xref: /linux/Documentation/admin-guide/sysctl/kernel.rst (revision dd047efbe0e086f6ea82774cb239ec4c64a86d76)
1===================================
2Documentation for /proc/sys/kernel/
3===================================
4
5.. See scripts/check-sysctl-docs to keep this up to date
6
7
8Copyright (c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>
9
10Copyright (c) 2009,        Shen Feng<shen@cn.fujitsu.com>
11
12For general info and legal blurb, please look in
13Documentation/admin-guide/sysctl/index.rst.
14
15------------------------------------------------------------------------------
16
17This file contains documentation for the sysctl files in
18``/proc/sys/kernel/``.
19
20The files in this directory can be used to tune and monitor
21miscellaneous and general things in the operation of the Linux
22kernel. Since some of the files *can* be used to screw up your
23system, it is advisable to read both documentation and source
24before actually making adjustments.
25
26Currently, these files might (depending on your configuration)
27show up in ``/proc/sys/kernel``:
28
29.. contents:: :local:
30
31
32acct
33====
34
35::
36
37    highwater lowwater frequency
38
39If BSD-style process accounting is enabled these values control
40its behaviour. If free space on filesystem where the log lives
41goes below ``lowwater``\ % accounting suspends. If free space gets
42above ``highwater``\ % accounting resumes. ``frequency`` determines
43how often do we check the amount of free space (value is in
44seconds). Default:
45
46::
47
48    4 2 30
49
50That is, suspend accounting if free space drops below 2%; resume it
51if it increases to at least 4%; consider information about amount of
52free space valid for 30 seconds.
53
54
55acpi_video_flags
56================
57
58See Documentation/power/video.rst. This allows the video resume mode to be set,
59in a similar fashion to the ``acpi_sleep`` kernel parameter, by
60combining the following values:
61
62= =======
631 s3_bios
642 s3_mode
654 s3_beep
66= =======
67
68arch
69====
70
71The machine hardware name, the same output as ``uname -m``
72(e.g. ``x86_64`` or ``aarch64``).
73
74auto_msgmni
75===========
76
77This variable has no effect and may be removed in future kernel
78releases. Reading it always returns 0.
79Up to Linux 3.17, it enabled/disabled automatic recomputing of
80`msgmni`_
81upon memory add/remove or upon IPC namespace creation/removal.
82Echoing "1" into this file enabled msgmni automatic recomputing.
83Echoing "0" turned it off. The default value was 1.
84
85
86bootloader_type (x86 only)
87==========================
88
89This gives the bootloader type number as indicated by the bootloader,
90shifted left by 4, and OR'd with the low four bits of the bootloader
91version.  The reason for this encoding is that this used to match the
92``type_of_loader`` field in the kernel header; the encoding is kept for
93backwards compatibility.  That is, if the full bootloader type number
94is 0x15 and the full version number is 0x234, this file will contain
95the value 340 = 0x154.
96
97See the ``type_of_loader`` and ``ext_loader_type`` fields in
98Documentation/arch/x86/boot.rst for additional information.
99
100
101bootloader_version (x86 only)
102=============================
103
104The complete bootloader version number.  In the example above, this
105file will contain the value 564 = 0x234.
106
107See the ``type_of_loader`` and ``ext_loader_ver`` fields in
108Documentation/arch/x86/boot.rst for additional information.
109
110
111bpf_stats_enabled
112=================
113
114Controls whether the kernel should collect statistics on BPF programs
115(total time spent running, number of times run...). Enabling
116statistics causes a slight reduction in performance on each program
117run. The statistics can be seen using ``bpftool``.
118
119= ===================================
1200 Don't collect statistics (default).
1211 Collect statistics.
122= ===================================
123
124
125cad_pid
126=======
127
128This is the pid which will be signalled on reboot (notably, by
129Ctrl-Alt-Delete). Writing a value to this file which doesn't
130correspond to a running process will result in ``-ESRCH``.
131
132See also `ctrl-alt-del`_.
133
134
135cap_last_cap
136============
137
138Highest valid capability of the running kernel.  Exports
139``CAP_LAST_CAP`` from the kernel.
140
141
142.. _core_pattern:
143
144core_pattern
145============
146
147``core_pattern`` is used to specify a core dumpfile pattern name.
148
149* max length 127 characters; default value is "core"
150* ``core_pattern`` is used as a pattern template for the output
151  filename; certain string patterns (beginning with '%') are
152  substituted with their actual values.
153* backward compatibility with ``core_uses_pid``:
154
155	If ``core_pattern`` does not include "%p" (default does not)
156	and ``core_uses_pid`` is set, then .PID will be appended to
157	the filename.
158
159* corename format specifiers
160
161	========	==========================================
162	%<NUL>		'%' is dropped
163	%%		output one '%'
164	%p		pid
165	%P		global pid (init PID namespace)
166	%i		tid
167	%I		global tid (init PID namespace)
168	%u		uid (in initial user namespace)
169	%g		gid (in initial user namespace)
170	%d		dump mode, matches ``PR_SET_DUMPABLE`` and
171			``/proc/sys/fs/suid_dumpable``
172	%s		signal number
173	%t		UNIX time of dump
174	%h		hostname
175	%e		executable filename (may be shortened, could be changed by prctl etc)
176	%f      	executable filename
177	%E		executable path
178	%c		maximum size of core file by resource limit RLIMIT_CORE
179	%C		CPU the task ran on
180	%<OTHER>	both are dropped
181	========	==========================================
182
183* If the first character of the pattern is a '|', the kernel will treat
184  the rest of the pattern as a command to run.  The core dump will be
185  written to the standard input of that program instead of to a file.
186
187
188core_pipe_limit
189===============
190
191This sysctl is only applicable when `core_pattern`_ is configured to
192pipe core files to a user space helper (when the first character of
193``core_pattern`` is a '|', see above).
194When collecting cores via a pipe to an application, it is occasionally
195useful for the collecting application to gather data about the
196crashing process from its ``/proc/pid`` directory.
197In order to do this safely, the kernel must wait for the collecting
198process to exit, so as not to remove the crashing processes proc files
199prematurely.
200This in turn creates the possibility that a misbehaving userspace
201collecting process can block the reaping of a crashed process simply
202by never exiting.
203This sysctl defends against that.
204It defines how many concurrent crashing processes may be piped to user
205space applications in parallel.
206If this value is exceeded, then those crashing processes above that
207value are noted via the kernel log and their cores are skipped.
2080 is a special value, indicating that unlimited processes may be
209captured in parallel, but that no waiting will take place (i.e. the
210collecting process is not guaranteed access to ``/proc/<crashing
211pid>/``).
212This value defaults to 0.
213
214
215core_sort_vma
216=============
217
218The default coredump writes VMAs in address order. By setting
219``core_sort_vma`` to 1, VMAs will be written from smallest size
220to largest size. This is known to break at least elfutils, but
221can be handy when dealing with very large (and truncated)
222coredumps where the more useful debugging details are included
223in the smaller VMAs.
