1 ===================================
2 Documentation for /proc/sys/kernel/
3 ===================================
7 Copyright (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
9 Copyright (c) 2009, Shen Feng<shen@cn.fujitsu.com>
11 For general info and legal blurb, please look in index.rst.
13 ------------------------------------------------------------------------------
15 This file contains documentation for the sysctl files in
16 /proc/sys/kernel/ and is valid for Linux kernel version 2.2.
18 The files in this directory can be used to tune and monitor
19 miscellaneous and general things in the operation of the Linux
20 kernel. Since some of the files _can_ be used to screw up your
21 system, it is advisable to read both documentation and source
22 before actually making adjustments.
24 Currently, these files might (depending on your configuration)
25 show up in /proc/sys/kernel:
30 - bootloader_type [ X86 only ]
31 - bootloader_version [ X86 only ]
41 - hardlockup_all_cpu_backtrace
44 - hung_task_check_count
45 - hung_task_timeout_secs
46 - hung_task_check_interval_secs
48 - hyperv_record_panic_msg
52 - modprobe ==> Documentation/debugging-modules.txt
54 - msg_next_id [ sysv ipc ]
65 - panic_on_stackoverflow
66 - panic_on_unrecovered_nmi
70 - perf_cpu_time_max_percent
72 - perf_event_max_stack
74 - perf_event_max_contexts_per_stack
76 - powersave-nap [ PPC only ]
80 - printk_ratelimit_burst
81 - pty ==> Documentation/filesystems/devpts.txt
83 - real-root-dev ==> Documentation/admin-guide/initrd.rst
84 - reboot-cmd [ SPARC only ]
88 - seccomp/ ==> Documentation/userspace-api/seccomp_filter.rst
90 - sem_next_id [ sysv ipc ]
91 - sg-big-buff [ generic SCSI device (sg) ]
92 - shm_next_id [ sysv ipc ]
97 - softlockup_all_cpu_backtrace
100 - stop-a [ SPARC only ]
101 - sysrq ==> Documentation/admin-guide/sysrq.rst
102 - sysctl_writes_strict
103 - tainted ==> Documentation/admin-guide/tainted-kernels.rst
114 highwater lowwater frequency
116 If BSD-style process accounting is enabled these values control
117 its behaviour. If free space on filesystem where the log lives
118 goes below <lowwater>% accounting suspends. If free space gets
119 above <highwater>% accounting resumes. <Frequency> determines
120 how often do we check the amount of free space (value is in
123 That is, suspend accounting if there left <= 2% free; resume it
124 if we got >=4%; consider information about amount of free space
125 valid for 30 seconds.
133 See Doc*/kernel/power/video.txt, it allows mode of video boot to be
140 This variable has no effect and may be removed in future kernel
141 releases. Reading it always returns 0.
142 Up to Linux 3.17, it enabled/disabled automatic recomputing of msgmni
143 upon memory add/remove or upon ipc namespace creation/removal.
144 Echoing "1" into this file enabled msgmni automatic recomputing.
145 Echoing "0" turned it off. auto_msgmni default value was 1.
151 x86 bootloader identification
153 This gives the bootloader type number as indicated by the bootloader,
154 shifted left by 4, and OR'd with the low four bits of the bootloader
155 version. The reason for this encoding is that this used to match the
156 type_of_loader field in the kernel header; the encoding is kept for
157 backwards compatibility. That is, if the full bootloader type number
158 is 0x15 and the full version number is 0x234, this file will contain
159 the value 340 = 0x154.
161 See the type_of_loader and ext_loader_type fields in
162 Documentation/x86/boot.rst for additional information.
168 x86 bootloader version
170 The complete bootloader version number. In the example above, this
171 file will contain the value 564 = 0x234.
173 See the type_of_loader and ext_loader_ver fields in
174 Documentation/x86/boot.rst for additional information.
180 Highest valid capability of the running kernel. Exports
181 CAP_LAST_CAP from the kernel.
187 core_pattern is used to specify a core dumpfile pattern name.
