About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / sysctl / kernel.txt




Custom Search

Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	Documentation for /proc/sys/kernel/*	kernel version 2.2.10
2		(c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>
3		(c) 2009,        Shen Feng<shen@cn.fujitsu.com>
4	
5	For general info and legal blurb, please look in README.
6	
7	==============================================================
8	
9	This file contains documentation for the sysctl files in
10	/proc/sys/kernel/ and is valid for Linux kernel version 2.2.
11	
12	The files in this directory can be used to tune and monitor
13	miscellaneous and general things in the operation of the Linux
14	kernel. Since some of the files _can_ be used to screw up your
15	system, it is advisable to read both documentation and source
16	before actually making adjustments.
17	
18	Currently, these files might (depending on your configuration)
19	show up in /proc/sys/kernel:
20	
21	- acct
22	- acpi_video_flags
23	- auto_msgmni
24	- bootloader_type	     [ X86 only ]
25	- bootloader_version	     [ X86 only ]
26	- callhome		     [ S390 only ]
27	- cap_last_cap
28	- core_pattern
29	- core_pipe_limit
30	- core_uses_pid
31	- ctrl-alt-del
32	- dmesg_restrict
33	- domainname
34	- hostname
35	- hotplug
36	- hardlockup_all_cpu_backtrace
37	- hardlockup_panic
38	- hung_task_panic
39	- hung_task_check_count
40	- hung_task_timeout_secs
41	- hung_task_warnings
42	- kexec_load_disabled
43	- kptr_restrict
44	- l2cr                        [ PPC only ]
45	- modprobe                    ==> Documentation/debugging-modules.txt
46	- modules_disabled
47	- msg_next_id		      [ sysv ipc ]
48	- msgmax
49	- msgmnb
50	- msgmni
51	- nmi_watchdog
52	- osrelease
53	- ostype
54	- overflowgid
55	- overflowuid
56	- panic
57	- panic_on_oops
58	- panic_on_stackoverflow
59	- panic_on_unrecovered_nmi
60	- panic_on_warn
61	- panic_on_rcu_stall
62	- perf_cpu_time_max_percent
63	- perf_event_paranoid
64	- perf_event_max_stack
65	- perf_event_mlock_kb
66	- perf_event_max_contexts_per_stack
67	- pid_max
68	- powersave-nap               [ PPC only ]
69	- printk
70	- printk_delay
71	- printk_ratelimit
72	- printk_ratelimit_burst
73	- pty                         ==> Documentation/filesystems/devpts.txt
74	- randomize_va_space
75	- real-root-dev               ==> Documentation/admin-guide/initrd.rst
76	- reboot-cmd                  [ SPARC only ]
77	- rtsig-max
78	- rtsig-nr
79	- seccomp/                    ==> Documentation/userspace-api/seccomp_filter.rst
80	- sem
81	- sem_next_id		      [ sysv ipc ]
82	- sg-big-buff                 [ generic SCSI device (sg) ]
83	- shm_next_id		      [ sysv ipc ]
84	- shm_rmid_forced
85	- shmall
86	- shmmax                      [ sysv ipc ]
87	- shmmni
88	- softlockup_all_cpu_backtrace
89	- soft_watchdog
90	- stop-a                      [ SPARC only ]
91	- sysrq                       ==> Documentation/admin-guide/sysrq.rst
92	- sysctl_writes_strict
93	- tainted
94	- threads-max
95	- unknown_nmi_panic
96	- watchdog
97	- watchdog_thresh
98	- version
99	
100	==============================================================
101	
102	acct:
103	
104	highwater lowwater frequency
105	
106	If BSD-style process accounting is enabled these values control
107	its behaviour. If free space on filesystem where the log lives
108	goes below <lowwater>% accounting suspends. If free space gets
109	above <highwater>% accounting resumes. <Frequency> determines
110	how often do we check the amount of free space (value is in
111	seconds). Default:
112	4 2 30
113	That is, suspend accounting if there left <= 2% free; resume it
114	if we got >=4%; consider information about amount of free space
115	valid for 30 seconds.
116	
117	==============================================================
118	
119	acpi_video_flags:
120	
121	flags
122	
123	See Doc*/kernel/power/video.txt, it allows mode of video boot to be
124	set during run time.
125	
126	==============================================================
127	
128	auto_msgmni:
129	
130	This variable has no effect and may be removed in future kernel
131	releases. Reading it always returns 0.
132	Up to Linux 3.17, it enabled/disabled automatic recomputing of msgmni
133	upon memory add/remove or upon ipc namespace creation/removal.