224
225
226core_uses_pid
227=============
228
229The default coredump filename is "core".  By setting
230``core_uses_pid`` to 1, the coredump filename becomes core.PID.
231If `core_pattern`_ does not include "%p" (default does not)
232and ``core_uses_pid`` is set, then .PID will be appended to
233the filename.
234
235
236ctrl-alt-del
237============
238
239When the value in this file is 0, ctrl-alt-del is trapped and
240sent to the ``init(1)`` program to handle a graceful restart.
241When, however, the value is > 0, Linux's reaction to a Vulcan
242Nerve Pinch (tm) will be an immediate reboot, without even
243syncing its dirty buffers.
244
245Note:
246  when a program (like dosemu) has the keyboard in 'raw'
247  mode, the ctrl-alt-del is intercepted by the program before it
248  ever reaches the kernel tty layer, and it's up to the program
249  to decide what to do with it.
250
251
252dmesg_restrict
253==============
254
255This toggle indicates whether unprivileged users are prevented
256from using ``dmesg(8)`` to view messages from the kernel's log
257buffer.
258When ``dmesg_restrict`` is set to 0 there are no restrictions.
259When ``dmesg_restrict`` is set to 1, users must have
260``CAP_SYSLOG`` to use ``dmesg(8)``.
261
262The kernel config option ``CONFIG_SECURITY_DMESG_RESTRICT`` sets the
263default value of ``dmesg_restrict``.
264
265
266domainname & hostname
267=====================
268
269These files can be used to set the NIS/YP domainname and the
270hostname of your box in exactly the same way as the commands
271domainname and hostname, i.e.::
272
273	# echo "darkstar" > /proc/sys/kernel/hostname
274	# echo "mydomain" > /proc/sys/kernel/domainname
275
276has the same effect as::
277
278	# hostname "darkstar"
279	# domainname "mydomain"
280
281Note, however, that the classic darkstar.frop.org has the
282hostname "darkstar" and DNS (Internet Domain Name Server)
283domainname "frop.org", not to be confused with the NIS (Network
284Information Service) or YP (Yellow Pages) domainname. These two
285domain names are in general different. For a detailed discussion
286see the ``hostname(1)`` man page.
287
288
289firmware_config
290===============
291
292See Documentation/driver-api/firmware/fallback-mechanisms.rst.
293
294The entries in this directory allow the firmware loader helper
295fallback to be controlled:
296
297* ``force_sysfs_fallback``, when set to 1, forces the use of the
298  fallback;
299* ``ignore_sysfs_fallback``, when set to 1, ignores any fallback.
300
301
302ftrace_dump_on_oops
303===================
304
305Determines whether ``ftrace_dump()`` should be called on an oops (or
306kernel panic). This will output the contents of the ftrace buffers to
307the console.  This is very useful for capturing traces that lead to
308crashes and outputting them to a serial console.
309
310======================= ===========================================
3110                       Disabled (default).
3121                       Dump buffers of all CPUs.
3132(orig_cpu)             Dump the buffer of the CPU that triggered the
314                        oops.
315<instance>              Dump the specific instance buffer on all CPUs.
316<instance>=2(orig_cpu)  Dump the specific instance buffer on the CPU
317                        that triggered the oops.
318======================= ===========================================
319
320Multiple instance dump is also supported, and instances are separated
321by commas. If global buffer also needs to be dumped, please specify
322the dump mode (1/2/orig_cpu) first for global buffer.
323
324So for example to dump "foo" and "bar" instance buffer on all CPUs,
325user can::
326
327  echo "foo,bar" > /proc/sys/kernel/ftrace_dump_on_oops
328
329To dump global buffer and "foo" instance buffer on all
330CPUs along with the "bar" instance buffer on CPU that triggered the
331oops, user can::
332
333  echo "1,foo,bar=2" > /proc/sys/kernel/ftrace_dump_on_oops
334
335ftrace_enabled, stack_tracer_enabled
336====================================
337
338See Documentation/trace/ftrace.rst.
339
340
341hardlockup_all_cpu_backtrace
342============================
343
344This value controls the hard lockup detector behavior when a hard
345lockup condition is detected as to whether or not to gather further
346debug information. If enabled, arch-specific all-CPU stack dumping
347will be initiated.
348
349= ============================================
3500 Do nothing. This is the default behavior.
3511 On detection capture more debug information.
352= ============================================
353
354
355hardlockup_panic
356================
357
358This parameter can be used to control whether the kernel panics
359when a hard lockup is detected.
360
361= ===========================
3620 Don't panic on hard lockup.
3631 Panic on hard lockup.
364= ===========================
365
366See Documentation/admin-guide/lockup-watchdogs.rst for more information.
367This can also be set using the nmi_watchdog kernel parameter.
368
369
370hotplug
371=======
372
373Path for the hotplug policy agent.
374Default value is ``CONFIG_UEVENT_HELPER_PATH``, which in turn defaults
375to the empty string.
376
377This file only exists when ``CONFIG_UEVENT_HELPER`` is enabled. Most
378modern systems rely exclusively on the netlink-based uevent source and
379don't need this.
380
381
382hung_task_all_cpu_backtrace
383===========================
384
385If this option is set, the kernel will send an NMI to all CPUs to dump
386their backtraces when a hung task is detected. This file shows up if
387CONFIG_DETECT_HUNG_TASK and CONFIG_SMP are enabled.
388
3890: Won't show all CPUs backtraces when a hung task is detected.
390This is the default behavior.
391
3921: Will non-maskably interrupt all CPUs and dump their backtraces when
393a hung task is detected.
394
395
396hung_task_panic
397===============
398
399Controls the kernel's behavior when a hung task is detected.
400This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
401
402= =================================================
4030 Continue operation. This is the default behavior.
4041 Panic immediately.
405= =================================================
406
407
408hung_task_check_count
409=====================
410
411The upper bound on the number of tasks that are checked.
412This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
413
414
415hung_task_detect_count
416======================
417
418Indicates the total number of tasks that have been detected as hung since
419the system boot.
420
421This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
422
423
424hung_task_timeout_secs
425======================
426
427When a task in D state did not get scheduled
428for more than this value report a warning.
429This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
430
4310 means infinite timeout, no checking is done.
432
433Possible values to set are in range {0:``LONG_MAX``/``HZ``}.
434
435
436hung_task_check_interval_secs
437=============================
438
439Hung task check interval. If hung task checking is enabled
440(see `hung_task_timeout_secs`_), the check is done every
441``hung_task_check_interval_secs`` seconds.
442This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
443
4440 (default) means use ``hung_task_timeout_secs`` as checking
445interval.
446
447Possible values to set are in range {0:``LONG_MAX``/``HZ``}.
448
449
450hung_task_warnings
451==================
452
453The maximum number of warnings to report. During a check interval
454if a hung task is detected, this value is decreased by 1.
455When this value reaches 0, no more warnings will be reported.
456This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
457
458-1: report an infinite number of warnings.
459
460
461hyperv_record_panic_msg
462=======================
463
464Controls whether the panic kmsg data should be reported to Hyper-V.
