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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 000000000000..a0c1d4ce403a --- /dev/null +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -0,0 +1,1177 @@ +=================================== +Documentation for /proc/sys/kernel/ +=================================== + +kernel version 2.2.10 + +Copyright (c) 1998, 1999, Rik van Riel <riel@nl.linux.org> + +Copyright (c) 2009, Shen Feng<shen@cn.fujitsu.com> + +For general info and legal blurb, please look in index.rst. + +------------------------------------------------------------------------------ + +This file contains documentation for the sysctl files in +/proc/sys/kernel/ and is valid for Linux kernel version 2.2. + +The files in this directory can be used to tune and monitor +miscellaneous and general things in the operation of the Linux +kernel. Since some of the files _can_ be used to screw up your +system, it is advisable to read both documentation and source +before actually making adjustments. + +Currently, these files might (depending on your configuration) +show up in /proc/sys/kernel: + +- acct +- acpi_video_flags +- auto_msgmni +- bootloader_type [ X86 only ] +- bootloader_version [ X86 only ] +- cap_last_cap +- core_pattern +- core_pipe_limit +- core_uses_pid +- ctrl-alt-del +- dmesg_restrict +- domainname +- hostname +- hotplug +- hardlockup_all_cpu_backtrace +- hardlockup_panic +- hung_task_panic +- hung_task_check_count +- hung_task_timeout_secs +- hung_task_check_interval_secs +- hung_task_warnings +- hyperv_record_panic_msg +- kexec_load_disabled +- kptr_restrict +- l2cr [ PPC only ] +- modprobe ==> Documentation/debugging-modules.txt +- modules_disabled +- msg_next_id [ sysv ipc ] +- msgmax +- msgmnb +- msgmni +- nmi_watchdog +- osrelease +- ostype +- overflowgid +- overflowuid +- panic +- panic_on_oops +- panic_on_stackoverflow +- panic_on_unrecovered_nmi +- panic_on_warn +- panic_print +- panic_on_rcu_stall +- perf_cpu_time_max_percent +- perf_event_paranoid +- perf_event_max_stack +- perf_event_mlock_kb +- perf_event_max_contexts_per_stack +- pid_max +- powersave-nap [ PPC only ] +- printk +- printk_delay +- printk_ratelimit +- printk_ratelimit_burst +- pty ==> Documentation/filesystems/devpts.txt +- randomize_va_space +- real-root-dev ==> Documentation/admin-guide/initrd.rst +- reboot-cmd [ SPARC only ] +- rtsig-max +- rtsig-nr +- sched_energy_aware +- seccomp/ ==> Documentation/userspace-api/seccomp_filter.rst +- sem +- sem_next_id [ sysv ipc ] +- sg-big-buff [ generic SCSI device (sg) ] +- shm_next_id [ sysv ipc ] +- shm_rmid_forced +- shmall +- shmmax [ sysv ipc ] +- shmmni +- softlockup_all_cpu_backtrace +- soft_watchdog +- stack_erasing +- stop-a [ SPARC only ] +- sysrq ==> Documentation/admin-guide/sysrq.rst +- sysctl_writes_strict +- tainted ==> Documentation/admin-guide/tainted-kernels.rst +- threads-max +- unknown_nmi_panic +- watchdog +- watchdog_thresh +- version + + +acct: +===== + +highwater lowwater frequency + +If BSD-style process accounting is enabled these values control +its behaviour. If free space on filesystem where the log lives +goes below <lowwater>% accounting suspends. If free space gets +above <highwater>% accounting resumes. <Frequency> determines +how often do we check the amount of free space (value is in +seconds). Default: +4 2 30 +That is, suspend accounting if there left <= 2% free; resume it +if we got >=4%; consider information about amount of free space +valid for 30 seconds. + + +acpi_video_flags: +================= + +flags + +See Doc*/kernel/power/video.txt, it allows mode of video boot to be +set during run time. + + +auto_msgmni: +============ + +This variable has no effect and may be removed in future kernel +releases. Reading it always returns 0. +Up to Linux 3.17, it enabled/disabled automatic recomputing of msgmni +upon memory add/remove or upon ipc namespace creation/removal. +Echoing "1" into this file enabled msgmni automatic recomputing. +Echoing "0" turned it off. auto_msgmni default value was 1. + + +bootloader_type: +================ + +x86 bootloader identification + +This gives the bootloader type number as indicated by the bootloader, +shifted left by 4, and OR'd with the low four bits of the bootloader +version. The reason for this encoding is that this used to match the +type_of_loader field in the kernel header; the encoding is kept for +backwards compatibility. That is, if the full bootloader type number +is 0x15 and the full version number is 0x234, this file will contain +the value 340 = 0x154. + +See the type_of_loader and ext_loader_type fields in +Documentation/x86/boot.rst for additional information. + + +bootloader_version: +=================== + +x86 bootloader version + +The complete bootloader version number. In the example above, this +file will contain the value 564 = 0x234. + +See the type_of_loader and ext_loader_ver fields in +Documentation/x86/boot.rst for additional information. + + +cap_last_cap: +============= + +Highest valid capability of the running