134	Echoing "1" into this file enabled msgmni automatic recomputing.
135	Echoing "0" turned it off. auto_msgmni default value was 1.
136	
137	
138	==============================================================
139	
140	bootloader_type:
141	
142	x86 bootloader identification
143	
144	This gives the bootloader type number as indicated by the bootloader,
145	shifted left by 4, and OR'd with the low four bits of the bootloader
146	version.  The reason for this encoding is that this used to match the
147	type_of_loader field in the kernel header; the encoding is kept for
148	backwards compatibility.  That is, if the full bootloader type number
149	is 0x15 and the full version number is 0x234, this file will contain
150	the value 340 = 0x154.
151	
152	See the type_of_loader and ext_loader_type fields in
153	Documentation/x86/boot.txt for additional information.
154	
155	==============================================================
156	
157	bootloader_version:
158	
159	x86 bootloader version
160	
161	The complete bootloader version number.  In the example above, this
162	file will contain the value 564 = 0x234.
163	
164	See the type_of_loader and ext_loader_ver fields in
165	Documentation/x86/boot.txt for additional information.
166	
167	==============================================================
168	
169	callhome:
170	
171	Controls the kernel's callhome behavior in case of a kernel panic.
172	
173	The s390 hardware allows an operating system to send a notification
174	to a service organization (callhome) in case of an operating system panic.
175	
176	When the value in this file is 0 (which is the default behavior)
177	nothing happens in case of a kernel panic. If this value is set to "1"
178	the complete kernel oops message is send to the IBM customer service
179	organization in case the mainframe the Linux operating system is running
180	on has a service contract with IBM.
181	
182	==============================================================
183	
184	cap_last_cap
185	
186	Highest valid capability of the running kernel.  Exports
187	CAP_LAST_CAP from the kernel.
188	
189	==============================================================
190	
191	core_pattern:
192	
193	core_pattern is used to specify a core dumpfile pattern name.
194	. max length 128 characters; default value is "core"
195	. core_pattern is used as a pattern template for the output filename;
196	  certain string patterns (beginning with '%') are substituted with
197	  their actual values.
198	. backward compatibility with core_uses_pid:
199		If core_pattern does not include "%p" (default does not)
200		and core_uses_pid is set, then .PID will be appended to
201		the filename.
202	. corename format specifiers:
203		%<NUL>	'%' is dropped
204		%%	output one '%'
205		%p	pid
206		%P	global pid (init PID namespace)
207		%i	tid
208		%I	global tid (init PID namespace)
209		%u	uid (in initial user namespace)
210		%g	gid (in initial user namespace)
211		%d	dump mode, matches PR_SET_DUMPABLE and
212			/proc/sys/fs/suid_dumpable
213		%s	signal number
214		%t	UNIX time of dump
215		%h	hostname
216		%e	executable filename (may be shortened)
217		%E	executable path
218		%<OTHER> both are dropped
219	. If the first character of the pattern is a '|', the kernel will treat
220	  the rest of the pattern as a command to run.  The core dump will be
221	  written to the standard input of that program instead of to a file.
222	
223	==============================================================
224	
225	core_pipe_limit:
226	
227	This sysctl is only applicable when core_pattern is configured to pipe
228	core files to a user space helper (when the first character of
229	core_pattern is a '|', see above).  When collecting cores via a pipe
230	to an application, it is occasionally useful for the collecting
231	application to gather data about the crashing process from its
232	/proc/pid directory.  In order to do this safely, the kernel must wait
233	for the collecting process to exit, so as not to remove the crashing
234	processes proc files prematurely.  This in turn creates the
235	possibility that a misbehaving userspace collecting process can block
236	the reaping of a crashed process simply by never exiting.  This sysctl
237	defends against that.  It defines how many concurrent crashing
238	processes may be piped to user space applications in parallel.  If
239	this value is exceeded, then those crashing processes above that value
240	are noted via the kernel log and their cores are skipped.  0 is a
241	special value, indicating that unlimited processes may be captured in
242	parallel, but that no waiting will take place (i.e. the collecting
243	process is not guaranteed access to /proc/<crashing pid>/).  This
244	value defaults to 0.
245	
246	==============================================================
247	
248	core_uses_pid:
249	
250	The default coredump filename is "core".  By setting
251	core_uses_pid to 1, the coredump filename becomes core.PID.
252	If core_pattern does not include "%p" (default does not)
253	and core_uses_pid is set, then .PID will be appended to
254	the filename.
255	
256	==============================================================
257	
258	ctrl-alt-del:
259	
260	When the value in this file is 0, ctrl-alt-del is trapped and
261	sent to the init(1) program to handle a graceful restart.
262	When, however, the value is > 0, Linux's reaction to a Vulcan
263	Nerve Pinch (tm) will be an immediate reboot, without even
264	syncing its dirty buffers.
265	
266	Note: when a program (like dosemu) has the keyboard in 'raw'
267	mode, the ctrl-alt-del is intercepted by the program before it
268	ever reaches the kernel tty layer, and it's up to the program
269	to decide what to do with it.