465
466= =========================================================
4670 Do not report panic kmsg data.
4681 Report the panic kmsg data. This is the default behavior.
469= =========================================================
470
471
472ignore-unaligned-usertrap
473=========================
474
475On architectures where unaligned accesses cause traps, and where this
476feature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_NO_WARN``;
477currently, ``arc``, ``parisc`` and ``loongarch``), controls whether all
478unaligned traps are logged.
479
480= =============================================================
4810 Log all unaligned accesses.
4821 Only warn the first time a process traps. This is the default
483  setting.
484= =============================================================
485
486See also `unaligned-trap`_.
487
488io_uring_disabled
489=================
490
491Prevents all processes from creating new io_uring instances. Enabling this
492shrinks the kernel's attack surface.
493
494= ======================================================================
4950 All processes can create io_uring instances as normal. This is the
496  default setting.
4971 io_uring creation is disabled (io_uring_setup() will fail with
498  -EPERM) for unprivileged processes not in the io_uring_group group.
499  Existing io_uring instances can still be used.  See the
500  documentation for io_uring_group for more information.
5012 io_uring creation is disabled for all processes. io_uring_setup()
502  always fails with -EPERM. Existing io_uring instances can still be
503  used.
504= ======================================================================
505
506
507io_uring_group
508==============
509
510When io_uring_disabled is set to 1, a process must either be
511privileged (CAP_SYS_ADMIN) or be in the io_uring_group group in order
512to create an io_uring instance.  If io_uring_group is set to -1 (the
513default), only processes with the CAP_SYS_ADMIN capability may create
514io_uring instances.
515
516
517kexec_load_disabled
518===================
519
520A toggle indicating if the syscalls ``kexec_load`` and
521``kexec_file_load`` have been disabled.
522This value defaults to 0 (false: ``kexec_*load`` enabled), but can be
523set to 1 (true: ``kexec_*load`` disabled).
524Once true, kexec can no longer be used, and the toggle cannot be set
525back to false.
526This allows a kexec image to be loaded before disabling the syscall,
527allowing a system to set up (and later use) an image without it being
528altered.
529Generally used together with the `modules_disabled`_ sysctl.
530
531kexec_load_limit_panic
532======================
533
534This parameter specifies a limit to the number of times the syscalls
535``kexec_load`` and ``kexec_file_load`` can be called with a crash
536image. It can only be set with a more restrictive value than the
537current one.
538
539== ======================================================
540-1 Unlimited calls to kexec. This is the default setting.
541N  Number of calls left.
542== ======================================================
543
544kexec_load_limit_reboot
545=======================
546
547Similar functionality as ``kexec_load_limit_panic``, but for a normal
548image.
549
550kptr_restrict
551=============
552
553This toggle indicates whether restrictions are placed on
554exposing kernel addresses via ``/proc`` and other interfaces.
555
556When ``kptr_restrict`` is set to 0 (the default) the address is hashed
557before printing.
558(This is the equivalent to %p.)
559
560When ``kptr_restrict`` is set to 1, kernel pointers printed using the
561%pK format specifier will be replaced with 0s unless the user has
562``CAP_SYSLOG`` and effective user and group ids are equal to the real
563ids.
564This is because %pK checks are done at read() time rather than open()
565time, so if permissions are elevated between the open() and the read()
566(e.g via a setuid binary) then %pK will not leak kernel pointers to
567unprivileged users.
568Note, this is a temporary solution only.
569The correct long-term solution is to do the permission checks at
570open() time.
571Consider removing world read permissions from files that use %pK, and
572using `dmesg_restrict`_ to protect against uses of %pK in ``dmesg(8)``
573if leaking kernel pointer values to unprivileged users is a concern.
574
575When ``kptr_restrict`` is set to 2, kernel pointers printed using
576%pK will be replaced with 0s regardless of privileges.
577
578
579modprobe
580========
581
582The full path to the usermode helper for autoloading kernel modules,
583by default ``CONFIG_MODPROBE_PATH``, which in turn defaults to
584"/sbin/modprobe".  This binary is executed when the kernel requests a
585module.  For example, if userspace passes an unknown filesystem type
586to mount(), then the kernel will automatically request the
587corresponding filesystem module by executing this usermode helper.
588This usermode helper should insert the needed module into the kernel.
589
590This sysctl only affects module autoloading.  It has no effect on the
591ability to explicitly insert modules.
592
593This sysctl can be used to debug module loading requests::
594
595    echo '#! /bin/sh' > /tmp/modprobe
596    echo 'echo "$@" >> /tmp/modprobe.log' >> /tmp/modprobe
597    echo 'exec /sbin/modprobe "$@"' >> /tmp/modprobe
598    chmod a+x /tmp/modprobe
599    echo /tmp/modprobe > /proc/sys/kernel/modprobe
600
601Alternatively, if this sysctl is set to the empty string, then module
602autoloading is completely disabled.  The kernel will not try to
603execute a usermode helper at all, nor will it call the
604kernel_module_request LSM hook.
605
606If CONFIG_STATIC_USERMODEHELPER=y is set in the kernel configuration,
607then the configured static usermode helper overrides this sysctl,
608except that the empty string is still accepted to completely disable
609module autoloading as described above.
610
611modules_disabled
612================
613
614A toggle value indicating if modules are allowed to be loaded
615in an otherwise modular kernel.  This toggle defaults to off
616(0), but can be set true (1).  Once true, modules can be
617neither loaded nor unloaded, and the toggle cannot be set back
618to false.  Generally used with the `kexec_load_disabled`_ toggle.
619
620
621.. _msgmni:
622
623msgmax, msgmnb, and msgmni
624==========================
625
626``msgmax`` is the maximum size of an IPC message, in bytes. 8192 by
627default (``MSGMAX``).
628
629``msgmnb`` is the maximum size of an IPC queue, in bytes. 16384 by
630default (``MSGMNB``).
631
632``msgmni`` is the maximum number of IPC queues. 32000 by default
633(``MSGMNI``).
634
635All of these parameters are set per ipc namespace. The maximum number of bytes
636in POSIX message queues is limited by ``RLIMIT_MSGQUEUE``. This limit is
637respected hierarchically in the each user namespace.
638
639msg_next_id, sem_next_id, and shm_next_id (System V IPC)
640========================================================
641
642These three toggles allows to specify desired id for next allocated IPC
643object: message, semaphore or shared memory respectively.
644
645By default they are equal to -1, which means generic allocation logic.
646Possible values to set are in range {0:``INT_MAX``}.
647
648Notes:
649  1) kernel doesn't guarantee, that new object will have desired id. So,
650     it's up to userspace, how to handle an object with "wrong" id.
651  2) Toggle with non-default value will be set back to -1 by kernel after
652     successful IPC object allocation. If an IPC object allocation syscall
653     fails, it is undefined if the value remains unmodified or is reset to -1.
654
655
656ngroups_max
657===========
658
659Maximum number of supplementary groups, _i.e._ the maximum size which
660``setgroups`` will accept. Exports ``NGROUPS_MAX`` from the kernel.
