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- ===================================
- Documentation for /proc/sys/kernel/
- ===================================
- .. See scripts/check-sysctl-docs to keep this up to date
- 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 :doc:`index`.
- ------------------------------------------------------------------------------
- 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``:
- .. contents:: :local:
- 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 free space drops below 2%; resume it
- if it increases to at least 4%; consider information about amount of
- free space valid for 30 seconds.
- acpi_video_flags
- ================
- See :doc:`/power/video`. This allows the video resume mode to be set,
- in a similar fashion to the ``acpi_sleep`` kernel parameter, by
- combining the following values:
- = =======
- 1 s3_bios
- 2 s3_mode
- 4 s3_beep
- = =======
- 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. The default value was 1.
- bootloader_type (x86 only)
- ==========================
- 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
- :doc:`/x86/boot` for additional information.
- bootloader_version (x86 only)
- =============================
- 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
- :doc:`/x86/boot` for additional information.
- bpf_stats_enabled
- =================
- Controls whether the kernel should collect statistics on BPF programs
- (total time spent running, number of times run...). Enabling
- statistics causes a slight reduction in performance on each program
- run. The statistics can be seen using ``bpftool``.
- = ===================================
- 0 Don't collect statistics (default).
- 1 Collect statistics.
- = ===================================
- cad_pid
- =======
- This is the pid which will be signalled on reboot (notably, by
- Ctrl-Alt-Delete). Writing a value to this file which doesn't
- correspond to a running process will result in ``-ESRCH``.
- See also `ctrl-alt-del`_.
- 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, could be changed by prctl etc)
- %f executable filename
- %E executable path
- %c maximum size of core file by resource limit RLIMIT_CORE
- %<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 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.
- firmware_config
- ===============
- See :doc:`/driver-api/firmware/fallback-mechanisms`.
- The entries in this directory allow the firmware loader helper
- fallback to be controlled:
- * ``force_sysfs_fallback``, when set to 1, forces the use of the
- fallback;
- * ``ignore_sysfs_fallback``, when set to 1, ignores any fallback.
- ftrace_dump_on_oops
- ===================
- Determines whether ``ftrace_dump()`` should be called on an oops (or
- kernel panic). This will output the contents of the ftrace buffers to
- the console. This is very useful for capturing traces that lead to
- crashes and outputting them to a serial console.
- = ===================================================
- 0 Disabled (default).
- 1 Dump buffers of all CPUs.
- 2 Dump the buffer of the CPU that triggered the oops.
- = ===================================================
- ftrace_enabled, stack_tracer_enabled
- ====================================
- See :doc:`/trace/ftrace`.
- 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 :doc:`/admin-guide/lockup-watchdogs` 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_all_cpu_backtrace
- ===========================
- If this option is set, the kernel will send an NMI to all CPUs to dump
- their backtraces when a hung task is detected. This file shows up if
- CONFIG_DETECT_HUNG_TASK and CONFIG_SMP are enabled.
- 0: Won't show all CPUs backtraces when a hung task is detected.
- This is the default behavior.
- 1: Will non-maskably interrupt all CPUs and dump their backtraces when
- a hung task is detected.
- 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 is 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.
- = =========================================================
- ignore-unaligned-usertrap
- =========================
- On architectures where unaligned accesses cause traps, and where this
- feature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_NO_WARN``;
- currently, ``arc`` and ``ia64``), controls whether all unaligned traps
- are logged.
- = =============================================================
- 0 Log all unaligned accesses.
- 1 Only warn the first time a process traps. This is the default
- setting.
- = =============================================================
- See also `unaligned-trap`_ and `unaligned-dump-stack`_. On ``ia64``,
- this allows system administrators to override the
- ``IA64_THREAD_UAC_NOPRINT`` ``prctl`` and avoid logs being flooded.
- 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 can no longer be used, and the toggle cannot be set
- back to false.
- This allows a kexec image to be loaded before disabling the syscall,
- allowing a system to set up (and later use) an image without it being
- altered.
- Generally used together with the `modules_disabled`_ sysctl.
- kptr_restrict
- =============
- This toggle indicates whether restrictions are placed on
- exposing kernel addresses via ``/proc`` and other interfaces.
- When ``kptr_restrict`` is set to 0 (the default) the address is hashed
- before printing.
- (This is the equivalent to %p.)
- When ``kptr_restrict`` is set to 1, kernel pointers printed using the
- %pK format specifier will be replaced with 0s unless the user has
- ``CAP_SYSLOG`` and effective user and group ids are equal to the real
- ids.
- This is because %pK checks are done at read() time rather than open()
- time, so if permissions are elevated between the open() and the read()
- (e.g via a setuid binary) then %pK will not leak kernel pointers to
- unprivileged users.
