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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 inDocumentation/admin-guide/sysctl/index.rst.
------------------------------------------------------------------------------
This file contains documentation for the sysctl files in``/proc/sys/kernel/``.
The files in this directory can be used to tune and monitormiscellaneous and general things in the operation of the Linuxkernel. Since some of the files *can* be used to screw up yoursystem, it is advisable to read both documentation and sourcebefore 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 controlits behaviour. If free space on filesystem where the log livesgoes below ``lowwater``\ % accounting suspends. If free space getsabove ``highwater``\ % accounting resumes. ``frequency`` determineshow often do we check the amount of free space (value is inseconds). Default:
::
4 2 30
That is, suspend accounting if free space drops below 2%; resume itif it increases to at least 4%; consider information about amount offree space valid for 30 seconds.
acpi_video_flags================
See Documentation/power/video.rst. This allows the video resume mode to be set,in a similar fashion to the ``acpi_sleep`` kernel parameter, bycombining the following values:
= =======1 s3_bios2 s3_mode4 s3_beep= =======
arch====
The machine hardware name, the same output as ``uname -m``(e.g. ``x86_64`` or ``aarch64``).
auto_msgmni===========
This variable has no effect and may be removed in future kernelreleases. 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 bootloaderversion. The reason for this encoding is that this used to match the``type_of_loader`` field in the kernel header; the encoding is kept forbackwards compatibility. That is, if the full bootloader type numberis 0x15 and the full version number is 0x234, this file will containthe value 340 = 0x154.
See the ``type_of_loader`` and ``ext_loader_type`` fields inDocumentation/arch/x86/boot.rst for additional information.
bootloader_version (x86 only)=============================
The complete bootloader version number. In the example above, thisfile will contain the value 564 = 0x234.
See the ``type_of_loader`` and ``ext_loader_ver`` fields inDocumentation/arch/x86/boot.rst for additional information.
bpf_stats_enabled=================
Controls whether the kernel should collect statistics on BPF programs(total time spent running, number of times run...). Enablingstatistics causes a slight reduction in performance on each programrun. 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, byCtrl-Alt-Delete). Writing a value to this file which doesn'tcorrespond 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============
``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 %C CPU the task ran on %F pidfd number %<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 topipe 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 occasionallyuseful for the collecting application to gather data about thecrashing process from its ``/proc/pid`` directory.In order to do this safely, the kernel must wait for the collectingprocess to exit, so as not to remove the crashing processes proc filesprematurely.This in turn creates the possibility that a misbehaving userspacecollecting process can block the reaping of a crashed process simplyby never exiting.This sysctl defends against that.It defines how many concurrent crashing processes may be piped to userspace applications in parallel.If this value is exceeded, then those crashing processes above thatvalue are noted via the kernel log and their cores are skipped.0 is a special value, indicating that unlimited processes may becaptured in parallel, but that no waiting will take place (i.e. thecollecting process is not guaranteed access to ``/proc/<crashingpid>/``).This value defaults to 0.
core_sort_vma=============
The default coredump writes VMAs in address order. By setting``core_sort_vma`` to 1, VMAs will be written from smallest sizeto largest size. This is known to break at least elfutils, butcan be handy when dealing with very large (and truncated)coredumps where the more useful debugging details are includedin the smaller VMAs.
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 tothe filename.
ctrl-alt-del============
When the value in this file is 0, ctrl-alt-del is trapped andsent to the ``init(1)`` program to handle a graceful restart.When, however, the value is > 0, Linux's reaction to a VulcanNerve Pinch (tm) will be an immediate reboot, without evensyncing 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 preventedfrom using ``dmesg(8)`` to view messages from the kernel's logbuffer.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 thedefault value of ``dmesg_restrict``.
domainname & hostname=====================
These files can be used to set the NIS/YP domainname and thehostname of your box in exactly the same way as the commandsdomainname 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 thehostname "darkstar" and DNS (Internet Domain Name Server)domainname "frop.org", not to be confused with the NIS (NetworkInformation Service) or YP (Yellow Pages) domainname. These twodomain names are in general different. For a detailed discussionsee the ``hostname(1)`` man page.
firmware_config===============
See Documentation/driver-api/firmware/fallback-mechanisms.rst.