661
662
663
664nmi_watchdog
665============
666
667This parameter can be used to control the NMI watchdog
668(i.e. the hard lockup detector) on x86 systems.
669
670= =================================
6710 Disable the hard lockup detector.
6721 Enable the hard lockup detector.
673= =================================
674
675The hard lockup detector monitors each CPU for its ability to respond to
676timer interrupts. The mechanism utilizes CPU performance counter registers
677that are programmed to generate Non-Maskable Interrupts (NMIs) periodically
678while a CPU is busy. Hence, the alternative name 'NMI watchdog'.
679
680The NMI watchdog is disabled by default if the kernel is running as a guest
681in a KVM virtual machine. This default can be overridden by adding::
682
683   nmi_watchdog=1
684
685to the guest kernel command line (see
686Documentation/admin-guide/kernel-parameters.rst).
687
688
689nmi_wd_lpm_factor (PPC only)
690============================
691
692Factor to apply to the NMI watchdog timeout (only when ``nmi_watchdog`` is
693set to 1). This factor represents the percentage added to
694``watchdog_thresh`` when calculating the NMI watchdog timeout during an
695LPM. The soft lockup timeout is not impacted.
696
697A value of 0 means no change. The default value is 200 meaning the NMI
698watchdog is set to 30s (based on ``watchdog_thresh`` equal to 10).
699
700
701numa_balancing
702==============
703
704Enables/disables and configures automatic page fault based NUMA memory
705balancing.  Memory is moved automatically to nodes that access it often.
706The value to set can be the result of ORing the following:
707
708= =================================
7090 NUMA_BALANCING_DISABLED
7101 NUMA_BALANCING_NORMAL
7112 NUMA_BALANCING_MEMORY_TIERING
712= =================================
713
714Or NUMA_BALANCING_NORMAL to optimize page placement among different
715NUMA nodes to reduce remote accessing.  On NUMA machines, there is a
716performance penalty if remote memory is accessed by a CPU. When this
717feature is enabled the kernel samples what task thread is accessing
718memory by periodically unmapping pages and later trapping a page
719fault. At the time of the page fault, it is determined if the data
720being accessed should be migrated to a local memory node.
721
722The unmapping of pages and trapping faults incur additional overhead that
723ideally is offset by improved memory locality but there is no universal
724guarantee. If the target workload is already bound to NUMA nodes then this
725feature should be disabled.
726
727Or NUMA_BALANCING_MEMORY_TIERING to optimize page placement among
728different types of memory (represented as different NUMA nodes) to
729place the hot pages in the fast memory.  This is implemented based on
730unmapping and page fault too.
731
732numa_balancing_promote_rate_limit_MBps
733======================================
734
735Too high promotion/demotion throughput between different memory types
736may hurt application latency.  This can be used to rate limit the
737promotion throughput.  The per-node max promotion throughput in MB/s
738will be limited to be no more than the set value.
739
740A rule of thumb is to set this to less than 1/10 of the PMEM node
741write bandwidth.
742
743oops_all_cpu_backtrace
744======================
745
746If this option is set, the kernel will send an NMI to all CPUs to dump
747their backtraces when an oops event occurs. It should be used as a last
748resort in case a panic cannot be triggered (to protect VMs running, for
749example) or kdump can't be collected. This file shows up if CONFIG_SMP
750is enabled.
751
7520: Won't show all CPUs backtraces when an oops is detected.
753This is the default behavior.
754
7551: Will non-maskably interrupt all CPUs and dump their backtraces when
756an oops event is detected.
757
758
759oops_limit
760==========
761
762Number of kernel oopses after which the kernel should panic when
763``panic_on_oops`` is not set. Setting this to 0 disables checking
764the count. Setting this to  1 has the same effect as setting
765``panic_on_oops=1``. The default value is 10000.
766
767
768osrelease, ostype & version
769===========================
770
771::
772
773  # cat osrelease
774  2.1.88
775  # cat ostype
776  Linux
777  # cat version
778  #5 Wed Feb 25 21:49:24 MET 1998
779
780The files ``osrelease`` and ``ostype`` should be clear enough.
781``version``
782needs a little more clarification however. The '#5' means that
783this is the fifth kernel built from this source base and the
784date behind it indicates the time the kernel was built.
785The only way to tune these values is to rebuild the kernel :-)
786
787
788overflowgid & overflowuid
789=========================
790
791if your architecture did not always support 32-bit UIDs (i.e. arm,
792i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
793applications that use the old 16-bit UID/GID system calls, if the
794actual UID or GID would exceed 65535.
795
796These sysctls allow you to change the value of the fixed UID and GID.
797The default is 65534.
798
799
800panic
801=====
802
803The value in this file determines the behaviour of the kernel on a
804panic:
805
806* if zero, the kernel will loop forever;
807* if negative, the kernel will reboot immediately;
808* if positive, the kernel will reboot after the corresponding number
809  of seconds.
810
811When you use the software watchdog, the recommended setting is 60.
812
813
814panic_on_io_nmi
815===============
816
817Controls the kernel's behavior when a CPU receives an NMI caused by
818an IO error.
819
820= ==================================================================
8210 Try to continue operation (default).
8221 Panic immediately. The IO error triggered an NMI. This indicates a
823  serious system condition which could result in IO data corruption.
824  Rather than continuing, panicking might be a better choice. Some
825  servers issue this sort of NMI when the dump button is pushed,
826  and you can use this option to take a crash dump.
827= ==================================================================
828
829
830panic_on_oops
831=============
832
833Controls the kernel's behaviour when an oops or BUG is encountered.
834
835= ===================================================================
8360 Try to continue operation.
8371 Panic immediately.  If the `panic` sysctl is also non-zero then the
838  machine will be rebooted.
839= ===================================================================
840
841
842panic_on_stackoverflow
843======================
844
845Controls the kernel's behavior when detecting the overflows of
846kernel, IRQ and exception stacks except a user stack.
847This file shows up if ``CONFIG_DEBUG_STACKOVERFLOW`` is enabled.
848
849= ==========================
8500 Try to continue operation.
8511 Panic immediately.
852= ==========================
853
854
855panic_on_unrecovered_nmi
856========================
857
858The default Linux behaviour on an NMI of either memory or unknown is
859to continue operation. For many environments such as scientific
860computing it is preferable that the box is taken out and the error
861dealt with than an uncorrected parity/ECC error get propagated.
862
863A small number of systems do generate NMIs for bizarre random reasons
864such as power management so the default is off. That sysctl works like
865the existing panic controls already in that directory.
866
867
868panic_on_warn
869=============
870
871Calls panic() in the WARN() path when set to 1.  This is useful to avoid
872a kernel rebuild when attempting to kdump at the location of a WARN().
