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