The entries in this directory allow the firmware loader helperfallback 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 (orkernel panic). This will output the contents of the ftrace buffers tothe console. This is very useful for capturing traces that lead tocrashes and outputting them to a serial console.
======================= ===========================================0 Disabled (default).1 Dump buffers of all CPUs.2(orig_cpu) Dump the buffer of the CPU that triggered the oops.<instance> Dump the specific instance buffer on all CPUs.<instance>=2(orig_cpu) Dump the specific instance buffer on the CPU that triggered the oops.======================= ===========================================
Multiple instance dump is also supported, and instances are separatedby commas. If global buffer also needs to be dumped, please specifythe dump mode (1/2/orig_cpu) first for global buffer.
So for example to dump "foo" and "bar" instance buffer on all CPUs,user can::
echo "foo,bar" > /proc/sys/kernel/ftrace_dump_on_oops
To dump global buffer and "foo" instance buffer on allCPUs along with the "bar" instance buffer on CPU that triggered theoops, user can::
echo "1,foo,bar=2" > /proc/sys/kernel/ftrace_dump_on_oops
ftrace_enabled, stack_tracer_enabled====================================
See Documentation/trace/ftrace.rst.
hardlockup_all_cpu_backtrace============================
This value controls the hard lockup detector behavior when a hardlockup condition is detected as to whether or not to gather furtherdebug information. If enabled, arch-specific all-CPU stack dumpingwill 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 panicswhen 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 ``CONFIG_UEVENT_HELPER_PATH``, which in turn defaultsto the empty string.
This file only exists when ``CONFIG_UEVENT_HELPER`` is enabled. Mostmodern systems rely exclusively on the netlink-based uevent source anddon't need this.
hung_task_all_cpu_backtrace===========================
If this option is set, the kernel will send an NMI to all CPUs to dumptheir backtraces when a hung task is detected. This file shows up ifCONFIG_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 whena hung task is detected.
hung_task_panic===============
When set to a non-zero value, a kernel panic will be triggered if thenumber of hung tasks found during a single scan reaches this value.This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
= =======================================================0 Continue operation. This is the default behavior.N Panic when N hung tasks are found during a single scan.= =======================================================
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_detect_count======================
Indicates the total number of tasks that have been detected as hung sincethe system boot or since the counter was reset. The counter is zeroed whena value of 0 is written.
This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
hung_task_sys_info==================A comma separated list of extra system information to be dumped whenhung task is detected, for example, "tasks,mem,timers,locks,...".Refer 'panic_sys_info' section below for more details.
hung_task_timeout_secs======================
When a task in D state did not get scheduledfor 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 checkinginterval.
Possible values to set are in range {0:``LONG_MAX``/``HZ``}.
hung_task_warnings==================
The maximum number of warnings to report. During a check intervalif 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 thisfeature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_NO_WARN``;currently, ``arc``, ``parisc`` and ``loongarch``), controls whether allunaligned 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`_.
io_uring_disabled=================
Prevents all processes from creating new io_uring instances. Enabling thisshrinks the kernel's attack surface.
= ======================================================================0 All processes can create io_uring instances as normal. This is the default setting.1 io_uring creation is disabled (io_uring_setup() will fail with -EPERM) for unprivileged processes not in the io_uring_group group. Existing io_uring instances can still be used. See the documentation for io_uring_group for more information.2 io_uring creation is disabled for all processes. io_uring_setup() always fails with -EPERM. Existing io_uring instances can still be used.= ======================================================================
io_uring_group==============
When io_uring_disabled is set to 1, a process must either beprivileged (CAP_SYS_ADMIN) or be in the io_uring_group group in orderto create an io_uring instance. If io_uring_group is set to -1 (thedefault), only processes with the CAP_SYS_ADMIN capability may createio_uring instances.
kernel_sys_info===============A comma separated list of extra system information to be dumped whensoft/hard lockup is detected, for example, "tasks,mem,timers,locks,...".Refer 'panic_sys_info' section below for more details.