873
874= ================================================
8750 Only WARN(), default behaviour.
8761 Call panic() after printing out WARN() location.
877= ================================================
878
879
880panic_print
881===========
882
883Bitmask for printing system info when panic happens. User can chose
884combination of the following bits:
885
886=====  ============================================
887bit 0  print all tasks info
888bit 1  print system memory info
889bit 2  print timer info
890bit 3  print locks info if ``CONFIG_LOCKDEP`` is on
891bit 4  print ftrace buffer
892bit 5  print all printk messages in buffer
893bit 6  print all CPUs backtrace (if available in the arch)
894bit 7  print only tasks in uninterruptible (blocked) state
895=====  ============================================
896
897So for example to print tasks and memory info on panic, user can::
898
899  echo 3 > /proc/sys/kernel/panic_print
900
901
902panic_on_rcu_stall
903==================
904
905When set to 1, calls panic() after RCU stall detection messages. This
906is useful to define the root cause of RCU stalls using a vmcore.
907
908= ============================================================
9090 Do not panic() when RCU stall takes place, default behavior.
9101 panic() after printing RCU stall messages.
911= ============================================================
912
913max_rcu_stall_to_panic
914======================
915
916When ``panic_on_rcu_stall`` is set to 1, this value determines the
917number of times that RCU can stall before panic() is called.
918
919When ``panic_on_rcu_stall`` is set to 0, this value is has no effect.
920
921perf_cpu_time_max_percent
922=========================
923
924Hints to the kernel how much CPU time it should be allowed to
925use to handle perf sampling events.  If the perf subsystem
926is informed that its samples are exceeding this limit, it
927will drop its sampling frequency to attempt to reduce its CPU
928usage.
929
930Some perf sampling happens in NMIs.  If these samples
931unexpectedly take too long to execute, the NMIs can become
932stacked up next to each other so much that nothing else is
933allowed to execute.
934
935===== ========================================================
9360     Disable the mechanism.  Do not monitor or correct perf's
937      sampling rate no matter how CPU time it takes.
938
9391-100 Attempt to throttle perf's sample rate to this
940      percentage of CPU.  Note: the kernel calculates an
941      "expected" length of each sample event.  100 here means
942      100% of that expected length.  Even if this is set to
943      100, you may still see sample throttling if this
944      length is exceeded.  Set to 0 if you truly do not care
945      how much CPU is consumed.
946===== ========================================================
947
948
949perf_event_paranoid
950===================
951
952Controls use of the performance events system by unprivileged
953users (without CAP_PERFMON).  The default value is 2.
954
955For backward compatibility reasons access to system performance
956monitoring and observability remains open for CAP_SYS_ADMIN
957privileged processes but CAP_SYS_ADMIN usage for secure system
958performance monitoring and observability operations is discouraged
959with respect to CAP_PERFMON use cases.
960
961===  ==================================================================
962 -1  Allow use of (almost) all events by all users.
963
964     Ignore mlock limit after perf_event_mlock_kb without
965     ``CAP_IPC_LOCK``.
966
967>=0  Disallow ftrace function tracepoint by users without
968     ``CAP_PERFMON``.
969
970     Disallow raw tracepoint access by users without ``CAP_PERFMON``.
971
972>=1  Disallow CPU event access by users without ``CAP_PERFMON``.
973
974>=2  Disallow kernel profiling by users without ``CAP_PERFMON``.
975===  ==================================================================
976
977
978perf_event_max_stack
979====================
980
981Controls maximum number of stack frames to copy for (``attr.sample_type &
982PERF_SAMPLE_CALLCHAIN``) configured events, for instance, when using
983'``perf record -g``' or '``perf trace --call-graph fp``'.
984
985This can only be done when no events are in use that have callchains
986enabled, otherwise writing to this file will return ``-EBUSY``.
987
988The default value is 127.
989
990
991perf_event_mlock_kb
992===================
993
994Control size of per-cpu ring buffer not counted against mlock limit.
995
996The default value is 512 + 1 page
997
998
999perf_event_max_contexts_per_stack
1000=================================
1001
1002Controls maximum number of stack frame context entries for
1003(``attr.sample_type & PERF_SAMPLE_CALLCHAIN``) configured events, for
1004instance, when using '``perf record -g``' or '``perf trace --call-graph fp``'.
1005
1006This can only be done when no events are in use that have callchains
1007enabled, otherwise writing to this file will return ``-EBUSY``.
1008
1009The default value is 8.
1010
1011
1012perf_user_access (arm64 and riscv only)
1013=======================================
1014
1015Controls user space access for reading perf event counters.
1016
1017arm64
1018=====
1019
1020The default value is 0 (access disabled).
1021
1022When set to 1, user space can read performance monitor counter registers
1023directly.
1024
1025See Documentation/arch/arm64/perf.rst for more information.
1026
1027riscv
1028=====
1029
1030When set to 0, user space access is disabled.
1031
1032The default value is 1, user space can read performance monitor counter
1033registers through perf, any direct access without perf intervention will trigger
1034an illegal instruction.
1035
1036When set to 2, which enables legacy mode (user space has direct access to cycle
1037and insret CSRs only). Note that this legacy value is deprecated and will be
1038removed once all user space applications are fixed.
1039
1040Note that the time CSR is always directly accessible to all modes.
1041
1042pid_max
1043=======
1044
1045PID allocation wrap value.  When the kernel's next PID value
1046reaches this value, it wraps back to a minimum PID value.
1047PIDs of value ``pid_max`` or larger are not allocated.
1048
1049
1050ns_last_pid
1051===========
1052
1053The last pid allocated in the current (the one task using this sysctl
1054lives in) pid namespace. When selecting a pid for a next task on fork
1055kernel tries to allocate a number starting from this one.
1056
1057
1058powersave-nap (PPC only)
1059========================
1060
1061If set, Linux-PPC will use the 'nap' mode of powersaving,
1062otherwise the 'doze' mode will be used.
1063
1064
1065==============================================================
1066
1067printk
1068======
1069
1070The four values in printk denote: ``console_loglevel``,
1071``default_message_loglevel``, ``minimum_console_loglevel`` and
1072``default_console_loglevel`` respectively.
1073
1074These values influence printk() behavior when printing or
1075logging error messages. See '``man 2 syslog``' for more info on
1076the different loglevels.
1077
1078======================== =====================================
1079console_loglevel         messages with a higher priority than
1080                         this will be printed to the console
1081default_message_loglevel messages without an explicit priority
1082                         will be printed with this priority
1083minimum_console_loglevel minimum (highest) value to which
1084                         console_loglevel can be set
1085default_console_loglevel default value for console_loglevel
1086======================== =====================================
1087
1088
1089printk_delay
1090============
1091
1092Delay each printk message in ``printk_delay`` milliseconds
1093
1094Value from 0 - 10000 is allowed.
1095
1096
1097printk_ratelimit
1098================
1099
1100Some warning messages are rate limited. ``printk_ratelimit`` specifies
1101the minimum length of time between these messages (in seconds).
1102The default value is 5 seconds.
1103
1104A value of 0 will disable rate limiting.
1105
1106
1107printk_ratelimit_burst
1108======================
1109
1110While long term we enforce one message per `printk_ratelimit`_
1111seconds, we do allow a burst of messages to pass through.