- Note, this is a temporary solution only.
- The correct long-term solution is to do the permission checks at
- open() time.
- Consider removing world read permissions from files that use %pK, and
- using `dmesg_restrict`_ to protect against uses of %pK in ``dmesg(8)``
- if leaking kernel pointer values to unprivileged users is a concern.
- When ``kptr_restrict`` is set to 2, kernel pointers printed using
- %pK will be replaced with 0s regardless of privileges.
- modprobe
- ========
- The full path to the usermode helper for autoloading kernel modules,
- by default "/sbin/modprobe". This binary is executed when the kernel
- requests a module. For example, if userspace passes an unknown
- filesystem type to mount(), then the kernel will automatically request
- the corresponding filesystem module by executing this usermode helper.
- This usermode helper should insert the needed module into the kernel.
- This sysctl only affects module autoloading. It has no effect on the
- ability to explicitly insert modules.
- This sysctl can be used to debug module loading requests::
- echo '#! /bin/sh' > /tmp/modprobe
- echo 'echo "$@" >> /tmp/modprobe.log' >> /tmp/modprobe
- echo 'exec /sbin/modprobe "$@"' >> /tmp/modprobe
- chmod a+x /tmp/modprobe
- echo /tmp/modprobe > /proc/sys/kernel/modprobe
- Alternatively, if this sysctl is set to the empty string, then module
- autoloading is completely disabled. The kernel will not try to
- execute a usermode helper at all, nor will it call the
- kernel_module_request LSM hook.
- If CONFIG_STATIC_USERMODEHELPER=y is set in the kernel configuration,
- then the configured static usermode helper overrides this sysctl,
- except that the empty string is still accepted to completely disable
- module autoloading as described above.
- modules_disabled
- ================
- A toggle value indicating if modules are allowed to be loaded
- in an otherwise modular kernel. This toggle defaults to off
- (0), but can be set true (1). Once true, modules can be
- neither loaded nor unloaded, and the toggle cannot be set back
- to false. Generally used with the `kexec_load_disabled`_ toggle.
- .. _msgmni:
- msgmax, msgmnb, and msgmni
- ==========================
- ``msgmax`` is the maximum size of an IPC message, in bytes. 8192 by
- default (``MSGMAX``).
- ``msgmnb`` is the maximum size of an IPC queue, in bytes. 16384 by
- default (``MSGMNB``).
- ``msgmni`` is the maximum number of IPC queues. 32000 by default
- (``MSGMNI``).
- msg_next_id, sem_next_id, and shm_next_id (System V IPC)
- ========================================================
- These three toggles allows to specify desired id for next allocated IPC
- object: message, semaphore or shared memory respectively.
- By default they are equal to -1, which means generic allocation logic.
- Possible values to set are in range {0:``INT_MAX``}.
- Notes:
- 1) kernel doesn't guarantee, that new object will have desired id. So,
- it's up to userspace, how to handle an object with "wrong" id.
- 2) Toggle with non-default value will be set back to -1 by kernel after
- successful IPC object allocation. If an IPC object allocation syscall
- fails, it is undefined if the value remains unmodified or is reset to -1.
- ngroups_max
- ===========
- Maximum number of supplementary groups, _i.e._ the maximum size which
- ``setgroups`` will accept. Exports ``NGROUPS_MAX`` from the kernel.
- nmi_watchdog
- ============
- This parameter can be used to control the NMI watchdog
- (i.e. the hard lockup detector) on x86 systems.
- = =================================
- 0 Disable the hard lockup detector.
- 1 Enable the hard lockup detector.
- = =================================
- The hard lockup detector monitors each CPU for its ability to respond to
- timer interrupts. The mechanism utilizes CPU performance counter registers
- that are programmed to generate Non-Maskable Interrupts (NMIs) periodically
- while a CPU is busy. Hence, the alternative name 'NMI watchdog'.
- The NMI watchdog is disabled by default if the kernel is running as a guest
- in a KVM virtual machine. This default can be overridden by adding::
- nmi_watchdog=1
- to the guest kernel command line (see :doc:`/admin-guide/kernel-parameters`).
- numa_balancing
- ==============
- Enables/disables automatic page fault based NUMA memory
- balancing. Memory is moved automatically to nodes
- that access it often.
- Enables/disables automatic NUMA memory balancing. On NUMA machines, there
- is a performance penalty if remote memory is accessed by a CPU. When this
- feature is enabled the kernel samples what task thread is accessing memory
- by periodically unmapping pages and later trapping a page fault. At the
- time of the page fault, it is determined if the data being accessed should
- be migrated to a local memory node.