189 * max length 127 characters; default value is "core"
190 * core_pattern is used as a pattern template for the output filename;
191 certain string patterns (beginning with '%') are substituted with
193 * backward compatibility with core_uses_pid:
195 If core_pattern does not include "%p" (default does not)
196 and core_uses_pid is set, then .PID will be appended to
199 * corename format specifiers::
201 %<NUL> '%' is dropped
204 %P global pid (init PID namespace)
206 %I global tid (init PID namespace)
207 %u uid (in initial user namespace)
208 %g gid (in initial user namespace)
209 %d dump mode, matches PR_SET_DUMPABLE and
210 /proc/sys/fs/suid_dumpable
214 %e executable filename (may be shortened)
216 %<OTHER> both are dropped
218 * If the first character of the pattern is a '|', the kernel will treat
219 the rest of the pattern as a command to run. The core dump will be
220 written to the standard input of that program instead of to a file.
226 This sysctl is only applicable when core_pattern is configured to pipe
227 core files to a user space helper (when the first character of
228 core_pattern is a '|', see above). When collecting cores via a pipe
229 to an application, it is occasionally useful for the collecting
230 application to gather data about the crashing process from its
231 /proc/pid directory. In order to do this safely, the kernel must wait
232 for the collecting process to exit, so as not to remove the crashing
233 processes proc files prematurely. This in turn creates the
234 possibility that a misbehaving userspace collecting process can block
235 the reaping of a crashed process simply by never exiting. This sysctl
236 defends against that. It defines how many concurrent crashing
237 processes may be piped to user space applications in parallel. If
238 this value is exceeded, then those crashing processes above that value
239 are noted via the kernel log and their cores are skipped. 0 is a
240 special value, indicating that unlimited processes may be captured in
241 parallel, but that no waiting will take place (i.e. the collecting
242 process is not guaranteed access to /proc/<crashing pid>/). This
249 The default coredump filename is "core". By setting
250 core_uses_pid to 1, the coredump filename becomes core.PID.
251 If core_pattern does not include "%p" (default does not)
252 and core_uses_pid is set, then .PID will be appended to
259 When the value in this file is 0, ctrl-alt-del is trapped and
260 sent to the init(1) program to handle a graceful restart.
261 When, however, the value is > 0, Linux's reaction to a Vulcan
262 Nerve Pinch (tm) will be an immediate reboot, without even
263 syncing its dirty buffers.
266 when a program (like dosemu) has the keyboard in 'raw'
267 mode, the ctrl-alt-del is intercepted by the program before it
268 ever reaches the kernel tty layer, and it's up to the program
269 to decide what to do with it.
275 This toggle indicates whether unprivileged users are prevented
276 from using dmesg(8) to view messages from the kernel's log buffer.
277 When dmesg_restrict is set to (0) there are no restrictions. When
278 dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
281 The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
282 default value of dmesg_restrict.
285 domainname & hostname:
286 ======================
288 These files can be used to set the NIS/YP domainname and the
289 hostname of your box in exactly the same way as the commands
290 domainname and hostname, i.e.::
292 # echo "darkstar" > /proc/sys/kernel/hostname
293 # echo "mydomain" > /proc/sys/kernel/domainname
295 has the same effect as::
297 # hostname "darkstar"
298 # domainname "mydomain"
300 Note, however, that the classic darkstar.frop.org has the
301 hostname "darkstar" and DNS (Internet Domain Name Server)
302 domainname "frop.org", not to be confused with the NIS (Network
303 Information Service) or YP (Yellow Pages) domainname. These two
304 domain names are in general different. For a detailed discussion
305 see the hostname(1) man page.
308 hardlockup_all_cpu_backtrace:
309 =============================
311 This value controls the hard lockup detector behavior when a hard
312 lockup condition is detected as to whether or not to gather further
313 debug information. If enabled, arch-specific all-CPU stack dumping
316 0: do nothing. This is the default behavior.
318 1: on detection capture more debug information.
324 This parameter can be used to control whether the kernel panics
325 when a hard lockup is detected.
327 0 - don't panic on hard lockup
328 1 - panic on hard lockup
330 See Documentation/admin-guide/lockup-watchdogs.rst for more information. This can
331 also be set using the nmi_watchdog kernel parameter.
337 Path for the hotplug policy agent.
338 Default value is "/sbin/hotplug".
344 Controls the kernel's behavior when a hung task is detected.
345 This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
347 0: continue operation. This is the default behavior.
349 1: panic immediately.
352 hung_task_check_count:
353 ======================
355 The upper bound on the number of tasks that are checked.
356 This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
359 hung_task_timeout_secs:
360 =======================
362 When a task in D state did not get scheduled
363 for more than this value report a warning.
364 This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
366 0: means infinite timeout - no checking done.
