restrict`_ to protect against uses of %pK in ``dmesg(8)``
482 if leaking kernel pointer values to unprivileged users is a concern.
484 When ``kptr_restrict`` is set to 2, kernel pointers printed using
485 %pK will be replaced with 0s regardless of privileges.
491 The full path to the usermode helper for autoloading kernel modules,
492 by default ``CONFIG_MODPROBE_PATH``, which in turn defaults to
493 "/sbin/modprobe". This binary is executed when the kernel requests a
494 module. For example, if userspace passes an unknown filesystem type
495 to mount(), then the kernel will automatically request the
496 corresponding filesystem module by executing this usermode helper.
497 This usermode helper should insert the needed module into the kernel.
499 This sysctl only affects module autoloading. It has no effect on the
500 ability to explicitly insert modules.
502 This sysctl can be used to debug module loading requests::
504 echo '#! /bin/sh' > /tmp/modprobe
505 echo 'echo "$@" >> /tmp/modprobe.log' >> /tmp/modprobe
506 echo 'exec /sbin/modprobe "$@"' >> /tmp/modprobe
507 chmod a+x /tmp/modprobe
508 echo /tmp/modprobe > /proc/sys/kernel/modprobe
510 Alternatively, if this sysctl is set to the empty string, then module
511 autoloading is completely disabled. The kernel will not try to
512 execute a usermode helper at all, nor will it call the
513 kernel_module_request LSM hook.
515 If CONFIG_STATIC_USERMODEHELPER=y is set in the kernel configuration,
516 then the configured static usermode helper overrides this sysctl,
517 except that the empty string is still accepted to completely disable
518 module autoloading as described above.
523 A toggle value indicating if modules are allowed to be loaded
524 in an otherwise modular kernel. This toggle defaults to off
525 (0), but can be set true (1). Once true, modules can be
526 neither loaded nor unloaded, and the toggle cannot be set back
527 to false. Generally used with the `kexec_load_disabled`_ toggle.
532 msgmax, msgmnb, and msgmni
533 ==========================
535 ``msgmax`` is the maximum size of an IPC message, in bytes. 8192 by
536 default (``MSGMAX``).
538 ``msgmnb`` is the maximum size of an IPC queue, in bytes. 16384 by
539 default (``MSGMNB``).
541 ``msgmni`` is the maximum number of IPC queues. 32000 by default
545 msg_next_id, sem_next_id, and shm_next_id (System V IPC)
546 ========================================================
548 These three toggles allows to specify desired id for next allocated IPC
549 object: message, semaphore or shared memory respectively.
551 By default they are equal to -1, which means generic allocation logic.
552 Possible values to set are in range {0:``INT_MAX``}.
555 1) kernel doesn't guarantee, that new object will have desired id. So,
556 it's up to userspace, how to handle an object with "wrong" id.
557 2) Toggle with non-default value will be set back to -1 by kernel after
558 successful IPC object allocation. If an IPC object allocation syscall
559 fails, it is undefined if the value remains unmodified or is reset to -1.
565 Maximum number of supplementary groups, _i.e._ the maximum size which
566 ``setgroups`` will accept. Exports ``NGROUPS_MAX`` from the kernel.
573 This parameter can be used to control the NMI watchdog
574 (i.e. the hard lockup detector) on x86 systems.
576 = =================================
577 0 Disable the hard lockup detector.
578 1 Enable the hard lockup detector.
579 = =================================
581 The hard lockup detector monitors each CPU for its ability to respond to
582 timer interrupts. The mechanism utilizes CPU performance counter registers
583 that are programmed to generate Non-Maskable Interrupts (NMIs) periodically
584 while a CPU is busy. Hence, the alternative name 'NMI watchdog'.
586 The NMI watchdog is disabled by default if the kernel is running as a guest
587 in a KVM virtual machine. This default can be overridden by adding::
591 to the guest kernel command line (see
592 Documentation/admin-guide/kernel-parameters.rst).
598 Enables/disables and configures automatic page fault based NUMA memory
599 balancing. Memory is moved automatically to nodes that access it often.