- The unmapping of pages and trapping faults incur additional overhead that
- ideally is offset by improved memory locality but there is no universal
- guarantee. If the target workload is already bound to NUMA nodes then this
- feature should be disabled. Otherwise, if the system overhead from the
- feature is too high then the rate the kernel samples for NUMA hinting
- faults may be controlled by the `numa_balancing_scan_period_min_ms,
- numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms,
- numa_balancing_scan_size_mb`_, and numa_balancing_settle_count sysctls.
- numa_balancing_scan_period_min_ms, numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms, numa_balancing_scan_size_mb
- ===============================================================================================================================
- Automatic NUMA balancing scans tasks address space and unmaps pages to
- detect if pages are properly placed or if the data should be migrated to a
- memory node local to where the task is running. Every "scan delay" the task
- scans the next "scan size" number of pages in its address space. When the
- end of the address space is reached the scanner restarts from the beginning.
- In combination, the "scan delay" and "scan size" determine the scan rate.
- When "scan delay" decreases, the scan rate increases. The scan delay and
- hence the scan rate of every task is adaptive and depends on historical
- behaviour. If pages are properly placed then the scan delay increases,
- otherwise the scan delay decreases. The "scan size" is not adaptive but
- the higher the "scan size", the higher the scan rate.
- Higher scan rates incur higher system overhead as page faults must be
- trapped and potentially data must be migrated. However, the higher the scan
- rate, the more quickly a tasks memory is migrated to a local node if the
- workload pattern changes and minimises performance impact due to remote
- memory accesses. These sysctls control the thresholds for scan delays and
- the number of pages scanned.
- ``numa_balancing_scan_period_min_ms`` is the minimum time in milliseconds to
- scan a tasks virtual memory. It effectively controls the maximum scanning
- rate for each task.
- ``numa_balancing_scan_delay_ms`` is the starting "scan delay" used for a task
- when it initially forks.
- ``numa_balancing_scan_period_max_ms`` is the maximum time in milliseconds to
- scan a tasks virtual memory. It effectively controls the minimum scanning
- rate for each task.
- ``numa_balancing_scan_size_mb`` is how many megabytes worth of pages are
- scanned for a given scan.
- oops_all_cpu_backtrace
- ======================
- If this option is set, the kernel will send an NMI to all CPUs to dump
- their backtraces when an oops event occurs. It should be used as a last
- resort in case a panic cannot be triggered (to protect VMs running, for
- example) or kdump can't be collected. This file shows up if CONFIG_SMP
- is enabled.
- 0: Won't show all CPUs backtraces when an oops is detected.
- This is the default behavior.
- 1: Will non-maskably interrupt all CPUs and dump their backtraces when
- an oops event is detected.
- osrelease, ostype & version
- ===========================
- ::
- # cat osrelease
- 2.1.88
- # cat ostype
- Linux
- # cat version
- #5 Wed Feb 25 21:49:24 MET 1998
- The files ``osrelease`` and ``ostype`` should be clear enough.
- ``version``
- needs a little more clarification however. The '#5' means that
- this is the fifth kernel built from this source base and the
- date behind it indicates the time the kernel was built.
- The only way to tune these values is to rebuild the kernel :-)
- overflowgid & overflowuid
- =========================
- if your architecture did not always support 32-bit UIDs (i.e. arm,
- i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
- applications that use the old 16-bit UID/GID system calls, if the
- actual UID or GID would exceed 65535.
- These sysctls allow you to change the value of the fixed UID and GID.
- The default is 65534.
- panic
- =====
- The value in this file determines the behaviour of the kernel on a
- panic:
- * if zero, the kernel will loop forever;
- * if negative, the kernel will reboot immediately;
- * if positive, the kernel will reboot after the corresponding number
- of seconds.
- When you use the software watchdog, the recommended setting is 60.
- panic_on_io_nmi
- ===============
- Controls the kernel's behavior when a CPU receives an NMI caused by
- an IO error.
- = ==================================================================
- 0 Try to continue operation (default).
- 1 Panic immediately. The IO error triggered an NMI. This indicates a
- serious system condition which could result in IO data corruption.
- Rather than continuing, panicking might be a better choice. Some
- servers issue this sort of NMI when the dump button is pushed,
- and you can use this option to take a crash dump.
- = ==================================================================
- panic_on_oops
- =============
- Controls the kernel's behaviour when an oops or BUG is encountered.
- = ===================================================================
- 0 Try to continue operation.
- 1 Panic immediately. If the `panic` sysctl is also non-zero then the
- machine will be rebooted.
- = ===================================================================
- panic_on_stackoverflow
- ======================
- Controls the kernel's behavior when detecting the overflows of
- kernel, IRQ and exception stacks except a user stack.
- This file shows up if ``CONFIG_DEBUG_STACKOVERFLOW`` is enabled.