It serves as the default kernel control knob, which will take effectwhen a kernel module calls sys_info() with parameter==0.
kexec_load_disabled===================
A toggle indicating if the syscalls ``kexec_load`` and``kexec_file_load`` have been disabled.This value defaults to 0 (false: ``kexec_*load`` enabled), but can beset to 1 (true: ``kexec_*load`` disabled).Once true, kexec can no longer be used, and the toggle cannot be setback 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 beingaltered.Generally used together with the `modules_disabled`_ sysctl.
kexec_load_limit_panic======================
This parameter specifies a limit to the number of times the syscalls``kexec_load`` and ``kexec_file_load`` can be called with a crashimage. It can only be set with a more restrictive value than thecurrent one.
== ======================================================-1 Unlimited calls to kexec. This is the default setting.N Number of calls left.== ======================================================
kexec_load_limit_reboot=======================
Similar functionality as ``kexec_load_limit_panic``, but for a normalimage.
kptr_restrict=============
This toggle indicates whether restrictions are placed onexposing kernel addresses via ``/proc`` and other interfaces.
When ``kptr_restrict`` is set to 0 (the default) the address is hashedbefore 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 realids.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 tounprivileged users.Note, this is a temporary solution only.The correct long-term solution is to do the permission checks atopen() time.Consider removing world read permissions from files that use %pK, andusing `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.
For disabling these security restrictions early at boot time (and oncefor all), use the ``hash_pointers`` boot parameter instead.
softlockup_sys_info & hardlockup_sys_info=========================================A comma separated list of extra system information to be dumped whensoft/hard lockup is detected, for example, "tasks,mem,timers,locks,...".Refer 'panic_sys_info' section below for more details.
modprobe========
The full path to the usermode helper for autoloading kernel modules,by default ``CONFIG_MODPROBE_PATH``, which in turn defaults to"/sbin/modprobe". This binary is executed when the kernel requests amodule. For example, if userspace passes an unknown filesystem typeto mount(), then the kernel will automatically request thecorresponding 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 theability 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 moduleautoloading is completely disabled. The kernel will not try toexecute a usermode helper at all, nor will it call thekernel_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 disablemodule autoloading as described above.
modules_disabled================
A toggle value indicating if modules are allowed to be loadedin an otherwise modular kernel. This toggle defaults to off(0), but can be set true (1). Once true, modules can beneither loaded nor unloaded, and the toggle cannot be set backto 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 bydefault (``MSGMAX``).
``msgmnb`` is the maximum size of an IPC queue, in bytes. 16384 bydefault (``MSGMNB``).
``msgmni`` is the maximum number of IPC queues. 32000 by default(``MSGMNI``).
All of these parameters are set per ipc namespace. The maximum number of bytesin POSIX message queues is limited by ``RLIMIT_MSGQUEUE``. This limit isrespected hierarchically in the each user namespace.
msg_next_id, sem_next_id, and shm_next_id (System V IPC)========================================================
These three toggles allows to specify desired id for next allocated IPCobject: 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 totimer interrupts. The mechanism utilizes CPU performance counter registersthat are programmed to generate Non-Maskable Interrupts (NMIs) periodicallywhile a CPU is busy. Hence, the alternative name 'NMI watchdog'.
The NMI watchdog is disabled by default if the kernel is running as a guestin a KVM virtual machine. This default can be overridden by adding::
nmi_watchdog=1
to the guest kernel command line (seeDocumentation/admin-guide/kernel-parameters.rst).
nmi_wd_lpm_factor (PPC only)============================
Factor to apply to the NMI watchdog timeout (only when ``nmi_watchdog`` isset to 1). This factor represents the percentage added to``watchdog_thresh`` when calculating the NMI watchdog timeout during anLPM. The soft lockup timeout is not impacted.