600 The value to set can be the result of ORing the following:
602 = =================================
603 0 NUMA_BALANCING_DISABLED
604 1 NUMA_BALANCING_NORMAL
605 2 NUMA_BALANCING_MEMORY_TIERING
606 = =================================
608 Or NUMA_BALANCING_NORMAL to optimize page placement among different
609 NUMA nodes to reduce remote accessing. On NUMA machines, there is a
610 performance penalty if remote memory is accessed by a CPU. When this
611 feature is enabled the kernel samples what task thread is accessing
612 memory by periodically unmapping pages and later trapping a page
613 fault. At the time of the page fault, it is determined if the data
614 being accessed should be migrated to a local memory node.
616 The unmapping of pages and trapping faults incur additional overhead that
617 ideally is offset by improved memory locality but there is no universal
618 guarantee. If the target workload is already bound to NUMA nodes then this
619 feature should be disabled.
621 Or NUMA_BALANCING_MEMORY_TIERING to optimize page placement among
622 different types of memory (represented as different NUMA nodes) to
623 place the hot pages in the fast memory. This is implemented based on
624 unmapping and page fault too.
626 oops_all_cpu_backtrace
627 ======================
629 If this option is set, the kernel will send an NMI to all CPUs to dump
630 their backtraces when an oops event occurs. It should be used as a last
631 resort in case a panic cannot be triggered (to protect VMs running, for
632 example) or kdump can't be collected. This file shows up if CONFIG_SMP
635 0: Won't show all CPUs backtraces when an oops is detected.
636 This is the default behavior.
638 1: Will non-maskably interrupt all CPUs and dump their backtraces when
639 an oops event is detected.
642 osrelease, ostype & version
643 ===========================
652 #5 Wed Feb 25 21:49:24 MET 1998
654 The files ``osrelease`` and ``ostype`` should be clear enough.
656 needs a little more clarification however. The '#5' means that
657 this is the fifth kernel built from this source base and the
658 date behind it indicates the time the kernel was built.
659 The only way to tune these values is to rebuild the kernel :-)
662 overflowgid & overflowuid
663 =========================
665 if your architecture did not always support 32-bit UIDs (i.e. arm,
666 i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
667 applications that use the old 16-bit UID/GID system calls, if the
668 actual UID or GID would exceed 65535.
670 These sysctls allow you to change the value of the fixed UID and GID.
671 The default is 65534.
677 The value in this file determines the behaviour of the kernel on a
680 * if zero, the kernel will loop forever;
681 * if negative, the kernel will reboot immediately;
682 * if positive, the kernel will reboot after the corresponding number
685 When you use the software watchdog, the recommended setting is 60.
691 Controls the kernel's behavior when a CPU receives an NMI caused by
694 = ==================================================================
695 0 Try to continue operation (default).
696 1 Panic immediately. The IO error triggered an NMI. This indicates a
697 serious system condition which could result in IO data corruption.
698 Rather than continuing, panicking might be a better choice. Some
699 servers issue this sort of NMI when the dump button is pushed,
700 and you can use this option to take a crash dump.
701 = ==================================================================
707 Controls the kernel's behaviour when an oops or BUG is encountered.
709 = ===================================================================
710 0 Try to continue operation.
711 1 Panic immediately. If the `panic` sysctl is also non-zero then the
712 machine will be rebooted.
713 = ===================================================================
716 panic_on_stackoverflow
717 ======================
719 Controls the kernel's behavior when detecting the overflows of
720 kernel, IRQ and exception stacks except a user stack.
721 This file shows up if ``CONFIG_DEBUG_STACKOVERFLOW`` is enabled.
723 = ==========================
724 0 Try to continue operation.
726 = ==========================
729 panic_on_unrecovered_nmi
730 ========================
732 The default Linux behaviour on an NMI of either memory or unknown is
733 to continue operation. For many environments such as scientific
734 computing it is preferable that the box is taken out and the error
735 dealt with than an uncorrected parity/ECC error get propagated.