- = ==========================
- 0 Try to continue operation.
- 1 Panic immediately.
- = ==========================
- panic_on_unrecovered_nmi
- ========================
- The default Linux behaviour on an NMI of either memory or unknown is
- to continue operation. For many environments such as scientific
- computing it is preferable that the box is taken out and the error
- dealt with than an uncorrected parity/ECC error get propagated.
- A small number of systems do generate NMIs for bizarre random reasons
- such as power management so the default is off. That sysctl works like
- the existing panic controls already in that directory.
- panic_on_warn
- =============
- Calls panic() in the WARN() path when set to 1. This is useful to avoid
- a kernel rebuild when attempting to kdump at the location of a WARN().
- = ================================================
- 0 Only WARN(), default behaviour.
- 1 Call panic() after printing out WARN() location.
- = ================================================
- panic_print
- ===========
- Bitmask for printing system info when panic happens. User can chose
- combination of the following bits:
- ===== ============================================
- bit 0 print all tasks info
- bit 1 print system memory info
- bit 2 print timer info
- bit 3 print locks info if ``CONFIG_LOCKDEP`` is on
- bit 4 print ftrace buffer
- bit 5 print all printk messages in buffer
- ===== ============================================
- So for example to print tasks and memory info on panic, user can::
- echo 3 > /proc/sys/kernel/panic_print
- panic_on_rcu_stall
- ==================
- When set to 1, calls panic() after RCU stall detection messages. This
- is useful to define the root cause of RCU stalls using a vmcore.
- = ============================================================
- 0 Do not panic() when RCU stall takes place, default behavior.
- 1 panic() after printing RCU stall messages.
- = ============================================================
- perf_cpu_time_max_percent
- =========================
- Hints to the kernel how much CPU time it should be allowed to
- use to handle perf sampling events. If the perf subsystem
- is informed that its samples are exceeding this limit, it
- will drop its sampling frequency to attempt to reduce its CPU
- usage.
- Some perf sampling happens in NMIs. If these samples
- unexpectedly take too long to execute, the NMIs can become
- stacked up next to each other so much that nothing else is
- allowed to execute.
- ===== ========================================================
- 0 Disable the mechanism. Do not monitor or correct perf's
- sampling rate no matter how CPU time it takes.
- 1-100 Attempt to throttle perf's sample rate to this
- percentage of CPU. Note: the kernel calculates an
- "expected" length of each sample event. 100 here means
- 100% of that expected length. Even if this is set to
- 100, you may still see sample throttling if this
- length is exceeded. Set to 0 if you truly do not care
- how much CPU is consumed.
- ===== ========================================================
- perf_event_paranoid
- ===================
- Controls use of the performance events system by unprivileged
- users (without CAP_PERFMON). The default value is 2.
- For backward compatibility reasons access to system performance
- monitoring and observability remains open for CAP_SYS_ADMIN
- privileged processes but CAP_SYS_ADMIN usage for secure system
- performance monitoring and observability operations is discouraged
- with respect to CAP_PERFMON use cases.
- === ==================================================================
- -1 Allow use of (almost) all events by all users.
- Ignore mlock limit after perf_event_mlock_kb without
- ``CAP_IPC_LOCK``.
- >=0 Disallow ftrace function tracepoint by users without
- ``CAP_PERFMON``.
- Disallow raw tracepoint access by users without ``CAP_PERFMON``.
- >=1 Disallow CPU event access by users without ``CAP_PERFMON``.
- >=2 Disallow kernel profiling by users without ``CAP_PERFMON``.
- === ==================================================================
- perf_event_max_stack
- ====================
- Controls maximum number of stack frames to copy for (``attr.sample_type &
- PERF_SAMPLE_CALLCHAIN``) configured events, for instance, when using
- '``perf record -g``' or '``perf trace --call-graph fp``'.
- This can only be done when no events are in use that have callchains
- enabled, otherwise writing to this file will return ``-EBUSY``.
- The default value is 127.
- perf_event_mlock_kb
- ===================
- Control size of per-cpu ring buffer not counted agains mlock limit.
- The default value is 512 + 1 page
- perf_event_max_contexts_per_stack
- =================================
- Controls maximum number of stack frame context entries for
- (``attr.sample_type & PERF_SAMPLE_CALLCHAIN``) configured events, for
- instance, when using '``perf record -g``' or '``perf trace --call-graph fp``'.
- This can only be done when no events are in use that have callchains
- enabled, otherwise writing to this file will return ``-EBUSY``.
- The default value is 8.
- pid_max
- =======
- PID allocation wrap value. When the kernel's next PID value
- reaches this value, it wraps back to a minimum PID value.
- PIDs of value ``pid_max`` or larger are not allocated.