270	
271	==============================================================
272	
273	dmesg_restrict:
274	
275	This toggle indicates whether unprivileged users are prevented
276	from using dmesg(8) to view messages from the kernel's log buffer.
277	When dmesg_restrict is set to (0) there are no restrictions. When
278	dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
279	dmesg(8).
280	
281	The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
282	default value of dmesg_restrict.
283	
284	==============================================================
285	
286	domainname & hostname:
287	
288	These files can be used to set the NIS/YP domainname and the
289	hostname of your box in exactly the same way as the commands
290	domainname and hostname, i.e.:
291	# echo "darkstar" > /proc/sys/kernel/hostname
292	# echo "mydomain" > /proc/sys/kernel/domainname
293	has the same effect as
294	# hostname "darkstar"
295	# domainname "mydomain"
296	
297	Note, however, that the classic darkstar.frop.org has the
298	hostname "darkstar" and DNS (Internet Domain Name Server)
299	domainname "frop.org", not to be confused with the NIS (Network
300	Information Service) or YP (Yellow Pages) domainname. These two
301	domain names are in general different. For a detailed discussion
302	see the hostname(1) man page.
303	
304	==============================================================
305	hardlockup_all_cpu_backtrace:
306	
307	This value controls the hard lockup detector behavior when a hard
308	lockup condition is detected as to whether or not to gather further
309	debug information. If enabled, arch-specific all-CPU stack dumping
310	will be initiated.
311	
312	0: do nothing. This is the default behavior.
313	
314	1: on detection capture more debug information.
315	==============================================================
316	
317	hardlockup_panic:
318	
319	This parameter can be used to control whether the kernel panics
320	when a hard lockup is detected.
321	
322	   0 - don't panic on hard lockup
323	   1 - panic on hard lockup
324	
325	See Documentation/lockup-watchdogs.txt for more information.  This can
326	also be set using the nmi_watchdog kernel parameter.
327	
328	==============================================================
329	
330	hotplug:
331	
332	Path for the hotplug policy agent.
333	Default value is "/sbin/hotplug".
334	
335	==============================================================
336	
337	hung_task_panic:
338	
339	Controls the kernel's behavior when a hung task is detected.
340	This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
341	
342	0: continue operation. This is the default behavior.
343	
344	1: panic immediately.
345	
346	==============================================================
347	
348	hung_task_check_count:
349	
350	The upper bound on the number of tasks that are checked.
351	This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
352	
353	==============================================================
354	
355	hung_task_timeout_secs:
356	
357	Check interval. When a task in D state did not get scheduled
358	for more than this value report a warning.
359	This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
360	
361	0: means infinite timeout - no checking done.
362	Possible values to set are in range {0..LONG_MAX/HZ}.
363	
364	==============================================================
365	
366	hung_task_warnings:
367	
368	The maximum number of warnings to report. During a check interval
369	if a hung task is detected, this value is decreased by 1.
370	When this value reaches 0, no more warnings will be reported.
371	This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
372	
373	-1: report an infinite number of warnings.
374	
375	==============================================================
376	
377	kexec_load_disabled:
378	
379	A toggle indicating if the kexec_load syscall has been disabled. This
380	value defaults to 0 (false: kexec_load enabled), but can be set to 1
381	(true: kexec_load disabled). Once true, kexec can no longer be used, and
382	the toggle cannot be set back to false. This allows a kexec image to be
383	loaded before disabling the syscall, allowing a system to set up (and
384	later use) an image without it being altered. Generally used together
385	with the "modules_disabled" sysctl.
386	
387	==============================================================
388	
389	kptr_restrict:
390	
391	This toggle indicates whether restrictions are placed on
392	exposing kernel addresses via /proc and other interfaces.
393	
394	When kptr_restrict is set to 0 (the default) the address is hashed before
395	printing. (This is the equivalent to %p.)
396	
397	When kptr_restrict is set to (1), kernel pointers printed using the %pK
398	format specifier will be replaced with 0's unless the user has CAP_SYSLOG
399	and effective user and group ids are equal to the real ids. This is
400	because %pK checks are done at read() time rather than open() time, so
401	if permissions are elevated between the open() and the read() (e.g via
402	a setuid binary) then %pK will not leak kernel pointers to unprivileged
403	users. Note, this is a temporary solution only. The correct long-term
404	solution is to do the permission checks at open() time. Consider removing
405	world read permissions from files that use %pK, and using dmesg_restrict
406	to protect against uses of %pK in dmesg(8) if leaking kernel pointer
407	values to unprivileged users is a concern.
408	
409	When kptr_restrict is set to (2), kernel pointers printed using
410	%pK will be replaced with 0's regardless of privileges.