737 A small number of systems do generate NMIs for bizarre random reasons
738 such as power management so the default is off. That sysctl works like
739 the existing panic controls already in that directory.
745 Calls panic() in the WARN() path when set to 1. This is useful to avoid
746 a kernel rebuild when attempting to kdump at the location of a WARN().
748 = ================================================
749 0 Only WARN(), default behaviour.
750 1 Call panic() after printing out WARN() location.
751 = ================================================
757 Bitmask for printing system info when panic happens. User can chose
758 combination of the following bits:
760 ===== ============================================
761 bit 0 print all tasks info
762 bit 1 print system memory info
763 bit 2 print timer info
764 bit 3 print locks info if ``CONFIG_LOCKDEP`` is on
765 bit 4 print ftrace buffer
766 bit 5 print all printk messages in buffer
767 bit 6 print all CPUs backtrace (if available in the arch)
768 ===== ============================================
770 So for example to print tasks and memory info on panic, user can::
772 echo 3 > /proc/sys/kernel/panic_print
778 When set to 1, calls panic() after RCU stall detection messages. This
779 is useful to define the root cause of RCU stalls using a vmcore.
781 = ============================================================
782 0 Do not panic() when RCU stall takes place, default behavior.
783 1 panic() after printing RCU stall messages.
784 = ============================================================
786 max_rcu_stall_to_panic
787 ======================
789 When ``panic_on_rcu_stall`` is set to 1, this value determines the
790 number of times that RCU can stall before panic() is called.
792 When ``panic_on_rcu_stall`` is set to 0, this value is has no effect.
794 perf_cpu_time_max_percent
795 =========================
797 Hints to the kernel how much CPU time it should be allowed to
798 use to handle perf sampling events. If the perf subsystem
799 is informed that its samples are exceeding this limit, it
800 will drop its sampling frequency to attempt to reduce its CPU
803 Some perf sampling happens in NMIs. If these samples
804 unexpectedly take too long to execute, the NMIs can become
805 stacked up next to each other so much that nothing else is
808 ===== ========================================================
809 0 Disable the mechanism. Do not monitor or correct perf's
810 sampling rate no matter how CPU time it takes.
812 1-100 Attempt to throttle perf's sample rate to this
813 percentage of CPU. Note: the kernel calculates an
814 "expected" length of each sample event. 100 here means
815 100% of that expected length. Even if this is set to
816 100, you may still see sample throttling if this
817 length is exceeded. Set to 0 if you truly do not care
818 how much CPU is consumed.
819 ===== ========================================================
825 Controls use of the performance events system by unprivileged
826 users (without CAP_PERFMON). The default value is 2.
828 For backward compatibility reasons access to system performance
829 monitoring and observability remains open for CAP_SYS_ADMIN
830 privileged processes but CAP_SYS_ADMIN usage for secure system
831 performance monitoring and observability operations is discouraged
832 with respect to CAP_PERFMON use cases.
834 === ==================================================================
835 -1 Allow use of (almost) all events by all users.
837 Ignore mlock limit after perf_event_mlock_kb without
840 >=0 Disallow ftrace function tracepoint by users without
843 Disallow raw tracepoint access by users without ``CAP_PERFMON``.
845 >=1 Disallow CPU event access by users without ``CAP_PERFMON``.
847 >=2 Disallow kernel profiling by users without ``CAP_PERFMON``.
848 === ==================================================================
854 Controls maximum number of stack frames to copy for (``attr.sample_type &
855 PERF_SAMPLE_CALLCHAIN``) configured events, for instance, when using
856 '``perf record -g``' or '``perf trace --call-graph fp``'.
858 This can only be done when no events are in use that have callchains
859 enabled, otherwise writing to this file will return ``-EBUSY``.