- ns_last_pid
- ===========
- The last pid allocated in the current (the one task using this sysctl
- lives in) pid namespace. When selecting a pid for a next task on fork
- kernel tries to allocate a number starting from this one.
- powersave-nap (PPC only)
- ========================
- If set, Linux-PPC will use the 'nap' mode of powersaving,
- otherwise the 'doze' mode will be used.
- ==============================================================
- printk
- ======
- The four values in printk denote: ``console_loglevel``,
- ``default_message_loglevel``, ``minimum_console_loglevel`` and
- ``default_console_loglevel`` respectively.
- These values influence printk() behavior when printing or
- logging error messages. See '``man 2 syslog``' for more info on
- the different loglevels.
- ======================== =====================================
- console_loglevel messages with a higher priority than
- this will be printed to the console
- default_message_loglevel messages without an explicit priority
- will be printed with this priority
- minimum_console_loglevel minimum (highest) value to which
- console_loglevel can be set
- default_console_loglevel default value for console_loglevel
- ======================== =====================================
- printk_delay
- ============
- Delay each printk message in ``printk_delay`` milliseconds
- Value from 0 - 10000 is allowed.
- printk_ratelimit
- ================
- Some warning messages are rate limited. ``printk_ratelimit`` specifies
- the minimum length of time between these messages (in seconds).
- The default value is 5 seconds.
- A value of 0 will disable rate limiting.
- printk_ratelimit_burst
- ======================
- While long term we enforce one message per `printk_ratelimit`_
- seconds, we do allow a burst of messages to pass through.
- ``printk_ratelimit_burst`` specifies the number of messages we can
- send before ratelimiting kicks in.
- The default value is 10 messages.
- printk_devkmsg
- ==============
- Control the logging to ``/dev/kmsg`` from userspace:
- ========= =============================================
- ratelimit default, ratelimited
- on unlimited logging to /dev/kmsg from userspace
- off logging to /dev/kmsg disabled
- ========= =============================================
- The kernel command line parameter ``printk.devkmsg=`` overrides this and is
- a one-time setting until next reboot: once set, it cannot be changed by
- this sysctl interface anymore.
- ==============================================================
- pty
- ===
- See Documentation/filesystems/devpts.rst.
- random
- ======
- This is a directory, with the following entries:
- * ``boot_id``: a UUID generated the first time this is retrieved, and
- unvarying after that;
- * ``entropy_avail``: the pool's entropy count, in bits;
- * ``poolsize``: the entropy pool size, in bits;
- * ``urandom_min_reseed_secs``: obsolete (used to determine the minimum
- number of seconds between urandom pool reseeding).
- * ``uuid``: a UUID generated every time this is retrieved (this can
- thus be used to generate UUIDs at will);
- * ``write_wakeup_threshold``: when the entropy count drops below this
- (as a number of bits), processes waiting to write to ``/dev/random``
- are woken up.
- If ``drivers/char/random.c`` is built with ``ADD_INTERRUPT_BENCH``
- defined, these additional entries are present:
- * ``add_interrupt_avg_cycles``: the average number of cycles between
- interrupts used to feed the pool;
- * ``add_interrupt_avg_deviation``: the standard deviation seen on the
- number of cycles between interrupts used to feed the pool.
- randomize_va_space
- ==================
- This option can be used to select the type of process address
- space randomization that is used in the system, for architectures
- that support this feature.
- == ===========================================================================
- 0 Turn the process address space randomization off. This is the
- default for architectures that do not support this feature anyways,
- and kernels that are booted with the "norandmaps" parameter.
- 1 Make the addresses of mmap base, stack and VDSO page randomized.
- This, among other things, implies that shared libraries will be
- loaded to random addresses. Also for PIE-linked binaries, the
- location of code start is randomized. This is the default if the
- ``CONFIG_COMPAT_BRK`` option is enabled.
- 2 Additionally enable heap randomization. This is the default if
- ``CONFIG_COMPAT_BRK`` is disabled.
- There are a few legacy applications out there (such as some ancient
- versions of libc.so.5 from 1996) that assume that brk area starts
- just after the end of the code+bss. These applications break when
- start of the brk area is randomized. There are however no known
- non-legacy applications that would be broken this way, so for most
- systems it is safe to choose full randomization.
- Systems with ancient and/or broken binaries should be configured
- with ``CONFIG_COMPAT_BRK`` enabled, which excludes the heap from process
- address space randomization.
- == ===========================================================================
- real-root-dev
- =============
- See :doc:`/admin-guide/initrd`.
- reboot-cmd (SPARC only)
- =======================
- ??? This seems to be a way to give an argument to the Sparc
- ROM/Flash boot loader. Maybe to tell it what to do after
- rebooting. ???