411	
412	==============================================================
413	
414	l2cr: (PPC only)
415	
416	This flag controls the L2 cache of G3 processor boards. If
417	0, the cache is disabled. Enabled if nonzero.
418	
419	==============================================================
420	
421	modules_disabled:
422	
423	A toggle value indicating if modules are allowed to be loaded
424	in an otherwise modular kernel.  This toggle defaults to off
425	(0), but can be set true (1).  Once true, modules can be
426	neither loaded nor unloaded, and the toggle cannot be set back
427	to false.  Generally used with the "kexec_load_disabled" toggle.
428	
429	==============================================================
430	
431	msg_next_id, sem_next_id, and shm_next_id:
432	
433	These three toggles allows to specify desired id for next allocated IPC
434	object: message, semaphore or shared memory respectively.
435	
436	By default they are equal to -1, which means generic allocation logic.
437	Possible values to set are in range {0..INT_MAX}.
438	
439	Notes:
440	1) kernel doesn't guarantee, that new object will have desired id. So,
441	it's up to userspace, how to handle an object with "wrong" id.
442	2) Toggle with non-default value will be set back to -1 by kernel after
443	successful IPC object allocation.
444	
445	==============================================================
446	
447	nmi_watchdog:
448	
449	This parameter can be used to control the NMI watchdog
450	(i.e. the hard lockup detector) on x86 systems.
451	
452	   0 - disable the hard lockup detector
453	   1 - enable the hard lockup detector
454	
455	The hard lockup detector monitors each CPU for its ability to respond to
456	timer interrupts. The mechanism utilizes CPU performance counter registers
457	that are programmed to generate Non-Maskable Interrupts (NMIs) periodically
458	while a CPU is busy. Hence, the alternative name 'NMI watchdog'.
459	
460	The NMI watchdog is disabled by default if the kernel is running as a guest
461	in a KVM virtual machine. This default can be overridden by adding
462	
463	   nmi_watchdog=1
464	
465	to the guest kernel command line (see Documentation/admin-guide/kernel-parameters.rst).
466	
467	==============================================================
468	
469	numa_balancing
470	
471	Enables/disables automatic page fault based NUMA memory
472	balancing. Memory is moved automatically to nodes
473	that access it often.
474	
475	Enables/disables automatic NUMA memory balancing. On NUMA machines, there
476	is a performance penalty if remote memory is accessed by a CPU. When this
477	feature is enabled the kernel samples what task thread is accessing memory
478	by periodically unmapping pages and later trapping a page fault. At the
479	time of the page fault, it is determined if the data being accessed should
480	be migrated to a local memory node.
481	
482	The unmapping of pages and trapping faults incur additional overhead that
483	ideally is offset by improved memory locality but there is no universal
484	guarantee. If the target workload is already bound to NUMA nodes then this
485	feature should be disabled. Otherwise, if the system overhead from the
486	feature is too high then the rate the kernel samples for NUMA hinting
487	faults may be controlled by the numa_balancing_scan_period_min_ms,
488	numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms,
489	numa_balancing_scan_size_mb, and numa_balancing_settle_count sysctls.
490	
491	==============================================================
492	
493	numa_balancing_scan_period_min_ms, numa_balancing_scan_delay_ms,
494	numa_balancing_scan_period_max_ms, numa_balancing_scan_size_mb
495	
496	Automatic NUMA balancing scans tasks address space and unmaps pages to
497	detect if pages are properly placed or if the data should be migrated to a
498	memory node local to where the task is running.  Every "scan delay" the task
499	scans the next "scan size" number of pages in its address space. When the
500	end of the address space is reached the scanner restarts from the beginning.
501	
502	In combination, the "scan delay" and "scan size" determine the scan rate.
503	When "scan delay" decreases, the scan rate increases.  The scan delay and
504	hence the scan rate of every task is adaptive and depends on historical
505	behaviour. If pages are properly placed then the scan delay increases,
506	otherwise the scan delay decreases.  The "scan size" is not adaptive but
507	the higher the "scan size", the higher the scan rate.
508	
509	Higher scan rates incur higher system overhead as page faults must be
510	trapped and potentially data must be migrated. However, the higher the scan
511	rate, the more quickly a tasks memory is migrated to a local node if the
512	workload pattern changes and minimises performance impact due to remote
513	memory accesses. These sysctls control the thresholds for scan delays and
514	the number of pages scanned.
515	
516	numa_balancing_scan_period_min_ms is the minimum time in milliseconds to
517	scan a tasks virtual memory. It effectively controls the maximum scanning
518	rate for each task.
519	
520	numa_balancing_scan_delay_ms is the starting "scan delay" used for a task
521	when it initially forks.
522	
523	numa_balancing_scan_period_max_ms is the maximum time in milliseconds to
524	scan a tasks virtual memory. It effectively controls the minimum scanning
525	rate for each task.