A value of 0 means no change. The default value is 200 meaning the NMIwatchdog is set to 30s (based on ``watchdog_thresh`` equal to 10).
numa_balancing==============
Enables/disables and configures automatic page fault based NUMA memorybalancing. Memory is moved automatically to nodes that access it often.The value to set can be the result of ORing the following:
= =================================0 NUMA_BALANCING_DISABLED1 NUMA_BALANCING_NORMAL2 NUMA_BALANCING_MEMORY_TIERING= =================================
Or NUMA_BALANCING_NORMAL to optimize page placement among differentNUMA nodes to reduce remote accessing. On NUMA machines, there is aperformance penalty if remote memory is accessed by a CPU. When thisfeature is enabled the kernel samples what task thread is accessingmemory by periodically unmapping pages and later trapping a pagefault. At the time of the page fault, it is determined if the databeing accessed should be migrated to a local memory node.
The unmapping of pages and trapping faults incur additional overhead thatideally is offset by improved memory locality but there is no universalguarantee. If the target workload is already bound to NUMA nodes then thisfeature should be disabled.
Or NUMA_BALANCING_MEMORY_TIERING to optimize page placement amongdifferent types of memory (represented as different NUMA nodes) toplace the hot pages in the fast memory. This is implemented based onunmapping and page fault too.
numa_balancing_promote_rate_limit_MBps======================================
Too high promotion/demotion throughput between different memory typesmay hurt application latency. This can be used to rate limit thepromotion throughput. The per-node max promotion throughput in MB/swill be limited to be no more than the set value.
A rule of thumb is to set this to less than 1/10 of the PMEM nodewrite bandwidth.
oops_all_cpu_backtrace======================
If this option is set, the kernel will send an NMI to all CPUs to dumptheir backtraces when an oops event occurs. It should be used as a lastresort in case a panic cannot be triggered (to protect VMs running, forexample) or kdump can't be collected. This file shows up if CONFIG_SMPis 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 whenan oops event is detected.
oops_limit==========
Number of kernel oopses after which the kernel should panic when``panic_on_oops`` is not set. Setting this to 0 disables checkingthe count. Setting this to 1 has the same effect as setting``panic_on_oops=1``. The default value is 10000.
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 thatthis is the fifth kernel built from this source base and thedate 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 toapplications that use the old 16-bit UID/GID system calls, if theactual 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 apanic:
* 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 byan 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 ofkernel, 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 isto continue operation. For many environments such as scientificcomputing it is preferable that the box is taken out and the errordealt with than an uncorrected parity/ECC error get propagated.
A small number of systems do generate NMIs for bizarre random reasonssuch as power management so the default is off. That sysctl works likethe existing panic controls already in that directory.
panic_on_warn=============
Calls panic() in the WARN() path when set to 1. This is useful to avoida 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 chosecombination of the following bits:
===== ============================================bit 0 print all tasks infobit 1 print system memory infobit 2 print timer infobit 3 print locks info if ``CONFIG_LOCKDEP`` is onbit 4 print ftrace bufferbit 5 replay all kernel messages on consoles at the end of panicbit 6 print all CPUs backtrace (if available in the arch)bit 7 print only tasks in uninterruptible (blocked) state===== ============================================
So for example to print tasks and memory info on panic, user can::
echo 3 > /proc/sys/kernel/panic_print
panic_sys_info==============
A comma separated list of extra information to be dumped on panic,for example, "tasks,mem,timers,...". It is a human readable alternativeto 'panic_print'. Possible values are:
============= ===================================================tasks print all tasks infomem print system memory infotimers print timers infolocks print locks info if CONFIG_LOCKDEP is onftrace print ftrace bufferall_bt print all CPUs backtrace (if available in the arch)blocked_tasks print only tasks in uninterruptible (blocked) state============= ===================================================
panic_on_rcu_stall==================
When set to 1, calls panic() after RCU stall detection messages. Thisis 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.= ============================================================
max_rcu_stall_to_panic======================
When ``panic_on_rcu_stall`` is set to 1, this value determines thenumber of times that RCU can stall before panic() is called.