368 Possible values to set are in range {0..LONG_MAX/HZ}.
371 hung_task_check_interval_secs:
372 ==============================
374 Hung task check interval. If hung task checking is enabled
375 (see hung_task_timeout_secs), the check is done every
376 hung_task_check_interval_secs seconds.
377 This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
379 0 (default): means use hung_task_timeout_secs as checking interval.
380 Possible values to set are in range {0..LONG_MAX/HZ}.
386 The maximum number of warnings to report. During a check interval
387 if a hung task is detected, this value is decreased by 1.
388 When this value reaches 0, no more warnings will be reported.
389 This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
391 -1: report an infinite number of warnings.
394 hyperv_record_panic_msg:
395 ========================
397 Controls whether the panic kmsg data should be reported to Hyper-V.
399 0: do not report panic kmsg data.
401 1: report the panic kmsg data. This is the default behavior.
407 A toggle indicating if the kexec_load syscall has been disabled. This
408 value defaults to 0 (false: kexec_load enabled), but can be set to 1
409 (true: kexec_load disabled). Once true, kexec can no longer be used, and
410 the toggle cannot be set back to false. This allows a kexec image to be
411 loaded before disabling the syscall, allowing a system to set up (and
412 later use) an image without it being altered. Generally used together
413 with the "modules_disabled" sysctl.
419 This toggle indicates whether restrictions are placed on
420 exposing kernel addresses via /proc and other interfaces.
422 When kptr_restrict is set to 0 (the default) the address is hashed before
423 printing. (This is the equivalent to %p.)
425 When kptr_restrict is set to (1), kernel pointers printed using the %pK
426 format specifier will be replaced with 0's unless the user has CAP_SYSLOG
427 and effective user and group ids are equal to the real ids. This is
428 because %pK checks are done at read() time rather than open() time, so
429 if permissions are elevated between the open() and the read() (e.g via
430 a setuid binary) then %pK will not leak kernel pointers to unprivileged
431 users. Note, this is a temporary solution only. The correct long-term
432 solution is to do the permission checks at open() time. Consider removing
433 world read permissions from files that use %pK, and using dmesg_restrict
434 to protect against uses of %pK in dmesg(8) if leaking kernel pointer
435 values to unprivileged users is a concern.
437 When kptr_restrict is set to (2), kernel pointers printed using
438 %pK will be replaced with 0's regardless of privileges.
444 This flag controls the L2 cache of G3 processor boards. If
445 0, the cache is disabled. Enabled if nonzero.
451 A toggle value indicating if modules are allowed to be loaded
452 in an otherwise modular kernel. This toggle defaults to off
453 (0), but can be set true (1). Once true, modules can be
454 neither loaded nor unloaded, and the toggle cannot be set back
455 to false. Generally used with the "kexec_load_disabled" toggle.
458 msg_next_id, sem_next_id, and shm_next_id:
459 ==========================================
461 These three toggles allows to specify desired id for next allocated IPC
462 object: message, semaphore or shared memory respectively.
464 By default they are equal to -1, which means generic allocation logic.
465 Possible values to set are in range {0..INT_MAX}.
468 1) kernel doesn't guarantee, that new object will have desired id. So,
469 it's up to userspace, how to handle an object with "wrong" id.
470 2) Toggle with non-default value will be set back to -1 by kernel after
471 successful IPC object allocation. If an IPC object allocation syscall
472 fails, it is undefined if the value remains unmodified or is reset to -1.
478 This parameter can be used to control the NMI watchdog
479 (i.e. the hard lockup detector) on x86 systems.
481 0 - disable the hard lockup detector
483 1 - enable the hard lockup detector
485 The hard lockup detector monitors each CPU for its ability to respond to
486 timer interrupts. The mechanism utilizes CPU performance counter registers
487 that are programmed to generate Non-Maskable Interrupts (NMIs) periodically
488 while a CPU is busy. Hence, the alternative name 'NMI watchdog'.
490 The NMI watchdog is disabled by default if the kernel is running as a guest
491 in a KVM virtual machine. This default can be overridden by adding::
495 to the guest kernel command line (see Documentation/admin-guide/kernel-parameters.rst).
501 Enables/disables automatic page fault based NUMA memory
502 balancing. Memory is moved automatically to nodes
503 that access it often.
505 Enables/disables automatic NUMA memory balancing. On NUMA machines, there
506 is a performance penalty if remote memory is accessed by a CPU. When this
507 feature is enabled the kernel samples what task thread is accessing memory
508 by periodically unmapping pages and later trapping a page fault. At the
509 time of the page fault, it is determined if the data being accessed should
510 be migrated to a local memory node.
