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diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst new file mode 100644 index 000000000..6b0c7b650 --- /dev/null +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -0,0 +1,1542 @@ +=================================== +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. + + +oops_limit +========== + +Number of kernel oopses after which the kernel should panic when +``panic_on_oops`` is not set. Setting this to 0 disables checking +the 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 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; + +* ``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 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 += ============================================================= + + +warn_limit +========== + +Number of kernel warnings after which the kernel should panic when +``panic_on_warn`` is not set. Setting this to 0 disables checking +the 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 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. |