861 The default value is 127.
867 Control size of per-cpu ring buffer not counted against mlock limit.
869 The default value is 512 + 1 page
872 perf_event_max_contexts_per_stack
873 =================================
875 Controls maximum number of stack frame context entries for
876 (``attr.sample_type & PERF_SAMPLE_CALLCHAIN``) configured events, for
877 instance, when using '``perf record -g``' or '``perf trace --call-graph fp``'.
879 This can only be done when no events are in use that have callchains
880 enabled, otherwise writing to this file will return ``-EBUSY``.
882 The default value is 8.
885 perf_user_access (arm64 only)
886 =================================
888 Controls user space access for reading perf event counters. When set to 1,
889 user space can read performance monitor counter registers directly.
891 The default value is 0 (access disabled).
893 See Documentation/arm64/perf.rst for more information.
899 PID allocation wrap value. When the kernel's next PID value
900 reaches this value, it wraps back to a minimum PID value.
901 PIDs of value ``pid_max`` or larger are not allocated.
907 The last pid allocated in the current (the one task using this sysctl
908 lives in) pid namespace. When selecting a pid for a next task on fork
909 kernel tries to allocate a number starting from this one.
912 powersave-nap (PPC only)
913 ========================
915 If set, Linux-PPC will use the 'nap' mode of powersaving,
916 otherwise the 'doze' mode will be used.
919 ==============================================================
924 The four values in printk denote: ``console_loglevel``,
925 ``default_message_loglevel``, ``minimum_console_loglevel`` and
926 ``default_console_loglevel`` respectively.
928 These values influence printk() behavior when printing or
929 logging error messages. See '``man 2 syslog``' for more info on
930 the different loglevels.
932 ======================== =====================================
933 console_loglevel messages with a higher priority than
934 this will be printed to the console
935 default_message_loglevel messages without an explicit priority
936 will be printed with this priority
937 minimum_console_loglevel minimum (highest) value to which
938 console_loglevel can be set
939 default_console_loglevel default value for console_loglevel
940 ======================== =====================================
946 Delay each printk message in ``printk_delay`` milliseconds
948 Value from 0 - 10000 is allowed.
954 Some warning messages are rate limited. ``printk_ratelimit`` specifies
955 the minimum length of time between these messages (in seconds).
956 The default value is 5 seconds.
958 A value of 0 will disable rate limiting.
961 printk_ratelimit_burst
962 ======================
964 While long term we enforce one message per `printk_ratelimit`_
965 seconds, we do allow a burst of messages to pass through.
966 ``printk_ratelimit_burst`` specifies the number of messages we can
967 send before ratelimiting kicks in.
969 The default value is 10 messages.
975 Control the logging to ``/dev/kmsg`` from userspace:
977 ========= =============================================
978 ratelimit default, ratelimited
979 on unlimited logging to /dev/kmsg from userspace
980 off logging to /dev/kmsg disabled
981 ========= =============================================
983 The kernel command line parameter ``printk.devkmsg=`` overrides this and is
984 a one-time setting until next reboot: once set, it cannot be changed by
985 this sysctl interface anymore.
987 ==============================================================
993 See Documentation/filesystems/devpts.rst.
999 This is a directory, with the following entries:
1001 * ``boot_id``: a UUID generated the first time this is retrieved, and
1002 unvarying after that;
1004 * ``uuid``: a UUID generated every time this is retrieved (this can
1005 thus be used to generate UUIDs at will);
1007 * ``entropy_avail``: the pool's entropy count, in bits;
1009 * ``poolsize``: the entropy pool size, in bits;
1011 * ``urandom_min_reseed_secs``: obsolete (used to determine the minimum
1012 number of seconds between urandom pool reseeding). This file is
1013 writable for compatibility purposes, but writing to it has no effect
1014 on any RNG behavior;
1016 * ``write_wakeup_threshold``: when the entropy count drops below this
1017 (as a number of bits), processes waiting to write to ``/dev/random``
1018 are woken up. This file is writable for compatibility purposes, but
1019 writing to it has no effect on any RNG behavior.
1025 This option can be used to select the type of process address
1026 space randomization that is used in the system, for architectures
1027 that support this feature.