kernel. Exports +CAP_LAST_CAP from the kernel. + + +core_pattern: +============= + +core_pattern is used to specify a core dumpfile pattern name. + +* max length 127 characters; default value is "core" +* core_pattern is used as a pattern template for the output filename; + certain string patterns (beginning with '%') are substituted with + their actual values. +* backward compatibility with core_uses_pid: + + If core_pattern does not include "%p" (default does not) + and core_uses_pid is set, then .PID will be appended to + the filename. + +* corename format specifiers:: + + %<NUL> '%' is dropped + %% output one '%' + %p pid + %P global pid (init PID namespace) + %i tid + %I global tid (init PID namespace) + %u uid (in initial user namespace) + %g gid (in initial user namespace) + %d dump mode, matches PR_SET_DUMPABLE and + /proc/sys/fs/suid_dumpable + %s signal number + %t UNIX time of dump + %h hostname + %e executable filename (may be shortened) + %E executable path + %<OTHER> both are dropped + +* If the first character of the pattern is a '|', the kernel will treat + the rest of the pattern as a command to run. The core dump will be + written to the standard input of that program instead of to a file. + + +core_pipe_limit: +================ + +This sysctl is only applicable when core_pattern is configured to pipe +core files to a user space helper (when the first character of +core_pattern is a '|', see above). When collecting cores via a pipe +to an application, it is occasionally useful for the collecting +application to gather data about the crashing process from its +/proc/pid directory. In order to do this safely, the kernel must wait +for the collecting process to exit, so as not to remove the crashing +processes proc files prematurely. This in turn creates the +possibility that a misbehaving userspace collecting process can block +the reaping of a crashed process simply by never exiting. This sysctl +defends against that. It defines how many concurrent crashing +processes may be piped to user space applications in parallel. If +this value is exceeded, then those crashing processes above that value +are noted via the kernel log and their cores are skipped. 0 is a +special value, indicating that unlimited processes may be captured in +parallel, but that no waiting will take place (i.e. the collecting +process is not guaranteed access to /proc/<crashing pid>/). This +value defaults to 0. + + +core_uses_pid: +============== + +The default coredump filename is "core". By setting +core_uses_pid to 1, the coredump filename becomes core.PID. +If core_pattern does not include "%p" (default does not) +and core_uses_pid is set, then .PID will be appended to +the filename. + + +ctrl-alt-del: +============= + +When the value in this file is 0, ctrl-alt-del is trapped and +sent to the init(1) program to handle a graceful restart. +When, however, the value is > 0, Linux's reaction to a Vulcan +Nerve Pinch (tm) will be an immediate reboot, without even +syncing its dirty buffers. + +Note: + when a program (like dosemu) has the keyboard in 'raw' + mode, the ctrl-alt-del is intercepted by the program before it + ever reaches the kernel tty layer, and it's up to the program + to decide what to do with it. + + +dmesg_restrict: +=============== + +This toggle indicates whether unprivileged users are prevented +from using dmesg(8) to view messages from the kernel's log buffer. +When dmesg_restrict is set to (0) there are no restrictions. When +dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use +dmesg(8). + +The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the +default value of dmesg_restrict. + + +domainname & hostname: +====================== + +These files can be used to set the NIS/YP domainname and the +hostname of your box in exactly the same way as the commands +domainname and hostname, i.e.:: + + # echo "darkstar" > /proc/sys/kernel/hostname + # echo "mydomain" > /proc/sys/kernel/domainname + +has the same effect as:: + + # hostname "darkstar" + # domainname "mydomain" + +Note, however, that the classic darkstar.frop.org has the +hostname "darkstar" and DNS (Internet Domain Name Server) +domainname "frop.org", not to be confused with the NIS (Network +Information Service) or YP (Yellow Pages) domainname. These two +domain names are in general different. For a detailed discussion +see the hostname(1) man page. + + +hardlockup_all_cpu_backtrace: +============================= + +This value controls the hard lockup detector behavior when a hard +lockup condition is detected as to whether or not to gather further +debug information. If enabled, arch-specific all-CPU stack dumping +will be initiated. + +0: do nothing. This is the default behavior. + +1: on detection capture more debug information. + + +hardlockup_panic: +================= + +This parameter can be used to control whether the kernel panics +when a hard lockup is detected. + + 0 - don't panic on hard lockup + 1 - panic on hard lockup + +See Documentation/lockup-watchdogs.txt for more information. This