526	
527	numa_balancing_scan_size_mb is how many megabytes worth of pages are
528	scanned for a given scan.
529	
530	==============================================================
531	
532	osrelease, ostype & version:
533	
534	# cat osrelease
535	2.1.88
536	# cat ostype
537	Linux
538	# cat version
539	#5 Wed Feb 25 21:49:24 MET 1998
540	
541	The files osrelease and ostype should be clear enough. Version
542	needs a little more clarification however. The '#5' means that
543	this is the fifth kernel built from this source base and the
544	date behind it indicates the time the kernel was built.
545	The only way to tune these values is to rebuild the kernel :-)
546	
547	==============================================================
548	
549	overflowgid & overflowuid:
550	
551	if your architecture did not always support 32-bit UIDs (i.e. arm,
552	i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
553	applications that use the old 16-bit UID/GID system calls, if the
554	actual UID or GID would exceed 65535.
555	
556	These sysctls allow you to change the value of the fixed UID and GID.
557	The default is 65534.
558	
559	==============================================================
560	
561	panic:
562	
563	The value in this file represents the number of seconds the kernel
564	waits before rebooting on a panic. When you use the software watchdog,
565	the recommended setting is 60.
566	
567	==============================================================
568	
569	panic_on_io_nmi:
570	
571	Controls the kernel's behavior when a CPU receives an NMI caused by
572	an IO error.
573	
574	0: try to continue operation (default)
575	
576	1: panic immediately. The IO error triggered an NMI. This indicates a
577	   serious system condition which could result in IO data corruption.
578	   Rather than continuing, panicking might be a better choice. Some
579	   servers issue this sort of NMI when the dump button is pushed,
580	   and you can use this option to take a crash dump.
581	
582	==============================================================
583	
584	panic_on_oops:
585	
586	Controls the kernel's behaviour when an oops or BUG is encountered.
587	
588	0: try to continue operation
589	
590	1: panic immediately.  If the `panic' sysctl is also non-zero then the
591	   machine will be rebooted.
592	
593	==============================================================
594	
595	panic_on_stackoverflow:
596	
597	Controls the kernel's behavior when detecting the overflows of
598	kernel, IRQ and exception stacks except a user stack.
599	This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.
600	
601	0: try to continue operation.
602	
603	1: panic immediately.
604	
605	==============================================================
606	
607	panic_on_unrecovered_nmi:
608	
609	The default Linux behaviour on an NMI of either memory or unknown is
610	to continue operation. For many environments such as scientific
611	computing it is preferable that the box is taken out and the error
612	dealt with than an uncorrected parity/ECC error get propagated.
613	
614	A small number of systems do generate NMI's for bizarre random reasons
615	such as power management so the default is off. That sysctl works like
616	the existing panic controls already in that directory.
617	
618	==============================================================
619	
620	panic_on_warn:
621	
622	Calls panic() in the WARN() path when set to 1.  This is useful to avoid
623	a kernel rebuild when attempting to kdump at the location of a WARN().
624	
625	0: only WARN(), default behaviour.
626	
627	1: call panic() after printing out WARN() location.
628	
629	==============================================================
630	
631	panic_on_rcu_stall:
632	
633	When set to 1, calls panic() after RCU stall detection messages. This
634	is useful to define the root cause of RCU stalls using a vmcore.
635	
636	0: do not panic() when RCU stall takes place, default behavior.
637	
638	1: panic() after printing RCU stall messages.
639	
640	==============================================================
641	
642	perf_cpu_time_max_percent:
643	
644	Hints to the kernel how much CPU time it should be allowed to
645	use to handle perf sampling events.  If the perf subsystem
646	is informed that its samples are exceeding this limit, it
647	will drop its sampling frequency to attempt to reduce its CPU
648	usage.
649	
650	Some perf sampling happens in NMIs.  If these samples
651	unexpectedly take too long to execute, the NMIs can become
652	stacked up next to each other so much that nothing else is
653	allowed to execute.
654	
655	0: disable the mechanism.  Do not monitor or correct perf's
656	   sampling rate no matter how CPU time it takes.
657	
658	1-100: attempt to throttle perf's sample rate to this
659	   percentage of CPU.  Note: the kernel calculates an
660	   "expected" length of each sample event.  100 here means
661	   100% of that expected length.  Even if this is set to
662	   100, you may still see sample throttling if this
663	   length is exceeded.  Set to 0 if you truly do not care
664	   how much CPU is consumed.
665	
666	==============================================================
667	
668	perf_event_paranoid:
669	
670	Controls use of the performance events system by unprivileged
671	users (without CAP_SYS_ADMIN).  The default value is 2.