1112``printk_ratelimit_burst`` specifies the number of messages we can
1113send before ratelimiting kicks in.
1114
1115The default value is 10 messages.
1116
1117
1118printk_devkmsg
1119==============
1120
1121Control the logging to ``/dev/kmsg`` from userspace:
1122
1123========= =============================================
1124ratelimit default, ratelimited
1125on        unlimited logging to /dev/kmsg from userspace
1126off       logging to /dev/kmsg disabled
1127========= =============================================
1128
1129The kernel command line parameter ``printk.devkmsg=`` overrides this and is
1130a one-time setting until next reboot: once set, it cannot be changed by
1131this sysctl interface anymore.
1132
1133==============================================================
1134
1135
1136pty
1137===
1138
1139See Documentation/filesystems/devpts.rst.
1140
1141
1142random
1143======
1144
1145This is a directory, with the following entries:
1146
1147* ``boot_id``: a UUID generated the first time this is retrieved, and
1148  unvarying after that;
1149
1150* ``uuid``: a UUID generated every time this is retrieved (this can
1151  thus be used to generate UUIDs at will);
1152
1153* ``entropy_avail``: the pool's entropy count, in bits;
1154
1155* ``poolsize``: the entropy pool size, in bits;
1156
1157* ``urandom_min_reseed_secs``: obsolete (used to determine the minimum
1158  number of seconds between urandom pool reseeding). This file is
1159  writable for compatibility purposes, but writing to it has no effect
1160  on any RNG behavior;
1161
1162* ``write_wakeup_threshold``: when the entropy count drops below this
1163  (as a number of bits), processes waiting to write to ``/dev/random``
1164  are woken up. This file is writable for compatibility purposes, but
1165  writing to it has no effect on any RNG behavior.
1166
1167
1168randomize_va_space
1169==================
1170
1171This option can be used to select the type of process address
1172space randomization that is used in the system, for architectures
1173that support this feature.
1174
1175==  ===========================================================================
11760   Turn the process address space randomization off.  This is the
1177    default for architectures that do not support this feature anyways,
1178    and kernels that are booted with the "norandmaps" parameter.
1179
11801   Make the addresses of mmap base, stack and VDSO page randomized.
1181    This, among other things, implies that shared libraries will be
1182    loaded to random addresses.  Also for PIE-linked binaries, the
1183    location of code start is randomized.  This is the default if the
1184    ``CONFIG_COMPAT_BRK`` option is enabled.
1185
11862   Additionally enable heap randomization.  This is the default if
1187    ``CONFIG_COMPAT_BRK`` is disabled.
1188
1189    There are a few legacy applications out there (such as some ancient
1190    versions of libc.so.5 from 1996) that assume that brk area starts
1191    just after the end of the code+bss.  These applications break when
1192    start of the brk area is randomized.  There are however no known
1193    non-legacy applications that would be broken this way, so for most
1194    systems it is safe to choose full randomization.
1195
1196    Systems with ancient and/or broken binaries should be configured
1197    with ``CONFIG_COMPAT_BRK`` enabled, which excludes the heap from process
1198    address space randomization.
1199==  ===========================================================================
1200
1201
1202real-root-dev
1203=============
1204
1205See Documentation/admin-guide/initrd.rst.
1206
1207
1208reboot-cmd (SPARC only)
1209=======================
1210
1211??? This seems to be a way to give an argument to the Sparc
1212ROM/Flash boot loader. Maybe to tell it what to do after
1213rebooting. ???
1214
1215
1216sched_energy_aware
1217==================
1218
1219Enables/disables Energy Aware Scheduling (EAS). EAS starts
1220automatically on platforms where it can run (that is,
1221platforms with asymmetric CPU topologies and having an Energy
1222Model available). If your platform happens to meet the
1223requirements for EAS but you do not want to use it, change
1224this value to 0. On Non-EAS platforms, write operation fails and
1225read doesn't return anything.
1226
1227task_delayacct
1228===============
1229
1230Enables/disables task delay accounting (see
1231Documentation/accounting/delay-accounting.rst. Enabling this feature incurs
1232a small amount of overhead in the scheduler but is useful for debugging
1233and performance tuning. It is required by some tools such as iotop.
1234
1235sched_schedstats
1236================
1237
1238Enables/disables scheduler statistics. Enabling this feature
1239incurs a small amount of overhead in the scheduler but is
1240useful for debugging and performance tuning.
1241
1242sched_util_clamp_min
1243====================
1244
1245Max allowed *minimum* utilization.
1246
1247Default value is 1024, which is the maximum possible value.
1248
1249It means that any requested uclamp.min value cannot be greater than
1250sched_util_clamp_min, i.e., it is restricted to the range
1251[0:sched_util_clamp_min].
1252
1253sched_util_clamp_max
1254====================
1255
1256Max allowed *maximum* utilization.
1257
1258Default value is 1024, which is the maximum possible value.
1259
1260It means that any requested uclamp.max value cannot be greater than
1261sched_util_clamp_max, i.e., it is restricted to the range
1262[0:sched_util_clamp_max].
1263
1264sched_util_clamp_min_rt_default
1265===============================
1266
1267By default Linux is tuned for performance. Which means that RT tasks always run
1268at the highest frequency and most capable (highest capacity) CPU (in
1269heterogeneous systems).
1270
1271Uclamp achieves this by setting the requested uclamp.min of all RT tasks to
12721024 by default, which effectively boosts the tasks to run at the highest
1273frequency and biases them to run on the biggest CPU.
1274
1275This knob allows admins to change the default behavior when uclamp is being
1276used. In battery powered devices particularly, running at the maximum
1277capacity and frequency will increase energy consumption and shorten the battery
1278life.
1279
1280This knob is only effective for RT tasks which the user hasn't modified their
1281requested uclamp.min value via sched_setattr() syscall.
1282
1283This knob will not escape the range constraint imposed by sched_util_clamp_min
1284defined above.
1285
1286For example if
1287
1288	sched_util_clamp_min_rt_default = 800
1289	sched_util_clamp_min = 600
1290
1291Then the boost will be clamped to 600 because 800 is outside of the permissible
1292range of [0:600]. This could happen for instance if a powersave mode will
1293restrict all boosts temporarily by modifying sched_util_clamp_min. As soon as
1294this restriction is lifted, the requested sched_util_clamp_min_rt_default
1295will take effect.
1296
1297seccomp
1298=======
1299
1300See Documentation/userspace-api/seccomp_filter.rst.
1301
1302
1303sg-big-buff
1304===========
1305
1306This file shows the size of the generic SCSI (sg) buffer.
1307You can't tune it just yet, but you could change it on
1308compile time by editing ``include/scsi/sg.h`` and changing
1309the value of ``SG_BIG_BUFF``.