When ``panic_on_rcu_stall`` is set to 0, this value is has no effect.
perf_cpu_time_max_percent=========================
Hints to the kernel how much CPU time it should be allowed touse to handle perf sampling events. If the perf subsystemis informed that its samples are exceeding this limit, itwill drop its sampling frequency to attempt to reduce its CPUusage.
Some perf sampling happens in NMIs. If these samplesunexpectedly take too long to execute, the NMIs can becomestacked up next to each other so much that nothing else isallowed 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 unprivilegedusers (without CAP_PERFMON). The default value is 2.
For backward compatibility reasons access to system performancemonitoring and observability remains open for CAP_SYS_ADMINprivileged processes but CAP_SYS_ADMIN usage for secure systemperformance monitoring and observability operations is discouragedwith 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 callchainsenabled, 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 against 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, forinstance, when using '``perf record -g``' or '``perf trace --call-graph fp``'.
This can only be done when no events are in use that have callchainsenabled, otherwise writing to this file will return ``-EBUSY``.
The default value is 8.
perf_user_access (arm64 and riscv only)=======================================
Controls user space access for reading perf event counters.
* for arm64 The default value is 0 (access disabled).
When set to 1, user space can read performance monitor counter registers directly.
See Documentation/arch/arm64/perf.rst for more information.
* for riscv When set to 0, user space access is disabled.
The default value is 1, user space can read performance monitor counter registers through perf, any direct access without perf intervention will trigger an illegal instruction.
When set to 2, which enables legacy mode (user space has direct access to cycle and insret CSRs only). Note that this legacy value is deprecated and will be removed once all user space applications are fixed.
Note that the time CSR is always directly accessible to all modes.
pid_max=======
PID allocation wrap value. When the kernel's next PID valuereaches 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 sysctllives in) pid namespace. When selecting a pid for a next task on forkkernel 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 orlogging error messages. See '``man 2 syslog``' for more info onthe different loglevels.
======================== =====================================console_loglevel messages with a higher priority than this will be printed to the consoledefault_message_loglevel messages without an explicit priority will be printed with this priorityminimum_console_loglevel minimum (highest) value to which console_loglevel can be setdefault_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`` specifiesthe 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 cansend before ratelimiting kicks in. After `printk_ratelimit`_ secondshave elapsed, another burst of messages may be sent.
The default value is 10 messages.
printk_devkmsg==============
Control the logging to ``/dev/kmsg`` from userspace:
========= =============================================ratelimit default, ratelimitedon unlimited logging to /dev/kmsg from userspaceoff logging to /dev/kmsg disabled========= =============================================
The kernel command line parameter ``printk.devkmsg=`` overrides this and isa one-time setting until next reboot: once set, it cannot be changed bythis 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;
* ``uuid``: a UUID generated every time this is retrieved (this can thus be used to generate UUIDs at will);
* ``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). This file is writable for compatibility purposes, but writing to it has no effect on any RNG behavior;
* ``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. This file is writable for compatibility purposes, but writing to it has no effect on any RNG behavior.
randomize_va_space==================
This option can be used to select the type of process addressspace randomization that is used in the system, for architecturesthat 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.== ===========================================================================
reboot-cmd (SPARC only)=======================
??? This seems to be a way to give an argument to the SparcROM/Flash boot loader. Maybe to tell it what to do afterrebooting. ???
sched_energy_aware==================
Enables/disables Energy Aware Scheduling (EAS). EAS startsautomatically on platforms where it can run (that is,platforms with asymmetric CPU topologies and having an EnergyModel available). If your platform happens to meet therequirements for EAS but you do not want to use it, changethis value to 0. On Non-EAS platforms, write operation fails andread doesn't return anything.