672	
673	 -1: Allow use of (almost) all events by all users
674	     Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK
675	>=0: Disallow ftrace function tracepoint by users without CAP_SYS_ADMIN
676	     Disallow raw tracepoint access by users without CAP_SYS_ADMIN
677	>=1: Disallow CPU event access by users without CAP_SYS_ADMIN
678	>=2: Disallow kernel profiling by users without CAP_SYS_ADMIN
679	
680	==============================================================
681	
682	perf_event_max_stack:
683	
684	Controls maximum number of stack frames to copy for (attr.sample_type &
685	PERF_SAMPLE_CALLCHAIN) configured events, for instance, when using
686	'perf record -g' or 'perf trace --call-graph fp'.
687	
688	This can only be done when no events are in use that have callchains
689	enabled, otherwise writing to this file will return -EBUSY.
690	
691	The default value is 127.
692	
693	==============================================================
694	
695	perf_event_mlock_kb:
696	
697	Control size of per-cpu ring buffer not counted agains mlock limit.
698	
699	The default value is 512 + 1 page
700	
701	==============================================================
702	
703	perf_event_max_contexts_per_stack:
704	
705	Controls maximum number of stack frame context entries for
706	(attr.sample_type & PERF_SAMPLE_CALLCHAIN) configured events, for
707	instance, when using 'perf record -g' or 'perf trace --call-graph fp'.
708	
709	This can only be done when no events are in use that have callchains
710	enabled, otherwise writing to this file will return -EBUSY.
711	
712	The default value is 8.
713	
714	==============================================================
715	
716	pid_max:
717	
718	PID allocation wrap value.  When the kernel's next PID value
719	reaches this value, it wraps back to a minimum PID value.
720	PIDs of value pid_max or larger are not allocated.
721	
722	==============================================================
723	
724	ns_last_pid:
725	
726	The last pid allocated in the current (the one task using this sysctl
727	lives in) pid namespace. When selecting a pid for a next task on fork
728	kernel tries to allocate a number starting from this one.
729	
730	==============================================================
731	
732	powersave-nap: (PPC only)
733	
734	If set, Linux-PPC will use the 'nap' mode of powersaving,
735	otherwise the 'doze' mode will be used.
736	
737	==============================================================
738	
739	printk:
740	
741	The four values in printk denote: console_loglevel,
742	default_message_loglevel, minimum_console_loglevel and
743	default_console_loglevel respectively.
744	
745	These values influence printk() behavior when printing or
746	logging error messages. See 'man 2 syslog' for more info on
747	the different loglevels.
748	
749	- console_loglevel: messages with a higher priority than
750	  this will be printed to the console
751	- default_message_loglevel: messages without an explicit priority
752	  will be printed with this priority
753	- minimum_console_loglevel: minimum (highest) value to which
754	  console_loglevel can be set
755	- default_console_loglevel: default value for console_loglevel
756	
757	==============================================================
758	
759	printk_delay:
760	
761	Delay each printk message in printk_delay milliseconds
762	
763	Value from 0 - 10000 is allowed.
764	
765	==============================================================
766	
767	printk_ratelimit:
768	
769	Some warning messages are rate limited. printk_ratelimit specifies
770	the minimum length of time between these messages (in jiffies), by
771	default we allow one every 5 seconds.
772	
773	A value of 0 will disable rate limiting.
774	
775	==============================================================
776	
777	printk_ratelimit_burst:
778	
779	While long term we enforce one message per printk_ratelimit
780	seconds, we do allow a burst of messages to pass through.
781	printk_ratelimit_burst specifies the number of messages we can
782	send before ratelimiting kicks in.
783	
784	==============================================================
785	
786	printk_devkmsg:
787	
788	Control the logging to /dev/kmsg from userspace:
789	
790	ratelimit: default, ratelimited
791	on: unlimited logging to /dev/kmsg from userspace
792	off: logging to /dev/kmsg disabled
793	
794	The kernel command line parameter printk.devkmsg= overrides this and is
795	a one-time setting until next reboot: once set, it cannot be changed by
796	this sysctl interface anymore.
797	
798	==============================================================
799	
800	randomize_va_space:
801	
802	This option can be used to select the type of process address
803	space randomization that is used in the system, for architectures
804	that support this feature.
805	
806	0 - Turn the process address space randomization off.  This is the
807	    default for architectures that do not support this feature anyways,
808	    and kernels that are booted with the "norandmaps" parameter.
809	
810	1 - Make the addresses of mmap base, stack and VDSO page randomized.
811	    This, among other things, implies that shared libraries will be
812	    loaded to random addresses.  Also for PIE-linked binaries, the
813	    location of code start is randomized.  This is the default if the
814	    CONFIG_COMPAT_BRK option is enabled.
815	
816	2 - Additionally enable heap randomization.  This is the default if
817	    CONFIG_COMPAT_BRK is disabled.
818	
819	    There are a few legacy applications out there (such as some ancient
820	    versions of libc.so.5 from 1996) that assume that brk area starts
821	    just after the end of the code+bss.  These applications break when
822	    start of the brk area is randomized.  There are however no known
823	    non-legacy applications that would be broken this way, so for most
824	    systems it is safe to choose full randomization.