512 The unmapping of pages and trapping faults incur additional overhead that
513 ideally is offset by improved memory locality but there is no universal
514 guarantee. If the target workload is already bound to NUMA nodes then this
515 feature should be disabled. Otherwise, if the system overhead from the
516 feature is too high then the rate the kernel samples for NUMA hinting
517 faults may be controlled by the numa_balancing_scan_period_min_ms,
518 numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms,
519 numa_balancing_scan_size_mb, and numa_balancing_settle_count sysctls.
521 numa_balancing_scan_period_min_ms, numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms, numa_balancing_scan_size_mb
522 ===============================================================================================================================
525 Automatic NUMA balancing scans tasks address space and unmaps pages to
526 detect if pages are properly placed or if the data should be migrated to a
527 memory node local to where the task is running. Every "scan delay" the task
528 scans the next "scan size" number of pages in its address space. When the
529 end of the address space is reached the scanner restarts from the beginning.
531 In combination, the "scan delay" and "scan size" determine the scan rate.
532 When "scan delay" decreases, the scan rate increases. The scan delay and
533 hence the scan rate of every task is adaptive and depends on historical
534 behaviour. If pages are properly placed then the scan delay increases,
535 otherwise the scan delay decreases. The "scan size" is not adaptive but
536 the higher the "scan size", the higher the scan rate.
538 Higher scan rates incur higher system overhead as page faults must be
539 trapped and potentially data must be migrated. However, the higher the scan
540 rate, the more quickly a tasks memory is migrated to a local node if the
541 workload pattern changes and minimises performance impact due to remote
542 memory accesses. These sysctls control the thresholds for scan delays and
543 the number of pages scanned.
545 numa_balancing_scan_period_min_ms is the minimum time in milliseconds to
546 scan a tasks virtual memory. It effectively controls the maximum scanning
549 numa_balancing_scan_delay_ms is the starting "scan delay" used for a task
550 when it initially forks.
552 numa_balancing_scan_period_max_ms is the maximum time in milliseconds to
553 scan a tasks virtual memory. It effectively controls the minimum scanning
556 numa_balancing_scan_size_mb is how many megabytes worth of pages are
557 scanned for a given scan.
563 Number of kernel oopses after which the kernel should panic when
564 ``panic_on_oops`` is not set. Setting this to 0 disables checking
565 the count. Setting this to 1 has the same effect as setting
566 ``panic_on_oops=1``. The default value is 10000.
569 osrelease, ostype & version:
570 ============================
579 #5 Wed Feb 25 21:49:24 MET 1998
581 The files osrelease and ostype should be clear enough. Version
582 needs a little more clarification however. The '#5' means that
583 this is the fifth kernel built from this source base and the
584 date behind it indicates the time the kernel was built.
585 The only way to tune these values is to rebuild the kernel :-)
588 overflowgid & overflowuid:
589 ==========================
591 if your architecture did not always support 32-bit UIDs (i.e. arm,
592 i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
593 applications that use the old 16-bit UID/GID system calls, if the
594 actual UID or GID would exceed 65535.
596 These sysctls allow you to change the value of the fixed UID and GID.
597 The default is 65534.
603 The value in this file represents the number of seconds the kernel
604 waits before rebooting on a panic. When you use the software watchdog,
605 the recommended setting is 60.
611 Controls the kernel's behavior when a CPU receives an NMI caused by
614 0: try to continue operation (default)
616 1: panic immediately. The IO error triggered an NMI. This indicates a
617 serious system condition which could result in IO data corruption.
618 Rather than continuing, panicking might be a better choice. Some
619 servers issue this sort of NMI when the dump button is pushed,
620 and you can use this option to take a crash dump.
626 Controls the kernel's behaviour when an oops or BUG is encountered.
628 0: try to continue operation
630 1: panic immediately. If the `panic` sysctl is also non-zero then the
631 machine will be rebooted.
634 panic_on_stackoverflow:
635 =======================
637 Controls the kernel's behavior when detecting the overflows of
638 kernel, IRQ and exception stacks except a user stack.
639 This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.
641 0: try to continue operation.
643 1: panic immediately.
646 panic_on_unrecovered_nmi:
647 =========================
649 The default Linux behaviour on an NMI of either memory or unknown is
650 to continue operation. For many environments such as scientific
651 computing it is preferable that the box is taken out and the error
652 dealt with than an uncorrected parity/ECC error get propagated.