task_delayacct===============
Enables/disables task delay accounting (seeDocumentation/accounting/delay-accounting.rst. Enabling this feature incursa small amount of overhead in the scheduler but is useful for debuggingand performance tuning. It is required by some tools such as iotop.
sched_schedstats================
Enables/disables scheduler statistics. Enabling this featureincurs a small amount of overhead in the scheduler but isuseful 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 thansched_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 thansched_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 runat the highest frequency and most capable (highest capacity) CPU (inheterogeneous systems).
Uclamp achieves this by setting the requested uclamp.min of all RT tasks to1024 by default, which effectively boosts the tasks to run at the highestfrequency and biases them to run on the biggest CPU.
This knob allows admins to change the default behavior when uclamp is beingused. In battery powered devices particularly, running at the maximumcapacity and frequency will increase energy consumption and shorten the batterylife.
This knob is only effective for RT tasks which the user hasn't modified theirrequested uclamp.min value via sched_setattr() syscall.
This knob will not escape the range constraint imposed by sched_util_clamp_mindefined 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 permissiblerange of [0:600]. This could happen for instance if a powersave mode willrestrict all boosts temporarily by modifying sched_util_clamp_min. As soon asthis restriction is lifted, the requested sched_util_clamp_min_rt_defaultwill take effect.
seccomp=======
See Documentation/userspace-api/seccomp_filter.rst.
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 oncompile time by editing ``include/scsi/sg.h`` and changingthe value of ``SG_BIG_BUFF``.
There shouldn't be any reason to change this value. Ifyou can come up with one, you probably know what youare doing anyway :)
shmall======
This parameter sets the total amount of shared memory pages that can be usedinside ipc namespace. The shared memory pages counting occurs for each ipcnamespace separately and is not inherited. Hence, ``shmall`` should always be atleast ``ceil(shmmax/PAGE_SIZE)``.
If you are not sure what the default ``PAGE_SIZE`` is on your Linuxsystem, you can run the following command::
# getconf PAGE_SIZE
To reduce or disable the ability to allocate shared memory, you must create anew ipc namespace, set this parameter to the required value and prohibit thecreation of a new ipc namespace in the current user namespace or cgroups canbe used.
shmmax======
This value can be used to query and set the run time limiton the maximum shared memory segment size that can be created.Shared memory segments up to 1Gb are now supported in thekernel. 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 oneprocess can consume, via ``setrlimit(2)``. Unfortunately, shared memorysegments are allowed to exist without association with any process, andthus might not be counted against any resource limits. If enabled,shared memory segments are automatically destroyed when their attachcount becomes zero after a detach or a process termination. It willalso destroy segments that were created, but never attached to, on exitfrom the process. The only use left for ``IPC_RMID`` is to immediatelydestroy an unattached segment. Of course, this breaks the way things aredefined, so some applications might stop working. Note that thisfeature will do you no good unless you also configure your resourcelimits (in particular, ``RLIMIT_AS`` and ``RLIMIT_NPROC``). Most systems don'tneed this.
Note that if you change this from 0 to 1, already created segmentswithout users and with a dead originative process will be destroyed.
sysctl_writes_strict====================
Control how file position affects the behavior of updating sysctl valuesvia 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 behaviorwhen a soft lockup condition is detected as to whether or notto gather further debug information. If enabled, each cpu willbe issued an NMI and instructed to capture stack trace.
This feature is only applicable for architectures which supportNMI.
= ============================================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 panicswhen 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 CPUswithout rescheduling voluntarily, and thus prevent the 'migration/N' threadsfrom running, causing the watchdog work fail to execute. The mechanism dependson the CPUs ability to respond to timer interrupts which are needed for thewatchdog work to be queued by the watchdog timer function, otherwise the NMIwatchdog — if enabled — can detect a hard lockup condition.
split_lock_mitigate (x86 only)==============================
On x86, each "split lock" imposes a system-wide performance penalty. On largersystems, large numbers of split locks from unprivileged users can result indenials of service to well-behaved and potentially more important users.