825	
826	    Systems with ancient and/or broken binaries should be configured
827	    with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
828	    address space randomization.
829	
830	==============================================================
831	
832	reboot-cmd: (Sparc only)
833	
834	??? This seems to be a way to give an argument to the Sparc
835	ROM/Flash boot loader. Maybe to tell it what to do after
836	rebooting. ???
837	
838	==============================================================
839	
840	rtsig-max & rtsig-nr:
841	
842	The file rtsig-max can be used to tune the maximum number
843	of POSIX realtime (queued) signals that can be outstanding
844	in the system.
845	
846	rtsig-nr shows the number of RT signals currently queued.
847	
848	==============================================================
849	
850	sched_schedstats:
851	
852	Enables/disables scheduler statistics. Enabling this feature
853	incurs a small amount of overhead in the scheduler but is
854	useful for debugging and performance tuning.
855	
856	==============================================================
857	
858	sg-big-buff:
859	
860	This file shows the size of the generic SCSI (sg) buffer.
861	You can't tune it just yet, but you could change it on
862	compile time by editing include/scsi/sg.h and changing
863	the value of SG_BIG_BUFF.
864	
865	There shouldn't be any reason to change this value. If
866	you can come up with one, you probably know what you
867	are doing anyway :)
868	
869	==============================================================
870	
871	shmall:
872	
873	This parameter sets the total amount of shared memory pages that
874	can be used system wide. Hence, SHMALL should always be at least
875	ceil(shmmax/PAGE_SIZE).
876	
877	If you are not sure what the default PAGE_SIZE is on your Linux
878	system, you can run the following command:
879	
880	# getconf PAGE_SIZE
881	
882	==============================================================
883	
884	shmmax:
885	
886	This value can be used to query and set the run time limit
887	on the maximum shared memory segment size that can be created.
888	Shared memory segments up to 1Gb are now supported in the
889	kernel.  This value defaults to SHMMAX.
890	
891	==============================================================
892	
893	shm_rmid_forced:
894	
895	Linux lets you set resource limits, including how much memory one
896	process can consume, via setrlimit(2).  Unfortunately, shared memory
897	segments are allowed to exist without association with any process, and
898	thus might not be counted against any resource limits.  If enabled,
899	shared memory segments are automatically destroyed when their attach
900	count becomes zero after a detach or a process termination.  It will
901	also destroy segments that were created, but never attached to, on exit
902	from the process.  The only use left for IPC_RMID is to immediately
903	destroy an unattached segment.  Of course, this breaks the way things are
904	defined, so some applications might stop working.  Note that this
905	feature will do you no good unless you also configure your resource
906	limits (in particular, RLIMIT_AS and RLIMIT_NPROC).  Most systems don't
907	need this.
908	
909	Note that if you change this from 0 to 1, already created segments
910	without users and with a dead originative process will be destroyed.
911	
912	==============================================================
913	
914	sysctl_writes_strict:
915	
916	Control how file position affects the behavior of updating sysctl values
917	via the /proc/sys interface:
918	
919	  -1 - Legacy per-write sysctl value handling, with no printk warnings.
920	       Each write syscall must fully contain the sysctl value to be
921	       written, and multiple writes on the same sysctl file descriptor
922	       will rewrite the sysctl value, regardless of file position.
923	   0 - Same behavior as above, but warn about processes that perform writes
924	       to a sysctl file descriptor when the file position is not 0.
925	   1 - (default) Respect file position when writing sysctl strings. Multiple
926	       writes will append to the sysctl value buffer. Anything past the max
927	       length of the sysctl value buffer will be ignored. Writes to numeric
928	       sysctl entries must always be at file position 0 and the value must
929	       be fully contained in the buffer sent in the write syscall.
930	
931	==============================================================
932	
933	softlockup_all_cpu_backtrace:
934	
935	This value controls the soft lockup detector thread's behavior
936	when a soft lockup condition is detected as to whether or not
937	to gather further debug information. If enabled, each cpu will
938	be issued an NMI and instructed to capture stack trace.
939	
940	This feature is only applicable for architectures which support
941	NMI.
942	
943	0: do nothing. This is the default behavior.
944	
945	1: on detection capture more debug information.
946	
947	==============================================================
948	
949	soft_watchdog
950	
951	This parameter can be used to control the soft lockup detector.
952	
953	   0 - disable the soft lockup detector
954	   1 - enable the soft lockup detector
955	
956	The soft lockup detector monitors CPUs for threads that are hogging the CPUs
957	without rescheduling voluntarily, and thus prevent the 'watchdog/N' threads
958	from running. The mechanism depends on the CPUs ability to respond to timer
959	interrupts which are needed for the 'watchdog/N' threads to be woken up by
960	the watchdog timer function, otherwise the NMI watchdog - if enabled - can
961	detect a hard lockup condition.
