path: root/Documentation/admin-guide/sysctl/kernel.rst

===================================
Documentation for /proc/sys/kernel/
===================================

kernel version 2.2.10

Copyright (c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>

Copyright (c) 2009,        Shen Feng<shen@cn.fujitsu.com>

For general info and legal blurb, please look in index.rst.

------------------------------------------------------------------------------

This file contains documentation for the sysctl files in
/proc/sys/kernel/ and is valid for Linux kernel version 2.2.

The files in this directory can be used to tune and monitor
miscellaneous and general things in the operation of the Linux
kernel. Since some of the files _can_ be used to screw up your
system, it is advisable to read both documentation and source
before actually making adjustments.

Currently, these files might (depending on your configuration)
show up in /proc/sys/kernel:

- acct
- acpi_video_flags
- auto_msgmni
- bootloader_type	     [ X86 only ]
- bootloader_version	     [ X86 only ]
- cap_last_cap
- core_pattern
- core_pipe_limit
- core_uses_pid
- ctrl-alt-del
- dmesg_restrict
- domainname
- hostname
- hotplug
- hardlockup_all_cpu_backtrace
- hardlockup_panic
- hung_task_panic
- hung_task_check_count
- hung_task_timeout_secs
- hung_task_check_interval_secs
- hung_task_warnings
- hyperv_record_panic_msg
- kexec_load_disabled
- kptr_restrict
- l2cr                        [ PPC only ]
- modprobe                    ==> Documentation/debugging-modules.txt
- modules_disabled
- msg_next_id		      [ sysv ipc ]
- msgmax
- msgmnb
- msgmni
- nmi_watchdog
- osrelease
- ostype
- overflowgid
- overflowuid
- panic
- panic_on_oops
- panic_on_stackoverflow
- panic_on_unrecovered_nmi
- panic_on_warn
- panic_print
- panic_on_rcu_stall
- perf_cpu_time_max_percent
- perf_event_paranoid
- perf_event_max_stack
- perf_event_mlock_kb
- perf_event_max_contexts_per_stack
- pid_max
- powersave-nap               [ PPC only ]
- printk
- printk_delay
- printk_ratelimit
- printk_ratelimit_burst
- pty                         ==> Documentation/filesystems/devpts.txt
- randomize_va_space
- real-root-dev               ==> Documentation/admin-guide/initrd.rst
- reboot-cmd                  [ SPARC only ]
- rtsig-max
- rtsig-nr
- sched_energy_aware
- seccomp/                    ==> Documentation/userspace-api/seccomp_filter.rst
- sem
- sem_next_id		      [ sysv ipc ]
- sg-big-buff                 [ generic SCSI device (sg) ]
- shm_next_id		      [ sysv ipc ]
- shm_rmid_forced
- shmall
- shmmax                      [ sysv ipc ]
- shmmni
- softlockup_all_cpu_backtrace
- soft_watchdog
- stack_erasing
- stop-a                      [ SPARC only ]
- sysrq                       ==> Documentation/admin-guide/sysrq.rst
- sysctl_writes_strict
- tainted                     ==> Documentation/admin-guide/tainted-kernels.rst
- threads-max
- unknown_nmi_panic
- watchdog
- watchdog_thresh
- version


acct:
=====

highwater lowwater frequency

If BSD-style process accounting is enabled these values control
its behaviour. If free space on filesystem where the log lives
goes below <lowwater>% accounting suspends. If free space gets
above <highwater>% accounting resumes. <Frequency> determines
how often do we check the amount of free space (value is in
seconds). Default:
4 2 30
That is, suspend accounting if there left <= 2% free; resume it
if we got >=4%; consider information about amount of free space
valid for 30 seconds.


acpi_video_flags:
=================

flags

See Doc*/kernel/power/video.txt, it allows mode of video boot to be
set during run time.


auto_msgmni:
============

This variable has no effect and may be removed in future kernel
releases. Reading it always returns 0.
Up to Linux 3.17, it enabled/disabled automatic recomputing of msgmni
upon memory add/remove or upon ipc namespace creation/removal.
Echoing "1" into this file enabled msgmni automatic recomputing.
Echoing "0" turned it off. auto_msgmni default value was 1.


bootloader_type:
================

x86 bootloader identification

This gives the bootloader type number as indicated by the bootloader,
shifted left by 4, and OR'd with the low four bits of the bootloader
version.  The reason for this encoding is that this used to match the
type_of_loader field in the kernel header; the encoding is kept for
backwards compatibility.  That is, if the full bootloader type number
is 0x15 and the full version number is 0x234, this file will contain
the value 340 = 0x154.

See the type_of_loader and ext_loader_type fields in
Documentation/x86/boot.rst for additional information.


bootloader_version:
===================

x86 bootloader version

The complete bootloader version number.  In the example above, this
file will contain the value 564 = 0x234.

See the type_of_loader and ext_loader_ver fields in
Documentation/x86/boot.rst for additional information.


cap_last_cap:
=============

Highest valid capability of the running kernel.  Exports
CAP_LAST_CAP from the kernel.


core_pattern:
=============

core_pattern is used to specify a core dumpfile pattern name.

* max length 127 characters; default value is "core"
* core_pattern is used as a pattern template for the output filename;
  certain string patterns (beginning with '%') are substituted with
  their actual values.
* backward compatibility with core_uses_pid:

	If core_pattern does not include "%p" (default does not)
	and core_uses_pid is set, then .PID will be appended to
	the filename.