The kernel mitigates these bad users by detecting split locks and imposingpenalties: forcing them to wait and only allowing one core to execute splitlocks at a time.
These mitigations can make those bad applications unbearably slow. Settingsplit_lock_mitigate=0 may restore some application performance, but will alsoincrease system exposure to denial of service attacks from split lock users.
= ===================================================================0 Disable the mitigation mode - just warns the split lock on kernel log and exposes the system to denials of service from the split lockers.1 Enable the mitigation mode (this is the default) - penalizes the split lockers with intentional performance degradation.= ===================================================================
stack_erasing=============
This parameter can be used to control kernel stack erasing at the endof syscalls for kernels built with ``CONFIG_KSTACK_ERASE``.
That erasing reduces the information which kernel stack leak bugscan reveal and blocks some uninitialized stack variable attacks.The tradeoff is the performance impact: on a single CPU system kernelcompilation sees a 1% slowdown, other systems and workloads may vary.
= ====================================================================0 Kernel stack erasing is disabled, KSTACK_ERASE_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 tothe boot PROM.
sysrq=====
See Documentation/admin-guide/sysrq.rst.
tainted=======
Non-zero if the kernel has been tainted. Numeric values, which can beORed 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)` kernel running on an out of specification system 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 distros131072 `(T)` The kernel was built with the struct randomization plugin====== ===== ==============================================================
See Documentation/admin-guide/tainted-kernels.rst 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 Documentation/admin-guide/kernel-parameters.rst 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 createdusing ``fork()``.
During initialization the kernel sets this value such that even if themaximum number of threads is created, the thread structures occupy onlya 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 theconstant ``FUTEX_TID_MASK`` (0x3fffffff).
If a value outside of this range is written to ``threads-max`` an``EINVAL`` error occurs.
timer_migration===============
When set to a non-zero value, attempt to migrate timers away from idle cpus toallow them to remain in low power states longer.
Default is set (1).
traceoff_on_warning===================
When set, disables tracing (see Documentation/trace/ftrace.rst) 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 Documentation/admin-guide/kernel-parameters.rst andDocumentation/trace/boottime-trace.rst.
unaligned-trap==============
On architectures where unaligned accesses cause traps, and where thisfeature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_ALLOW``; currently,``arc``, ``parisc`` and ``loongarch``), controls whether unaligned trapsare 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 thevalue is non-zero, unknown NMI is trapped and then panic occurs. Atthat time, kernel debugging information is displayed on console.
NMI switch that most IA32 servers have fires unknown NMI up, forexample. 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 therunning 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, bywriting 0 or 1 to this entry.
If ``BPF_UNPRIV_DEFAULT_OFF`` is enabled in the kernel config, then thisentry will default to 2 instead of 0.
= =============================================================0 Unprivileged calls to ``bpf()`` are enabled1 Unprivileged calls to ``bpf()`` are disabled without recovery2 Unprivileged calls to ``bpf()`` are disabled= =============================================================
warn_limit==========
Number of kernel warnings after which the kernel should panic when``panic_on_warn`` is not set. Setting this to 0 disables checkingthe warning count. Setting this to 1 has the same effect as setting``panic_on_warn=1``. The default value is 0.
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 orenabled 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`` isenabled 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 laterbrought online, the watchdog will be started based on the mask value.
Typically this value would only be touched in the ``nohz_full`` caseto 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 youmight say::
echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask
watchdog_thresh===============
This value can be used to control the frequency of hrtimer and NMIevents and the soft and hard lockup thresholds. The default thresholdis 10 seconds.
The softlockup threshold is (``2 * watchdog_thresh``). Setting thistunable to zero will disable lockup detection altogether.