- sched_energy_aware
- ==================
- Enables/disables Energy Aware Scheduling (EAS). EAS starts
- automatically on platforms where it can run (that is,
- platforms with asymmetric CPU topologies and having an Energy
- Model available). If your platform happens to meet the
- requirements for EAS but you do not want to use it, change
- this value to 0.
- sched_schedstats
- ================
- Enables/disables scheduler statistics. Enabling this feature
- incurs a small amount of overhead in the scheduler but is
- useful for debugging and performance tuning.
- sched_util_clamp_min:
- =====================
- Max allowed *minimum* utilization.
- Default value is 1024, which is the maximum possible value.
- It means that any requested uclamp.min value cannot be greater than
- sched_util_clamp_min, i.e., it is restricted to the range
- [0:sched_util_clamp_min].
- sched_util_clamp_max:
- =====================
- Max allowed *maximum* utilization.
- Default value is 1024, which is the maximum possible value.
- It means that any requested uclamp.max value cannot be greater than
- sched_util_clamp_max, i.e., it is restricted to the range
- [0:sched_util_clamp_max].
- sched_util_clamp_min_rt_default:
- ================================
- By default Linux is tuned for performance. Which means that RT tasks always run
- at the highest frequency and most capable (highest capacity) CPU (in
- heterogeneous systems).
- Uclamp achieves this by setting the requested uclamp.min of all RT tasks to
- 1024 by default, which effectively boosts the tasks to run at the highest
- frequency and biases them to run on the biggest CPU.
- This knob allows admins to change the default behavior when uclamp is being
- used. In battery powered devices particularly, running at the maximum
- capacity and frequency will increase energy consumption and shorten the battery
- life.
- This knob is only effective for RT tasks which the user hasn't modified their
- requested uclamp.min value via sched_setattr() syscall.
- This knob will not escape the range constraint imposed by sched_util_clamp_min
- defined above.
- For example if
- sched_util_clamp_min_rt_default = 800
- sched_util_clamp_min = 600
- Then the boost will be clamped to 600 because 800 is outside of the permissible
- range of [0:600]. This could happen for instance if a powersave mode will
- restrict all boosts temporarily by modifying sched_util_clamp_min. As soon as
- this restriction is lifted, the requested sched_util_clamp_min_rt_default
- will take effect.
- seccomp
- =======
- See :doc:`/userspace-api/seccomp_filter`.
- sg-big-buff
- ===========
- This file shows the size of the generic SCSI (sg) buffer.
- You can't tune it just yet, but you could change it on
- compile time by editing ``include/scsi/sg.h`` and changing
- the value of ``SG_BIG_BUFF``.
- There shouldn't be any reason to change this value. If
- you can come up with one, you probably know what you
- are doing anyway :)
- shmall
- ======
- This parameter sets the total amount of shared memory pages that
- can be used system wide. Hence, ``shmall`` should always be at least
- ``ceil(shmmax/PAGE_SIZE)``.
- If you are not sure what the default ``PAGE_SIZE`` is on your Linux
- system, you can run the following command::
- # getconf PAGE_SIZE
- shmmax
- ======
- This value can be used to query and set the run time limit
- on the maximum shared memory segment size that can be created.
- Shared memory segments up to 1Gb are now supported in the
- kernel. This value defaults to ``SHMMAX``.
- shmmni
- ======
- This value determines the maximum number of shared memory segments.
- 4096 by default (``SHMMNI``).
- shm_rmid_forced
- ===============
- Linux lets you set resource limits, including how much memory one
- process can consume, via ``setrlimit(2)``. Unfortunately, shared memory
- segments are allowed to exist without association with any process, and
- thus might not be counted against any resource limits. If enabled,
- shared memory segments are automatically destroyed when their attach
- count becomes zero after a detach or a process termination. It will
- also destroy segments that were created, but never attached to, on exit
- from the process. The only use left for ``IPC_RMID`` is to immediately
- destroy an unattached segment. Of course, this breaks the way things are
- defined, so some applications might stop working. Note that this
- feature will do you no good unless you also configure your resource
- limits (in particular, ``RLIMIT_AS`` and ``RLIMIT_NPROC``). Most systems don't
- need this.
- Note that if you change this from 0 to 1, already created segments
- without users and with a dead originative process will be destroyed.
- sysctl_writes_strict
- ====================
- Control how file position affects the behavior of updating sysctl values
- via the ``/proc/sys`` interface:
- == ======================================================================
- -1 Legacy per-write sysctl value handling, with no printk warnings.
- Each write syscall must fully contain the sysctl value to be
- written, and multiple writes on the same sysctl file descriptor
- will rewrite the sysctl value, regardless of file position.