1310
1311There shouldn't be any reason to change this value. If
1312you can come up with one, you probably know what you
1313are doing anyway :)
1314
1315
1316shmall
1317======
1318
1319This parameter sets the total amount of shared memory pages that can be used
1320inside ipc namespace. The shared memory pages counting occurs for each ipc
1321namespace separately and is not inherited. Hence, ``shmall`` should always be at
1322least ``ceil(shmmax/PAGE_SIZE)``.
1323
1324If you are not sure what the default ``PAGE_SIZE`` is on your Linux
1325system, you can run the following command::
1326
1327	# getconf PAGE_SIZE
1328
1329To reduce or disable the ability to allocate shared memory, you must create a
1330new ipc namespace, set this parameter to the required value and prohibit the
1331creation of a new ipc namespace in the current user namespace or cgroups can
1332be used.
1333
1334shmmax
1335======
1336
1337This value can be used to query and set the run time limit
1338on the maximum shared memory segment size that can be created.
1339Shared memory segments up to 1Gb are now supported in the
1340kernel.  This value defaults to ``SHMMAX``.
1341
1342
1343shmmni
1344======
1345
1346This value determines the maximum number of shared memory segments.
13474096 by default (``SHMMNI``).
1348
1349
1350shm_rmid_forced
1351===============
1352
1353Linux lets you set resource limits, including how much memory one
1354process can consume, via ``setrlimit(2)``.  Unfortunately, shared memory
1355segments are allowed to exist without association with any process, and
1356thus might not be counted against any resource limits.  If enabled,
1357shared memory segments are automatically destroyed when their attach
1358count becomes zero after a detach or a process termination.  It will
1359also destroy segments that were created, but never attached to, on exit
1360from the process.  The only use left for ``IPC_RMID`` is to immediately
1361destroy an unattached segment.  Of course, this breaks the way things are
1362defined, so some applications might stop working.  Note that this
1363feature will do you no good unless you also configure your resource
1364limits (in particular, ``RLIMIT_AS`` and ``RLIMIT_NPROC``).  Most systems don't
1365need this.
1366
1367Note that if you change this from 0 to 1, already created segments
1368without users and with a dead originative process will be destroyed.
1369
1370
1371sysctl_writes_strict
1372====================
1373
1374Control how file position affects the behavior of updating sysctl values
1375via the ``/proc/sys`` interface:
1376
1377  ==   ======================================================================
1378  -1   Legacy per-write sysctl value handling, with no printk warnings.
1379       Each write syscall must fully contain the sysctl value to be
1380       written, and multiple writes on the same sysctl file descriptor
1381       will rewrite the sysctl value, regardless of file position.
1382   0   Same behavior as above, but warn about processes that perform writes
1383       to a sysctl file descriptor when the file position is not 0.
1384   1   (default) Respect file position when writing sysctl strings. Multiple
1385       writes will append to the sysctl value buffer. Anything past the max
1386       length of the sysctl value buffer will be ignored. Writes to numeric
1387       sysctl entries must always be at file position 0 and the value must
1388       be fully contained in the buffer sent in the write syscall.
1389  ==   ======================================================================
1390
1391
1392softlockup_all_cpu_backtrace
1393============================
1394
1395This value controls the soft lockup detector thread's behavior
1396when a soft lockup condition is detected as to whether or not
1397to gather further debug information. If enabled, each cpu will
1398be issued an NMI and instructed to capture stack trace.
1399
1400This feature is only applicable for architectures which support
1401NMI.
1402
1403= ============================================
14040 Do nothing. This is the default behavior.
14051 On detection capture more debug information.
1406= ============================================
1407
1408
1409softlockup_panic
1410=================
1411
1412This parameter can be used to control whether the kernel panics
1413when a soft lockup is detected.
1414
1415= ============================================
14160 Don't panic on soft lockup.
14171 Panic on soft lockup.
1418= ============================================
1419
1420This can also be set using the softlockup_panic kernel parameter.
1421
1422
1423soft_watchdog
1424=============
1425
1426This parameter can be used to control the soft lockup detector.
1427
1428= =================================
14290 Disable the soft lockup detector.
14301 Enable the soft lockup detector.
1431= =================================
1432
1433The soft lockup detector monitors CPUs for threads that are hogging the CPUs
1434without rescheduling voluntarily, and thus prevent the 'migration/N' threads
1435from running, causing the watchdog work fail to execute. The mechanism depends
1436on the CPUs ability to respond to timer interrupts which are needed for the
1437watchdog work to be queued by the watchdog timer function, otherwise the NMI
1438watchdog — if enabled — can detect a hard lockup condition.
1439
1440
1441split_lock_mitigate (x86 only)
1442==============================
1443
1444On x86, each "split lock" imposes a system-wide performance penalty. On larger
1445systems, large numbers of split locks from unprivileged users can result in
1446denials of service to well-behaved and potentially more important users.
1447
1448The kernel mitigates these bad users by detecting split locks and imposing
1449penalties: forcing them to wait and only allowing one core to execute split
1450locks at a time.
1451
1452These mitigations can make those bad applications unbearably slow. Setting
1453split_lock_mitigate=0 may restore some application performance, but will also
1454increase system exposure to denial of service attacks from split lock users.
1455
1456= ===================================================================
14570 Disable the mitigation mode - just warns the split lock on kernel log
1458  and exposes the system to denials of service from the split lockers.
14591 Enable the mitigation mode (this is the default) - penalizes the split
1460  lockers with intentional performance degradation.
1461= ===================================================================
1462
1463
1464stack_erasing
1465=============
1466
1467This parameter can be used to control kernel stack erasing at the end
1468of syscalls for kernels built with ``CONFIG_GCC_PLUGIN_STACKLEAK``.
1469
1470That erasing reduces the information which kernel stack leak bugs
1471can reveal and blocks some uninitialized stack variable attacks.
1472The tradeoff is the performance impact: on a single CPU system kernel
1473compilation sees a 1% slowdown, other systems and workloads may vary.
1474
1475= ====================================================================
14760 Kernel stack erasing is disabled, STACKLEAK_METRICS are not updated.
14771 Kernel stack erasing is enabled (default), it is performed before
1478  returning to the userspace at the end of syscalls.
1479= ====================================================================
1480
1481
1482stop-a (SPARC only)
1483===================
1484
1485Controls Stop-A:
1486
1487= ====================================
14880 Stop-A has no effect.
14891 Stop-A breaks to the PROM (default).
1490= ====================================
1491
1492Stop-A is always enabled on a panic, so that the user can return to
1493the boot PROM.
1494
1495
1496sysrq
1497=====
1498
1499See Documentation/admin-guide/sysrq.rst.
1500
1501
1502tainted
1503=======
1504
1505Non-zero if the kernel has been tainted. Numeric values, which can be
1506ORed together. The letters are seen in "Tainted" line of Oops reports.