* corename format specifiers::

	%<NUL>	'%' is dropped
	%%	output one '%'
	%p	pid
	%P	global pid (init PID namespace)
	%i	tid
	%I	global tid (init PID namespace)
	%u	uid (in initial user namespace)
	%g	gid (in initial user namespace)
	%d	dump mode, matches PR_SET_DUMPABLE and
		/proc/sys/fs/suid_dumpable
	%s	signal number
	%t	UNIX time of dump
	%h	hostname
	%e	executable filename (may be shortened)
	%E	executable path
	%<OTHER> both are dropped

* If the first character of the pattern is a '|', the kernel will treat
  the rest of the pattern as a command to run.  The core dump will be
  written to the standard input of that program instead of to a file.


core_pipe_limit:
================

This sysctl is only applicable when core_pattern is configured to pipe
core files to a user space helper (when the first character of
core_pattern is a '|', see above).  When collecting cores via a pipe
to an application, it is occasionally useful for the collecting
application to gather data about the crashing process from its
/proc/pid directory.  In order to do this safely, the kernel must wait
for the collecting process to exit, so as not to remove the crashing
processes proc files prematurely.  This in turn creates the
possibility that a misbehaving userspace collecting process can block
the reaping of a crashed process simply by never exiting.  This sysctl
defends against that.  It defines how many concurrent crashing
processes may be piped to user space applications in parallel.  If
this value is exceeded, then those crashing processes above that value
are noted via the kernel log and their cores are skipped.  0 is a
special value, indicating that unlimited processes may be captured in
parallel, but that no waiting will take place (i.e. the collecting
process is not guaranteed access to /proc/<crashing pid>/).  This
value defaults to 0.


core_uses_pid:
==============

The default coredump filename is "core".  By setting
core_uses_pid to 1, the coredump filename becomes core.PID.
If core_pattern does not include "%p" (default does not)
and core_uses_pid is set, then .PID will be appended to
the filename.


ctrl-alt-del:
=============

When the value in this file is 0, ctrl-alt-del is trapped and
sent to the init(1) program to handle a graceful restart.
When, however, the value is > 0, Linux's reaction to a Vulcan
Nerve Pinch (tm) will be an immediate reboot, without even
syncing its dirty buffers.

Note:
  when a program (like dosemu) has the keyboard in 'raw'
  mode, the ctrl-alt-del is intercepted by the program before it
  ever reaches the kernel tty layer, and it's up to the program
  to decide what to do with it.


dmesg_restrict:
===============

This toggle indicates whether unprivileged users are prevented
from using dmesg(8) to view messages from the kernel's log buffer.
When dmesg_restrict is set to (0) there are no restrictions. When
dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
dmesg(8).

The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
default value of dmesg_restrict.


domainname & hostname:
======================

These files can be used to set the NIS/YP domainname and the
hostname of your box in exactly the same way as the commands
domainname and hostname, i.e.::

	# echo "darkstar" > /proc/sys/kernel/hostname
	# echo "mydomain" > /proc/sys/kernel/domainname

has the same effect as::

	# hostname "darkstar"
	# domainname "mydomain"

Note, however, that the classic darkstar.frop.org has the
hostname "darkstar" and DNS (Internet Domain Name Server)
domainname "frop.org", not to be confused with the NIS (Network
Information Service) or YP (Yellow Pages) domainname. These two
domain names are in general different. For a detailed discussion
see the hostname(1) man page.


hardlockup_all_cpu_backtrace:
=============================

This value controls the hard lockup detector behavior when a hard
lockup condition is detected as to whether or not to gather further
debug information. If enabled, arch-specific all-CPU stack dumping
will be initiated.

0: do nothing. This is the default behavior.

1: on detection capture more debug information.


hardlockup_panic:
=================

This parameter can be used to control whether the kernel panics
when a hard lockup is detected.

   0 - don't panic on hard lockup
   1 - panic on hard lockup

See Documentation/admin-guide/lockup-watchdogs.rst for more information.  This can
also be set using the nmi_watchdog kernel parameter.


hotplug:
========

Path for the hotplug policy agent.
Default value is "/sbin/hotplug".


hung_task_panic:
================

Controls the kernel's behavior when a hung task is detected.
This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.

0: continue operation. This is the default behavior.

1: panic immediately.


hung_task_check_count:
======================

The upper bound on the number of tasks that are checked.
This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.


hung_task_timeout_secs:
=======================

When a task in D state did not get scheduled
for more than this value report a warning.
This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.

0: means infinite timeout - no checking done.

Possible values to set are in range {0..LONG_MAX/HZ}.


hung_task_check_interval_secs:
==============================

Hung task check interval. If hung task checking is enabled
(see hung_task_timeout_secs), the check is done every
hung_task_check_interval_secs seconds.
This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.

0 (default): means use hung_task_timeout_secs as checking interval.
Possible values to set are in range {0..LONG_MAX/HZ}.


hung_task_warnings:
===================

The maximum number of warnings to report. During a check interval
if a hung task is detected, this value is decreased by 1.
When this value reaches 0, no more warnings will be reported.
This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.

-1: report an infinite number of warnings.


hyperv_record_panic_msg:
========================

Controls whether the panic kmsg data should be reported to Hyper-V.

0: do not report panic kmsg data.

1: report the panic kmsg data. This is the default behavior.


kexec_load_disabled:
====================

A toggle indicating if the kexec_load syscall has been disabled. This
value defaults to 0 (false: kexec_load enabled), but can be set to 1
(true: kexec_load disabled). Once true, kexec