1029 == ===========================================================================
1030 0 Turn the process address space randomization off. This is the
1031 default for architectures that do not support this feature anyways,
1032 and kernels that are booted with the "norandmaps" parameter.
1034 1 Make the addresses of mmap base, stack and VDSO page randomized.
1035 This, among other things, implies that shared libraries will be
1036 loaded to random addresses. Also for PIE-linked binaries, the
1037 location of code start is randomized. This is the default if the
1038 ``CONFIG_COMPAT_BRK`` option is enabled.
1040 2 Additionally enable heap randomization. This is the default if
1041 ``CONFIG_COMPAT_BRK`` is disabled.
1043 There are a few legacy applications out there (such as some ancient
1044 versions of libc.so.5 from 1996) that assume that brk area starts
1045 just after the end of the code+bss. These applications break when
1046 start of the brk area is randomized. There are however no known
1047 non-legacy applications that would be broken this way, so for most
1048 systems it is safe to choose full randomization.
1050 Systems with ancient and/or broken binaries should be configured
1051 with ``CONFIG_COMPAT_BRK`` enabled, which excludes the heap from process
1052 address space randomization.
1053 == ===========================================================================
1059 See Documentation/admin-guide/initrd.rst.
1062 reboot-cmd (SPARC only)
1063 =======================
1065 ??? This seems to be a way to give an argument to the Sparc
1066 ROM/Flash boot loader. Maybe to tell it what to do after
1073 Enables/disables Energy Aware Scheduling (EAS). EAS starts
1074 automatically on platforms where it can run (that is,
1075 platforms with asymmetric CPU topologies and having an Energy
1076 Model available). If your platform happens to meet the
1077 requirements for EAS but you do not want to use it, change
1083 Enables/disables task delay accounting (see
1084 Documentation/accounting/delay-accounting.rst. Enabling this feature incurs
1085 a small amount of overhead in the scheduler but is useful for debugging
1086 and performance tuning. It is required by some tools such as iotop.
1091 Enables/disables scheduler statistics. Enabling this feature
1092 incurs a small amount of overhead in the scheduler but is
1093 useful for debugging and performance tuning.
1095 sched_util_clamp_min
1096 ====================
1098 Max allowed *minimum* utilization.
1100 Default value is 1024, which is the maximum possible value.
1102 It means that any requested uclamp.min value cannot be greater than
1103 sched_util_clamp_min, i.e., it is restricted to the range
1104 [0:sched_util_clamp_min].
1106 sched_util_clamp_max
1107 ====================
1109 Max allowed *maximum* utilization.
1111 Default value is 1024, which is the maximum possible value.
1113 It means that any requested uclamp.max value cannot be greater than
1114 sched_util_clamp_max, i.e., it is restricted to the range
1115 [0:sched_util_clamp_max].
1117 sched_util_clamp_min_rt_default
1118 ===============================
1120 By default Linux is tuned for performance. Which means that RT tasks always run
1121 at the highest frequency and most capable (highest capacity) CPU (in
1122 heterogeneous systems).
1124 Uclamp achieves this by setting the requested uclamp.min of all RT tasks to
1125 1024 by default, which effectively boosts the tasks to run at the highest
1126 frequency and biases them to run on the biggest CPU.
1128 This knob allows admins to change the default behavior when uclamp is being
1129 used. In battery powered devices particularly, running at the maximum
1130 capacity and frequency will increase energy consumption and shorten the battery
1133 This knob is only effective for RT tasks which the user hasn't modified their
1134 requested uclamp.min value via sched_setattr() syscall.
1136 This knob will not escape the range constraint imposed by sched_util_clamp_min
1141 sched_util_clamp_min_rt_default = 800
1142 sched_util_clamp_min = 600
1144 Then the boost will be clamped to 600 because 800 is outside of the permissible
1145 range of [0:600]. This could happen for instance if a powersave mode will
1146 restrict all boosts temporarily by modifying sched_util_clamp_min. As soon as
1147 this restriction is lifted, the requested sched_util_clamp_min_rt_default
1153 See Documentation/userspace-api/seccomp_filter.rst.
1159 This file shows the size of the generic SCSI (sg) buffer.
1160 You can't tune it just yet, but you could change it on
1161 compile time by editing ``include/scsi/sg.h`` and changing
1162 the value of ``SG_BIG_BUFF``.