654 A small number of systems do generate NMI's for bizarre random reasons
655 such as power management so the default is off. That sysctl works like
656 the existing panic controls already in that directory.
662 Calls panic() in the WARN() path when set to 1. This is useful to avoid
663 a kernel rebuild when attempting to kdump at the location of a WARN().
665 0: only WARN(), default behaviour.
667 1: call panic() after printing out WARN() location.
673 Bitmask for printing system info when panic happens. User can chose
674 combination of the following bits:
676 ===== ========================================
677 bit 0 print all tasks info
678 bit 1 print system memory info
679 bit 2 print timer info
680 bit 3 print locks info if CONFIG_LOCKDEP is on
681 bit 4 print ftrace buffer
682 ===== ========================================
684 So for example to print tasks and memory info on panic, user can::
686 echo 3 > /proc/sys/kernel/panic_print
692 When set to 1, calls panic() after RCU stall detection messages. This
693 is useful to define the root cause of RCU stalls using a vmcore.
695 0: do not panic() when RCU stall takes place, default behavior.
697 1: panic() after printing RCU stall messages.
700 perf_cpu_time_max_percent:
701 ==========================
703 Hints to the kernel how much CPU time it should be allowed to
704 use to handle perf sampling events. If the perf subsystem
705 is informed that its samples are exceeding this limit, it
706 will drop its sampling frequency to attempt to reduce its CPU
709 Some perf sampling happens in NMIs. If these samples
710 unexpectedly take too long to execute, the NMIs can become
711 stacked up next to each other so much that nothing else is
715 disable the mechanism. Do not monitor or correct perf's
716 sampling rate no matter how CPU time it takes.
719 attempt to throttle perf's sample rate to this
720 percentage of CPU. Note: the kernel calculates an
721 "expected" length of each sample event. 100 here means
722 100% of that expected length. Even if this is set to
723 100, you may still see sample throttling if this
724 length is exceeded. Set to 0 if you truly do not care
725 how much CPU is consumed.
731 Controls use of the performance events system by unprivileged
732 users (without CAP_SYS_ADMIN). The default value is 2.
734 === ==================================================================
735 -1 Allow use of (almost) all events by all users
737 Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK
739 >=0 Disallow ftrace function tracepoint by users without CAP_SYS_ADMIN
741 Disallow raw tracepoint access by users without CAP_SYS_ADMIN
743 >=1 Disallow CPU event access by users without CAP_SYS_ADMIN
745 >=2 Disallow kernel profiling by users without CAP_SYS_ADMIN
746 === ==================================================================
749 perf_event_max_stack:
750 =====================
752 Controls maximum number of stack frames to copy for (attr.sample_type &
753 PERF_SAMPLE_CALLCHAIN) configured events, for instance, when using
754 'perf record -g' or 'perf trace --call-graph fp'.
756 This can only be done when no events are in use that have callchains
757 enabled, otherwise writing to this file will return -EBUSY.
759 The default value is 127.
765 Control size of per-cpu ring buffer not counted agains mlock limit.
767 The default value is 512 + 1 page
770 perf_event_max_contexts_per_stack:
771 ==================================
773 Controls maximum number of stack frame context entries for
774 (attr.sample_type & PERF_SAMPLE_CALLCHAIN) configured events, for
775 instance, when using 'perf record -g' or 'perf trace --call-graph fp'.
777 This can only be done when no events are in use that have callchains
778 enabled, otherwise writing to this file will return -EBUSY.
780 The default value is 8.
786 PID allocation wrap value. When the kernel's next PID value
787 reaches this value, it wraps back to a minimum PID value.
788 PIDs of value pid_max or larger are not allocated.
794 The last pid allocated in the current (the one task using this sysctl
795 lives in) pid namespace. When selecting a pid for a next task on fork
796 kernel tries to allocate a number starting from this one.
799 powersave-nap: (PPC only)
800 =========================
802 If set, Linux-PPC will use the 'nap' mode of powersaving,
803 otherwise the 'doze' mode will be used.
805 ==============================================================
810 The four values in printk denote: console_loglevel,
811 default_message_loglevel, minimum_console_loglevel and
812 default_console_loglevel respectively.
814 These values influence printk() behavior when printing or
815 logging error messages. See 'man 2 syslog' for more info on
816 the different loglevels.