can +also be set using the nmi_watchdog kernel parameter. + + +hotplug: +======== + +Path for the hotplug policy agent. +Default value is "/sbin/hotplug". + + +hung_task_panic: +================ + +Controls the kernel's behavior when a hung task is detected. +This file shows up if CONFIG_DETECT_HUNG_TASK is enabled. + +0: continue operation. This is the default behavior. + +1: panic immediately. + + +hung_task_check_count: +====================== + +The upper bound on the number of tasks that are checked. +This file shows up if CONFIG_DETECT_HUNG_TASK is enabled. + + +hung_task_timeout_secs: +======================= + +When a task in D state did not get scheduled +for more than this value report a warning. +This file shows up if CONFIG_DETECT_HUNG_TASK is enabled. + +0: means infinite timeout - no checking done. + +Possible values to set are in range {0..LONG_MAX/HZ}. + + +hung_task_check_interval_secs: +============================== + +Hung task check interval. If hung task checking is enabled +(see hung_task_timeout_secs), the check is done every +hung_task_check_interval_secs seconds. +This file shows up if CONFIG_DETECT_HUNG_TASK is enabled. + +0 (default): means use hung_task_timeout_secs as checking interval. +Possible values to set are in range {0..LONG_MAX/HZ}. + + +hung_task_warnings: +=================== + +The maximum number of warnings to report. During a check interval +if a hung task is detected, this value is decreased by 1. +When this value reaches 0, no more warnings will be reported. +This file shows up if CONFIG_DETECT_HUNG_TASK is enabled. + +-1: report an infinite number of warnings. + + +hyperv_record_panic_msg: +======================== + +Controls whether the panic kmsg data should be reported to Hyper-V. + +0: do not report panic kmsg data. + +1: report the panic kmsg data. This is the default behavior. + + +kexec_load_disabled: +==================== + +A toggle indicating if the kexec_load syscall has been disabled. This +value defaults to 0 (false: kexec_load enabled), but can be set to 1 +(true: kexec_load disabled). Once true, kexec 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 0's 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 0's regardless of privileges. + + +l2cr: (PPC only) +================ + +This flag controls the L2 cache of G3 processor boards. If +0, the cache is disabled. Enabled if nonzero. + + +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. + + +msg_next_id, sem_next_id, and shm_next_id: +========================================== + +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. + + +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 Documentation/admin-guide/kernel-parameters.rst). + + +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. + + +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 represents the number of seconds the kernel +waits before rebooting on a panic. 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 NMI's 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 +===== ======================================== + +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_SYS_ADMIN). The default value is 2. + +=== ================================================================== + -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_SYS_ADMIN + + Disallow raw tracepoint access by users without CAP_SYS_ADMIN + +>=1 Disallow CPU event access by users without CAP_SYS_ADMIN + +>=2 Disallow kernel profiling by users without CAP_SYS_ADMIN +=== ================================================================== + + +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 jiffies), by +default we allow one every 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. + + +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. + + +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. +== =========================================================================== + + +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. ??? + + +rtsig-max & rtsig-nr: +===================== + +The file rtsig-max can be used to tune the maximum number +of POSIX realtime (queued) signals that can be outstanding +in the system. + +rtsig-nr shows the number of RT signals currently queued. + + +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. + + +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. + + +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. + + +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. + + +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 Documentation/admin-guide/tainted-kernels.rst for more information. + + +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 20. + +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 error +EINVAL occurs. + +The value written is checked against the available RAM pages. If the +thread structures would occupy too much (more than 1/8th) of the +available RAM pages threads-max is reduced accordingly. + + +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. + + +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. |