1164 There shouldn't be any reason to change this value. If
1165 you can come up with one, you probably know what you
1172 This parameter sets the total amount of shared memory pages that
1173 can be used system wide. Hence, ``shmall`` should always be at least
1174 ``ceil(shmmax/PAGE_SIZE)``.
1176 If you are not sure what the default ``PAGE_SIZE`` is on your Linux
1177 system, you can run the following command::
1185 This value can be used to query and set the run time limit
1186 on the maximum shared memory segment size that can be created.
1187 Shared memory segments up to 1Gb are now supported in the
1188 kernel. This value defaults to ``SHMMAX``.
1194 This value determines the maximum number of shared memory segments.
1195 4096 by default (``SHMMNI``).
1201 Linux lets you set resource limits, including how much memory one
1202 process can consume, via ``setrlimit(2)``. Unfortunately, shared memory
1203 segments are allowed to exist without association with any process, and
1204 thus might not be counted against any resource limits. If enabled,
1205 shared memory segments are automatically destroyed when their attach
1206 count becomes zero after a detach or a process termination. It will
1207 also destroy segments that were created, but never attached to, on exit
1208 from the process. The only use left for ``IPC_RMID`` is to immediately
1209 destroy an unattached segment. Of course, this breaks the way things are
1210 defined, so some applications might stop working. Note that this
1211 feature will do you no good unless you also configure your resource
1212 limits (in particular, ``RLIMIT_AS`` and ``RLIMIT_NPROC``). Most systems don't
1215 Note that if you change this from 0 to 1, already created segments
1216 without users and with a dead originative process will be destroyed.
1219 sysctl_writes_strict
1220 ====================
1222 Control how file position affects the behavior of updating sysctl values
1223 via the ``/proc/sys`` interface:
1225 == ======================================================================
1226 -1 Legacy per-write sysctl value handling, with no printk warnings.
1227 Each write syscall must fully contain the sysctl value to be
1228 written, and multiple writes on the same sysctl file descriptor
1229 will rewrite the sysctl value, regardless of file position.
1230 0 Same behavior as above, but warn about processes that perform writes
1231 to a sysctl file descriptor when the file position is not 0.
1232 1 (default) Respect file position when writing sysctl strings. Multiple
1233 writes will append to the sysctl value buffer. Anything past the max
1234 length of the sysctl value buffer will be ignored. Writes to numeric
1235 sysctl entries must always be at file position 0 and the value must
1236 be fully contained in the buffer sent in the write syscall.
1237 == ======================================================================
1240 softlockup_all_cpu_backtrace
1241 ============================
1243 This value controls the soft lockup detector thread's behavior
1244 when a soft lockup condition is detected as to whether or not
1245 to gather further debug information. If enabled, each cpu will
1246 be issued an NMI and instructed to capture stack trace.
1248 This feature is only applicable for architectures which support
1251 = ============================================
1252 0 Do nothing. This is the default behavior.
1253 1 On detection capture more debug information.
1254 = ============================================
1260 This parameter can be used to control whether the kernel panics
1261 when a soft lockup is detected.
1263 = ============================================
1264 0 Don't panic on soft lockup.
1265 1 Panic on soft lockup.
1266 = ============================================
1268 This can also be set using the softlockup_panic kernel parameter.
1274 This parameter can be used to control the soft lockup detector.
1276 = =================================
1277 0 Disable the soft lockup detector.
1278 1 Enable the soft lockup detector.