819 messages with a higher priority than
820 this will be printed to the console
821 - default_message_loglevel:
822 messages without an explicit priority
823 will be printed with this priority
824 - minimum_console_loglevel:
825 minimum (highest) value to which
826 console_loglevel can be set
827 - default_console_loglevel:
828 default value for console_loglevel
834 Delay each printk message in printk_delay milliseconds
836 Value from 0 - 10000 is allowed.
842 Some warning messages are rate limited. printk_ratelimit specifies
843 the minimum length of time between these messages (in jiffies), by
844 default we allow one every 5 seconds.
846 A value of 0 will disable rate limiting.
849 printk_ratelimit_burst:
850 =======================
852 While long term we enforce one message per printk_ratelimit
853 seconds, we do allow a burst of messages to pass through.
854 printk_ratelimit_burst specifies the number of messages we can
855 send before ratelimiting kicks in.
861 Control the logging to /dev/kmsg from userspace:
866 on: unlimited logging to /dev/kmsg from userspace
868 off: logging to /dev/kmsg disabled
870 The kernel command line parameter printk.devkmsg= overrides this and is
871 a one-time setting until next reboot: once set, it cannot be changed by
872 this sysctl interface anymore.
877 See Documentation/filesystems/devpts.rst.
883 This is a directory, with the following entries:
885 * ``boot_id``: a UUID generated the first time this is retrieved, and
886 unvarying after that;
888 * ``uuid``: a UUID generated every time this is retrieved (this can
889 thus be used to generate UUIDs at will);
891 * ``entropy_avail``: the pool's entropy count, in bits;
893 * ``poolsize``: the entropy pool size, in bits;
895 * ``urandom_min_reseed_secs``: obsolete (used to determine the minimum
896 number of seconds between urandom pool reseeding). This file is
897 writable for compatibility purposes, but writing to it has no effect
900 * ``write_wakeup_threshold``: when the entropy count drops below this
901 (as a number of bits), processes waiting to write to ``/dev/random``
902 are woken up. This file is writable for compatibility purposes, but
903 writing to it has no effect on any RNG behavior.
909 This option can be used to select the type of process address
910 space randomization that is used in the system, for architectures
911 that support this feature.
913 == ===========================================================================
914 0 Turn the process address space randomization off. This is the
915 default for architectures that do not support this feature anyways,
916 and kernels that are booted with the "norandmaps" parameter.
918 1 Make the addresses of mmap base, stack and VDSO page randomized.
919 This, among other things, implies that shared libraries will be
920 loaded to random addresses. Also for PIE-linked binaries, the
921 location of code start is randomized. This is the default if the
922 CONFIG_COMPAT_BRK option is enabled.
924 2 Additionally enable heap randomization. This is the default if
925 CONFIG_COMPAT_BRK is disabled.
927 There are a few legacy applications out there (such as some ancient
928 versions of libc.so.5 from 1996) that assume that brk area starts
929 just after the end of the code+bss. These applications break when
930 start of the brk area is randomized. There are however no known
931 non-legacy applications that would be broken this way, so for most
932 systems it is safe to choose full randomization.
934 Systems with ancient and/or broken binaries should be configured
935 with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
936 address space randomization.
937 == ===========================================================================
940 reboot-cmd: (Sparc only)
941 ========================
943 ??? This seems to be a way to give an argument to the Sparc
944 ROM/Flash boot loader. Maybe to tell it what to do after
948 rtsig-max & rtsig-nr:
949 =====================
951 The file rtsig-max can be used to tune the maximum number
952 of POSIX realtime (queued) signals that can be outstanding
955 rtsig-nr shows the number of RT signals currently queued.
961 Enables/disables Energy Aware Scheduling (EAS). EAS starts
962 automatically on platforms where it can run (that is,
963 platforms with asymmetric CPU topologies and having an Energy
964 Model available). If your platform happens to meet the
965 requirements for EAS but you do not want to use it, change
972 Enables/disables scheduler statistics. Enabling this feature
973 incurs a small amount of overhead in the scheduler but is
974 useful for debugging and performance tuning.
980 This file shows the size of the generic SCSI (sg) buffer.
981 You can't tune it just yet, but you could change it on
982 compile time by editing include/scsi/sg.h and changing
983 the value of SG_BIG_BUFF.
985 There shouldn't be any reason to change this value. If
986 you can come up with one, you probably know what you
993 This parameter sets the total amount of shared memory pages that
994 can be used system wide. Hence, SHMALL should always be at least
995 ceil(shmmax/PAGE_SIZE).