1507
1508======  =====  ==============================================================
1509     1  `(P)`  proprietary module was loaded
1510     2  `(F)`  module was force loaded
1511     4  `(S)`  kernel running on an out of specification system
1512     8  `(R)`  module was force unloaded
1513    16  `(M)`  processor reported a Machine Check Exception (MCE)
1514    32  `(B)`  bad page referenced or some unexpected page flags
1515    64  `(U)`  taint requested by userspace application
1516   128  `(D)`  kernel died recently, i.e. there was an OOPS or BUG
1517   256  `(A)`  an ACPI table was overridden by user
1518   512  `(W)`  kernel issued warning
1519  1024  `(C)`  staging driver was loaded
1520  2048  `(I)`  workaround for bug in platform firmware applied
1521  4096  `(O)`  externally-built ("out-of-tree") module was loaded
1522  8192  `(E)`  unsigned module was loaded
1523 16384  `(L)`  soft lockup occurred
1524 32768  `(K)`  kernel has been live patched
1525 65536  `(X)`  Auxiliary taint, defined and used by for distros
1526131072  `(T)`  The kernel was built with the struct randomization plugin
1527======  =====  ==============================================================
1528
1529See Documentation/admin-guide/tainted-kernels.rst for more information.
1530
1531Note:
1532  writes to this sysctl interface will fail with ``EINVAL`` if the kernel is
1533  booted with the command line option ``panic_on_taint=<bitmask>,nousertaint``
1534  and any of the ORed together values being written to ``tainted`` match with
1535  the bitmask declared on panic_on_taint.
1536  See Documentation/admin-guide/kernel-parameters.rst for more details on
1537  that particular kernel command line option and its optional
1538  ``nousertaint`` switch.
1539
1540threads-max
1541===========
1542
1543This value controls the maximum number of threads that can be created
1544using ``fork()``.
1545
1546During initialization the kernel sets this value such that even if the
1547maximum number of threads is created, the thread structures occupy only
1548a part (1/8th) of the available RAM pages.
1549
1550The minimum value that can be written to ``threads-max`` is 1.
1551
1552The maximum value that can be written to ``threads-max`` is given by the
1553constant ``FUTEX_TID_MASK`` (0x3fffffff).
1554
1555If a value outside of this range is written to ``threads-max`` an
1556``EINVAL`` error occurs.
1557
1558timer_migration
1559===============
1560
1561When set to a non-zero value, attempt to migrate timers away from idle cpus to
1562allow them to remain in low power states longer.
1563
1564Default is set (1).
1565
1566traceoff_on_warning
1567===================
1568
1569When set, disables tracing (see Documentation/trace/ftrace.rst) when a
1570``WARN()`` is hit.
1571
1572
1573tracepoint_printk
1574=================
1575
1576When tracepoints are sent to printk() (enabled by the ``tp_printk``
1577boot parameter), this entry provides runtime control::
1578
1579    echo 0 > /proc/sys/kernel/tracepoint_printk
1580
1581will stop tracepoints from being sent to printk(), and::
1582
1583    echo 1 > /proc/sys/kernel/tracepoint_printk
1584
1585will send them to printk() again.
1586
1587This only works if the kernel was booted with ``tp_printk`` enabled.
1588
1589See Documentation/admin-guide/kernel-parameters.rst and
1590Documentation/trace/boottime-trace.rst.
1591
1592
1593unaligned-trap
1594==============
1595
1596On architectures where unaligned accesses cause traps, and where this
1597feature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_ALLOW``; currently,
1598``arc``, ``parisc`` and ``loongarch``), controls whether unaligned traps
1599are caught and emulated (instead of failing).
1600
1601= ========================================================
16020 Do not emulate unaligned accesses.
16031 Emulate unaligned accesses. This is the default setting.
1604= ========================================================
1605
1606See also `ignore-unaligned-usertrap`_.
1607
1608
1609unknown_nmi_panic
1610=================
1611
1612The value in this file affects behavior of handling NMI. When the
1613value is non-zero, unknown NMI is trapped and then panic occurs. At
1614that time, kernel debugging information is displayed on console.
1615
1616NMI switch that most IA32 servers have fires unknown NMI up, for
1617example.  If a system hangs up, try pressing the NMI switch.
1618
1619
1620unprivileged_bpf_disabled
1621=========================
1622
1623Writing 1 to this entry will disable unprivileged calls to ``bpf()``;
1624once disabled, calling ``bpf()`` without ``CAP_SYS_ADMIN`` or ``CAP_BPF``
1625will return ``-EPERM``. Once set to 1, this can't be cleared from the
1626running kernel anymore.
1627
1628Writing 2 to this entry will also disable unprivileged calls to ``bpf()``,
1629however, an admin can still change this setting later on, if needed, by
1630writing 0 or 1 to this entry.
1631
1632If ``BPF_UNPRIV_DEFAULT_OFF`` is enabled in the kernel config, then this
1633entry will default to 2 instead of 0.
1634
1635= =============================================================
16360 Unprivileged calls to ``bpf()`` are enabled
16371 Unprivileged calls to ``bpf()`` are disabled without recovery
16382 Unprivileged calls to ``bpf()`` are disabled
1639= =============================================================
1640
1641
1642warn_limit
1643==========
1644
1645Number of kernel warnings after which the kernel should panic when
1646``panic_on_warn`` is not set. Setting this to 0 disables checking
1647the warning count. Setting this to 1 has the same effect as setting
1648``panic_on_warn=1``. The default value is 0.
1649
1650
1651watchdog
1652========
1653
1654This parameter can be used to disable or enable the soft lockup detector
1655*and* the NMI watchdog (i.e. the hard lockup detector) at the same time.
1656
1657= ==============================
16580 Disable both lockup detectors.
16591 Enable both lockup detectors.
1660= ==============================
1661
1662The soft lockup detector and the NMI watchdog can also be disabled or
1663enabled individually, using the ``soft_watchdog`` and ``nmi_watchdog``
1664parameters.
1665If the ``watchdog`` parameter is read, for example by executing::
1666
1667   cat /proc/sys/kernel/watchdog
1668
1669the output of this command (0 or 1) shows the logical OR of
1670``soft_watchdog`` and ``nmi_watchdog``.
1671
1672
1673watchdog_cpumask
1674================
1675
1676This value can be used to control on which cpus the watchdog may run.
1677The default cpumask is all possible cores, but if ``NO_HZ_FULL`` is
1678enabled in the kernel config, and cores are specified with the
1679``nohz_full=`` boot argument, those cores are excluded by default.
1680Offline cores can be included in this mask, and if the core is later
1681brought online, the watchdog will be started based on the mask value.
1682
1683Typically this value would only be touched in the ``nohz_full`` case
1684to re-enable cores that by default were not running the watchdog,
1685if a kernel lockup was suspected on those cores.
1686
1687The argument value is the standard cpulist format for cpumasks,
1688so for example to enable the watchdog on cores 0, 2, 3, and 4 you
1689might say::
1690
1691  echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask
1692
1693
1694watchdog_thresh
1695===============
1696
1697This value can be used to control the frequency of hrtimer and NMI
1698events and the soft and hard lockup thresholds. The default threshold
1699is 10 seconds.
1700
1701The softlockup threshold is (``2 * watchdog_thresh``). Setting this
1702tunable to zero will disable lockup detection altogether.
1703