- 0 Same behavior as above, but warn about processes that perform writes
- to a sysctl file descriptor when the file position is not 0.
- 1 (default) Respect file position when writing sysctl strings. Multiple
- writes will append to the sysctl value buffer. Anything past the max
- length of the sysctl value buffer will be ignored. Writes to numeric
- sysctl entries must always be at file position 0 and the value must
- be fully contained in the buffer sent in the write syscall.
- == ======================================================================
- softlockup_all_cpu_backtrace
- ============================
- This value controls the soft lockup detector thread's behavior
- when a soft lockup condition is detected as to whether or not
- to gather further debug information. If enabled, each cpu will
- be issued an NMI and instructed to capture stack trace.
- This feature is only applicable for architectures which support
- NMI.
- = ============================================
- 0 Do nothing. This is the default behavior.
- 1 On detection capture more debug information.
- = ============================================
- softlockup_panic
- =================
- This parameter can be used to control whether the kernel panics
- when a soft lockup is detected.
- = ============================================
- 0 Don't panic on soft lockup.
- 1 Panic on soft lockup.
- = ============================================
- This can also be set using the softlockup_panic kernel parameter.
- soft_watchdog
- =============
- This parameter can be used to control the soft lockup detector.
- = =================================
- 0 Disable the soft lockup detector.
- 1 Enable the soft lockup detector.
- = =================================
- The soft lockup detector monitors CPUs for threads that are hogging the CPUs
- without rescheduling voluntarily, and thus prevent the 'watchdog/N' threads
- from running. The mechanism depends on the CPUs ability to respond to timer
- interrupts which are needed for the 'watchdog/N' threads to be woken up by
- the watchdog timer function, otherwise the NMI watchdog — if enabled — can
- detect a hard lockup condition.
- stack_erasing
- =============
- This parameter can be used to control kernel stack erasing at the end
- of syscalls for kernels built with ``CONFIG_GCC_PLUGIN_STACKLEAK``.
- That erasing reduces the information which kernel stack leak bugs
- can reveal and blocks some uninitialized stack variable attacks.
- The tradeoff is the performance impact: on a single CPU system kernel
- compilation sees a 1% slowdown, other systems and workloads may vary.
- = ====================================================================
- 0 Kernel stack erasing is disabled, STACKLEAK_METRICS are not updated.
- 1 Kernel stack erasing is enabled (default), it is performed before
- returning to the userspace at the end of syscalls.
- = ====================================================================
- stop-a (SPARC only)
- ===================
- Controls Stop-A:
- = ====================================
- 0 Stop-A has no effect.
- 1 Stop-A breaks to the PROM (default).
- = ====================================
- Stop-A is always enabled on a panic, so that the user can return to
- the boot PROM.
- sysrq
- =====
- See :doc:`/admin-guide/sysrq`.
- tainted
- =======
- Non-zero if the kernel has been tainted. Numeric values, which can be
- ORed together. The letters are seen in "Tainted" line of Oops reports.
- ====== ===== ==============================================================
- 1 `(P)` proprietary module was loaded
- 2 `(F)` module was force loaded
- 4 `(S)` SMP kernel oops on an officially SMP incapable processor
- 8 `(R)` module was force unloaded
- 16 `(M)` processor reported a Machine Check Exception (MCE)
- 32 `(B)` bad page referenced or some unexpected page flags
- 64 `(U)` taint requested by userspace application
- 128 `(D)` kernel died recently, i.e. there was an OOPS or BUG
- 256 `(A)` an ACPI table was overridden by user
- 512 `(W)` kernel issued warning
- 1024 `(C)` staging driver was loaded
- 2048 `(I)` workaround for bug in platform firmware applied
- 4096 `(O)` externally-built ("out-of-tree") module was loaded
- 8192 `(E)` unsigned module was loaded
- 16384 `(L)` soft lockup occurred
- 32768 `(K)` kernel has been live patched
- 65536 `(X)` Auxiliary taint, defined and used by for distros
- 131072 `(T)` The kernel was built with the struct randomization plugin
- ====== ===== ==============================================================
- See :doc:`/admin-guide/tainted-kernels` for more information.
- Note:
- writes to this sysctl interface will fail with ``EINVAL`` if the kernel is
- booted with the command line option ``panic_on_taint=<bitmask>,nousertaint``
- and any of the ORed together values being written to ``tainted`` match with
- the bitmask declared on panic_on_taint.
- See :doc:`/admin-guide/kernel-parameters` for more details on that particular
- kernel command line option and its optional ``nousertaint`` switch.
- threads-max
- ===========
- This value controls the maximum number of threads that can be created
- using ``fork()``.
- During initialization the kernel sets this value such that even if the
- maximum number of threads is created, the thread structures occupy only
- a part (1/8th) of the available RAM pages.