1279 = =================================
1281 The soft lockup detector monitors CPUs for threads that are hogging the CPUs
1282 without rescheduling voluntarily, and thus prevent the 'migration/N' threads
1283 from running, causing the watchdog work fail to execute. The mechanism depends
1284 on the CPUs ability to respond to timer interrupts which are needed for the
1285 watchdog work to be queued by the watchdog timer function, otherwise the NMI
1286 watchdog — if enabled — can detect a hard lockup condition.
1292 This parameter can be used to control kernel stack erasing at the end
1293 of syscalls for kernels built with ``CONFIG_GCC_PLUGIN_STACKLEAK``.
1295 That erasing reduces the information which kernel stack leak bugs
1296 can reveal and blocks some uninitialized stack variable attacks.
1297 The tradeoff is the performance impact: on a single CPU system kernel
1298 compilation sees a 1% slowdown, other systems and workloads may vary.
1300 = ====================================================================
1301 0 Kernel stack erasing is disabled, STACKLEAK_METRICS are not updated.
1302 1 Kernel stack erasing is enabled (default), it is performed before
1303 returning to the userspace at the end of syscalls.
1304 = ====================================================================
1312 = ====================================
1313 0 Stop-A has no effect.
1314 1 Stop-A breaks to the PROM (default).
1315 = ====================================
1317 Stop-A is always enabled on a panic, so that the user can return to
1324 See Documentation/admin-guide/sysrq.rst.
1330 Non-zero if the kernel has been tainted. Numeric values, which can be
1331 ORed together. The letters are seen in "Tainted" line of Oops reports.
1333 ====== ===== ==============================================================
1334 1 `(P)` proprietary module was loaded
1335 2 `(F)` module was force loaded
1336 4 `(S)` kernel running on an out of specification system
1337 8 `(R)` module was force unloaded
1338 16 `(M)` processor reported a Machine Check Exception (MCE)
1339 32 `(B)` bad page referenced or some unexpected page flags
1340 64 `(U)` taint requested by userspace application
1341 128 `(D)` kernel died recently, i.e. there was an OOPS or BUG
1342 256 `(A)` an ACPI table was overridden by user
1343 512 `(W)` kernel issued warning
1344 1024 `(C)` staging driver was loaded
1345 2048 `(I)` workaround for bug in platform firmware applied
1346 4096 `(O)` externally-built ("out-of-tree") module was loaded
1347 8192 `(E)` unsigned module was loaded
1348 16384 `(L)` soft lockup occurred
1349 32768 `(K)` kernel has been live patched
1350 65536 `(X)` Auxiliary taint, defined and used by for distros
1351 131072 `(T)` The kernel was built with the struct randomization plugin
1352 ====== ===== ==============================================================
1354 See Documentation/admin-guide/tainted-kernels.rst for more information.
1357 writes to this sysctl interface will fail with ``EINVAL`` if the kernel is
1358 booted with the command line option ``panic_on_taint=<bitmask>,nousertaint``
1359 and any of the ORed together values being written to ``tainted`` match with
1360 the bitmask declared on panic_on_taint.
1361 See Documentation/admin-guide/kernel-parameters.rst for more details on
1362 that particular kernel command line option and its optional
1363 ``nousertaint`` switch.
1368 This value controls the maximum number of threads that can be created
1371 During initialization the kernel sets this value such that even if the
1372 maximum number of threads is created, the thread structures occupy only
1373 a part (1/8th) of the available RAM pages.
1375 The minimum value that can be written to ``threads-max`` is 1.
1377 The maximum value that can be written to ``threads-max`` is given by the
1378 constant ``FUTEX_TID_MASK`` (0x3fffffff).
1380 If a value outside of this range is written to ``threads-max`` an
1381 ``EINVAL`` error occurs.
1387 When set, disables tracing (see Documentation/trace/ftrace.rst) when a
1394 When tracepoints are sent to printk() (enabled by the ``tp_printk``
1395 boot parameter), this entry provides runtime control::
1397 echo 0 > /proc/sys/kernel/tracepoint_printk
1399 will stop tracepoints from being sent to printk(), and::
1401 echo 1 > /proc/sys/kernel/tracepoint_printk
1403 will send them to printk() again.