997 If you are not sure what the default PAGE_SIZE is on your Linux
998 system, you can run the following command:
1006 This value can be used to query and set the run time limit
1007 on the maximum shared memory segment size that can be created.
1008 Shared memory segments up to 1Gb are now supported in the
1009 kernel. This value defaults to SHMMAX.
1015 Linux lets you set resource limits, including how much memory one
1016 process can consume, via setrlimit(2). Unfortunately, shared memory
1017 segments are allowed to exist without association with any process, and
1018 thus might not be counted against any resource limits. If enabled,
1019 shared memory segments are automatically destroyed when their attach
1020 count becomes zero after a detach or a process termination. It will
1021 also destroy segments that were created, but never attached to, on exit
1022 from the process. The only use left for IPC_RMID is to immediately
1023 destroy an unattached segment. Of course, this breaks the way things are
1024 defined, so some applications might stop working. Note that this
1025 feature will do you no good unless you also configure your resource
1026 limits (in particular, RLIMIT_AS and RLIMIT_NPROC). Most systems don't
1029 Note that if you change this from 0 to 1, already created segments
1030 without users and with a dead originative process will be destroyed.
1033 sysctl_writes_strict:
1034 =====================
1036 Control how file position affects the behavior of updating sysctl values
1037 via the /proc/sys interface:
1039 == ======================================================================
1040 -1 Legacy per-write sysctl value handling, with no printk warnings.
1041 Each write syscall must fully contain the sysctl value to be
1042 written, and multiple writes on the same sysctl file descriptor
1043 will rewrite the sysctl value, regardless of file position.
1044 0 Same behavior as above, but warn about processes that perform writes
1045 to a sysctl file descriptor when the file position is not 0.
1046 1 (default) Respect file position when writing sysctl strings. Multiple
1047 writes will append to the sysctl value buffer. Anything past the max
1048 length of the sysctl value buffer will be ignored. Writes to numeric
1049 sysctl entries must always be at file position 0 and the value must
1050 be fully contained in the buffer sent in the write syscall.
1051 == ======================================================================
1054 softlockup_all_cpu_backtrace:
1055 =============================
1057 This value controls the soft lockup detector thread's behavior
1058 when a soft lockup condition is detected as to whether or not
1059 to gather further debug information. If enabled, each cpu will
1060 be issued an NMI and instructed to capture stack trace.
1062 This feature is only applicable for architectures which support
1065 0: do nothing. This is the default behavior.
1067 1: on detection capture more debug information.
1073 This parameter can be used to control the soft lockup detector.
1075 0 - disable the soft lockup detector
1077 1 - enable the soft lockup detector
1079 The soft lockup detector monitors CPUs for threads that are hogging the CPUs
1080 without rescheduling voluntarily, and thus prevent the 'watchdog/N' threads
1081 from running. The mechanism depends on the CPUs ability to respond to timer
1082 interrupts which are needed for the 'watchdog/N' threads to be woken up by
1083 the watchdog timer function, otherwise the NMI watchdog - if enabled - can
1084 detect a hard lockup condition.
1090 This parameter can be used to control kernel stack erasing at the end
1091 of syscalls for kernels built with CONFIG_GCC_PLUGIN_STACKLEAK.
1093 That erasing reduces the information which kernel stack leak bugs
1094 can reveal and blocks some uninitialized stack variable attacks.
1095 The tradeoff is the performance impact: on a single CPU system kernel
1096 compilation sees a 1% slowdown, other systems and workloads may vary.
1098 0: kernel stack erasing is disabled, STACKLEAK_METRICS are not updated.
1100 1: kernel stack erasing is enabled (default), it is performed before
1101 returning to the userspace at the end of syscalls.