- The minimum value that can be written to ``threads-max`` is 1.
- The maximum value that can be written to ``threads-max`` is given by the
- constant ``FUTEX_TID_MASK`` (0x3fffffff).
- If a value outside of this range is written to ``threads-max`` an
- ``EINVAL`` error occurs.
- traceoff_on_warning
- ===================
- When set, disables tracing (see :doc:`/trace/ftrace`) when a
- ``WARN()`` is hit.
- tracepoint_printk
- =================
- When tracepoints are sent to printk() (enabled by the ``tp_printk``
- boot parameter), this entry provides runtime control::
- echo 0 > /proc/sys/kernel/tracepoint_printk
- will stop tracepoints from being sent to printk(), and::
- echo 1 > /proc/sys/kernel/tracepoint_printk
- will send them to printk() again.
- This only works if the kernel was booted with ``tp_printk`` enabled.
- See :doc:`/admin-guide/kernel-parameters` and
- :doc:`/trace/boottime-trace`.
- .. _unaligned-dump-stack:
- unaligned-dump-stack (ia64)
- ===========================
- When logging unaligned accesses, controls whether the stack is
- dumped.
- = ===================================================
- 0 Do not dump the stack. This is the default setting.
- 1 Dump the stack.
- = ===================================================
- See also `ignore-unaligned-usertrap`_.
- unaligned-trap
- ==============
- On architectures where unaligned accesses cause traps, and where this
- feature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_ALLOW``; currently,
- ``arc`` and ``parisc``), controls whether unaligned traps are caught
- and emulated (instead of failing).
- = ========================================================
- 0 Do not emulate unaligned accesses.
- 1 Emulate unaligned accesses. This is the default setting.
- = ========================================================
- See also `ignore-unaligned-usertrap`_.
- unknown_nmi_panic
- =================
- The value in this file affects behavior of handling NMI. When the
- value is non-zero, unknown NMI is trapped and then panic occurs. At
- that time, kernel debugging information is displayed on console.
- NMI switch that most IA32 servers have fires unknown NMI up, for
- example. If a system hangs up, try pressing the NMI switch.
- unprivileged_bpf_disabled
- =========================
- Writing 1 to this entry will disable unprivileged calls to ``bpf()``;
- once disabled, calling ``bpf()`` without ``CAP_SYS_ADMIN`` or ``CAP_BPF``
- will return ``-EPERM``. Once set to 1, this can't be cleared from the
- running kernel anymore.
- Writing 2 to this entry will also disable unprivileged calls to ``bpf()``,
- however, an admin can still change this setting later on, if needed, by
- writing 0 or 1 to this entry.
- If ``BPF_UNPRIV_DEFAULT_OFF`` is enabled in the kernel config, then this
- entry will default to 2 instead of 0.
- = =============================================================
- 0 Unprivileged calls to ``bpf()`` are enabled
- 1 Unprivileged calls to ``bpf()`` are disabled without recovery
- 2 Unprivileged calls to ``bpf()`` are disabled
- = =============================================================
- watchdog
- ========
- This parameter can be used to disable or enable the soft lockup detector
- *and* the NMI watchdog (i.e. the hard lockup detector) at the same time.
- = ==============================
- 0 Disable both lockup detectors.
- 1 Enable both lockup detectors.
- = ==============================
- The soft lockup detector and the NMI watchdog can also be disabled or
- enabled individually, using the ``soft_watchdog`` and ``nmi_watchdog``
- parameters.
- If the ``watchdog`` parameter is read, for example by executing::
- cat /proc/sys/kernel/watchdog
- the output of this command (0 or 1) shows the logical OR of
- ``soft_watchdog`` and ``nmi_watchdog``.
- watchdog_cpumask
- ================
- This value can be used to control on which cpus the watchdog may run.
- The default cpumask is all possible cores, but if ``NO_HZ_FULL`` is
- enabled in the kernel config, and cores are specified with the
- ``nohz_full=`` boot argument, those cores are excluded by default.
- Offline cores can be included in this mask, and if the core is later
- brought online, the watchdog will be started based on the mask value.
- Typically this value would only be touched in the ``nohz_full`` case
- to re-enable cores that by default were not running the watchdog,
- if a kernel lockup was suspected on those cores.
- The argument value is the standard cpulist format for cpumasks,
- so for example to enable the watchdog on cores 0, 2, 3, and 4 you
- might say::
- echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask
- watchdog_thresh
- ===============
- This value can be used to control the frequency of hrtimer and NMI
- events and the soft and hard lockup thresholds. The default threshold
- is 10 seconds.
- The softlockup threshold is (``2 * watchdog_thresh``). Setting this
- tunable to zero will disable lockup detection altogether.
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