962	
963	==============================================================
964	
965	tainted:
966	
967	Non-zero if the kernel has been tainted.  Numeric values, which
968	can be ORed together:
969	
970	   1 - A module with a non-GPL license has been loaded, this
971	       includes modules with no license.
972	       Set by modutils >= 2.4.9 and module-init-tools.
973	   2 - A module was force loaded by insmod -f.
974	       Set by modutils >= 2.4.9 and module-init-tools.
975	   4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
976	   8 - A module was forcibly unloaded from the system by rmmod -f.
977	  16 - A hardware machine check error occurred on the system.
978	  32 - A bad page was discovered on the system.
979	  64 - The user has asked that the system be marked "tainted".  This
980	       could be because they are running software that directly modifies
981	       the hardware, or for other reasons.
982	 128 - The system has died.
983	 256 - The ACPI DSDT has been overridden with one supplied by the user
984	        instead of using the one provided by the hardware.
985	 512 - A kernel warning has occurred.
986	1024 - A module from drivers/staging was loaded.
987	2048 - The system is working around a severe firmware bug.
988	4096 - An out-of-tree module has been loaded.
989	8192 - An unsigned module has been loaded in a kernel supporting module
990	       signature.
991	16384 - A soft lockup has previously occurred on the system.
992	32768 - The kernel has been live patched.
993	
994	==============================================================
995	
996	threads-max
997	
998	This value controls the maximum number of threads that can be created
999	using fork().
1000	
1001	During initialization the kernel sets this value such that even if the
1002	maximum number of threads is created, the thread structures occupy only
1003	a part (1/8th) of the available RAM pages.
1004	
1005	The minimum value that can be written to threads-max is 20.
1006	The maximum value that can be written to threads-max is given by the
1007	constant FUTEX_TID_MASK (0x3fffffff).
1008	If a value outside of this range is written to threads-max an error
1009	EINVAL occurs.
1010	
1011	The value written is checked against the available RAM pages. If the
1012	thread structures would occupy too much (more than 1/8th) of the
1013	available RAM pages threads-max is reduced accordingly.
1014	
1015	==============================================================
1016	
1017	unknown_nmi_panic:
1018	
1019	The value in this file affects behavior of handling NMI. When the
1020	value is non-zero, unknown NMI is trapped and then panic occurs. At
1021	that time, kernel debugging information is displayed on console.
1022	
1023	NMI switch that most IA32 servers have fires unknown NMI up, for
1024	example.  If a system hangs up, try pressing the NMI switch.
1025	
1026	==============================================================
1027	
1028	watchdog:
1029	
1030	This parameter can be used to disable or enable the soft lockup detector
1031	_and_ the NMI watchdog (i.e. the hard lockup detector) at the same time.
1032	
1033	   0 - disable both lockup detectors
1034	   1 - enable both lockup detectors
1035	
1036	The soft lockup detector and the NMI watchdog can also be disabled or
1037	enabled individually, using the soft_watchdog and nmi_watchdog parameters.
1038	If the watchdog parameter is read, for example by executing
1039	
1040	   cat /proc/sys/kernel/watchdog
1041	
1042	the output of this command (0 or 1) shows the logical OR of soft_watchdog
1043	and nmi_watchdog.
1044	
1045	==============================================================
1046	
1047	watchdog_cpumask:
1048	
1049	This value can be used to control on which cpus the watchdog may run.
1050	The default cpumask is all possible cores, but if NO_HZ_FULL is
1051	enabled in the kernel config, and cores are specified with the
1052	nohz_full= boot argument, those cores are excluded by default.
1053	Offline cores can be included in this mask, and if the core is later
1054	brought online, the watchdog will be started based on the mask value.
1055	
1056	Typically this value would only be touched in the nohz_full case
1057	to re-enable cores that by default were not running the watchdog,
1058	if a kernel lockup was suspected on those cores.
1059	
1060	The argument value is the standard cpulist format for cpumasks,
1061	so for example to enable the watchdog on cores 0, 2, 3, and 4 you
1062	might say:
1063	
1064	  echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask
1065	
1066	==============================================================
1067	
1068	watchdog_thresh:
1069	
1070	This value can be used to control the frequency of hrtimer and NMI
1071	events and the soft and hard lockup thresholds. The default threshold
1072	is 10 seconds.
1073	
1074	The softlockup threshold is (2 * watchdog_thresh). Setting this
1075	tunable to zero will disable lockup detection altogether.
1076	
1077	==============================================================
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.