1405 This only works if the kernel was booted with ``tp_printk`` enabled.
1407 See Documentation/admin-guide/kernel-parameters.rst and
1408 Documentation/trace/boottime-trace.rst.
1411 .. _unaligned-dump-stack:
1413 unaligned-dump-stack (ia64)
1414 ===========================
1416 When logging unaligned accesses, controls whether the stack is
1419 = ===================================================
1420 0 Do not dump the stack. This is the default setting.
1422 = ===================================================
1424 See also `ignore-unaligned-usertrap`_.
1430 On architectures where unaligned accesses cause traps, and where this
1431 feature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_ALLOW``; currently,
1432 ``arc`` and ``parisc``), controls whether unaligned traps are caught
1433 and emulated (instead of failing).
1435 = ========================================================
1436 0 Do not emulate unaligned accesses.
1437 1 Emulate unaligned accesses. This is the default setting.
1438 = ========================================================
1440 See also `ignore-unaligned-usertrap`_.
1446 The value in this file affects behavior of handling NMI. When the
1447 value is non-zero, unknown NMI is trapped and then panic occurs. At
1448 that time, kernel debugging information is displayed on console.
1450 NMI switch that most IA32 servers have fires unknown NMI up, for
1451 example. If a system hangs up, try pressing the NMI switch.
1454 unprivileged_bpf_disabled
1455 =========================
1457 Writing 1 to this entry will disable unprivileged calls to ``bpf()``;
1458 once disabled, calling ``bpf()`` without ``CAP_SYS_ADMIN`` or ``CAP_BPF``
1459 will return ``-EPERM``. Once set to 1, this can't be cleared from the
1460 running kernel anymore.
1462 Writing 2 to this entry will also disable unprivileged calls to ``bpf()``,
1463 however, an admin can still change this setting later on, if needed, by
1464 writing 0 or 1 to this entry.
1466 If ``BPF_UNPRIV_DEFAULT_OFF`` is enabled in the kernel config, then this
1467 entry will default to 2 instead of 0.
1469 = =============================================================
1470 0 Unprivileged calls to ``bpf()`` are enabled
1471 1 Unprivileged calls to ``bpf()`` are disabled without recovery
1472 2 Unprivileged calls to ``bpf()`` are disabled
1473 = =============================================================
1478 This parameter can be used to disable or enable the soft lockup detector
1479 *and* the NMI watchdog (i.e. the hard lockup detector) at the same time.
1481 = ==============================
1482 0 Disable both lockup detectors.
1483 1 Enable both lockup detectors.
1484 = ==============================
1486 The soft lockup detector and the NMI watchdog can also be disabled or
1487 enabled individually, using the ``soft_watchdog`` and ``nmi_watchdog``
1489 If the ``watchdog`` parameter is read, for example by executing::
1491 cat /proc/sys/kernel/watchdog
1493 the output of this command (0 or 1) shows the logical OR of
1494 ``soft_watchdog`` and ``nmi_watchdog``.
1500 This value can be used to control on which cpus the watchdog may run.
1501 The default cpumask is all possible cores, but if ``NO_HZ_FULL`` is
1502 enabled in the kernel config, and cores are specified with the
1503 ``nohz_full=`` boot argument, those cores are excluded by default.
1504 Offline cores can be included in this mask, and if the core is later
1505 brought online, the watchdog will be started based on the mask value.
1507 Typically this value would only be touched in the ``nohz_full`` case
1508 to re-enable cores that by default were not running the watchdog,
1509 if a kernel lockup was suspected on those cores.
1511 The argument value is the standard cpulist format for cpumasks,
1512 so for example to enable the watchdog on cores 0, 2, 3, and 4 you
1515 echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask
1521 This value can be used to control the frequency of hrtimer and NMI
1522 events and the soft and hard lockup thresholds. The default threshold
1525 The softlockup threshold is (``2 * watchdog_thresh``). Setting this
1526 tunable to zero will disable lockup detection altogether.