1107 Non-zero if the kernel has been tainted. Numeric values, which can be
1108 ORed together. The letters are seen in "Tainted" line of Oops reports.
1110 ====== ===== ==============================================================
1111 1 `(P)` proprietary module was loaded
1112 2 `(F)` module was force loaded
1113 4 `(S)` SMP kernel oops on an officially SMP incapable processor
1114 8 `(R)` module was force unloaded
1115 16 `(M)` processor reported a Machine Check Exception (MCE)
1116 32 `(B)` bad page referenced or some unexpected page flags
1117 64 `(U)` taint requested by userspace application
1118 128 `(D)` kernel died recently, i.e. there was an OOPS or BUG
1119 256 `(A)` an ACPI table was overridden by user
1120 512 `(W)` kernel issued warning
1121 1024 `(C)` staging driver was loaded
1122 2048 `(I)` workaround for bug in platform firmware applied
1123 4096 `(O)` externally-built ("out-of-tree") module was loaded
1124 8192 `(E)` unsigned module was loaded
1125 16384 `(L)` soft lockup occurred
1126 32768 `(K)` kernel has been live patched
1127 65536 `(X)` Auxiliary taint, defined and used by for distros
1128 131072 `(T)` The kernel was built with the struct randomization plugin
1129 ====== ===== ==============================================================
1131 See Documentation/admin-guide/tainted-kernels.rst for more information.
1137 This value controls the maximum number of threads that can be created
1140 During initialization the kernel sets this value such that even if the
1141 maximum number of threads is created, the thread structures occupy only
1142 a part (1/8th) of the available RAM pages.
1144 The minimum value that can be written to threads-max is 20.
1146 The maximum value that can be written to threads-max is given by the
1147 constant FUTEX_TID_MASK (0x3fffffff).
1149 If a value outside of this range is written to threads-max an error
1152 The value written is checked against the available RAM pages. If the
1153 thread structures would occupy too much (more than 1/8th) of the
1154 available RAM pages threads-max is reduced accordingly.
1160 The value in this file affects behavior of handling NMI. When the
1161 value is non-zero, unknown NMI is trapped and then panic occurs. At
1162 that time, kernel debugging information is displayed on console.
1164 NMI switch that most IA32 servers have fires unknown NMI up, for
1165 example. If a system hangs up, try pressing the NMI switch.
1168 unprivileged_bpf_disabled:
1169 ==========================
1171 Writing 1 to this entry will disable unprivileged calls to ``bpf()``;
1172 once disabled, calling ``bpf()`` without ``CAP_SYS_ADMIN`` will return
1173 ``-EPERM``. Once set to 1, this can't be cleared from the running kernel
1176 Writing 2 to this entry will also disable unprivileged calls to ``bpf()``,
1177 however, an admin can still change this setting later on, if needed, by
1178 writing 0 or 1 to this entry.
1180 If ``BPF_UNPRIV_DEFAULT_OFF`` is enabled in the kernel config, then this
1181 entry will default to 2 instead of 0.
1183 = =============================================================
1184 0 Unprivileged calls to ``bpf()`` are enabled
1185 1 Unprivileged calls to ``bpf()`` are disabled without recovery
1186 2 Unprivileged calls to ``bpf()`` are disabled
1187 = =============================================================
1193 Number of kernel warnings after which the kernel should panic when
1194 ``panic_on_warn`` is not set. Setting this to 0 disables checking
1195 the warning count. Setting this to 1 has the same effect as setting
1196 ``panic_on_warn=1``. The default value is 0.
1202 This parameter can be used to disable or enable the soft lockup detector
1203 _and_ the NMI watchdog (i.e. the hard lockup detector) at the same time.
1205 0 - disable both lockup detectors
1207 1 - enable both lockup detectors
1209 The soft lockup detector and the NMI watchdog can also be disabled or
1210 enabled individually, using the soft_watchdog and nmi_watchdog parameters.
1211 If the watchdog parameter is read, for example by executing::
1213 cat /proc/sys/kernel/watchdog
1215 the output of this command (0 or 1) shows the logical OR of soft_watchdog
1222 This value can be used to control on which cpus the watchdog may run.
1223 The default cpumask is all possible cores, but if NO_HZ_FULL is
1224 enabled in the kernel config, and cores are specified with the
1225 nohz_full= boot argument, those cores are excluded by default.
1226 Offline cores can be included in this mask, and if the core is later
1227 brought online, the watchdog will be started based on the mask value.
1229 Typically this value would only be touched in the nohz_full case
1230 to re-enable cores that by default were not running the watchdog,
1231 if a kernel lockup was suspected on those cores.
1233 The argument value is the standard cpulist format for cpumasks,
1234 so for example to enable the watchdog on cores 0, 2, 3, and 4 you
1237 echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask
1243 This value can be used to control the frequency of hrtimer and NMI
1244 events and the soft and hard lockup thresholds. The default threshold
1247 The softlockup threshold is (2 * watchdog_thresh). Setting this
1248 tunable to zero will disable lockup detection altogether.