aboutsummaryrefslogtreecommitdiffstats
path: root/Documentation/sysctl/kernel.txt
blob: bac23c198360507dbc00db7ef106498666826495 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
Documentation for /proc/sys/kernel/*	kernel version 2.2.10
	(c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>
	(c) 2009,        Shen Feng<shen@cn.fujitsu.com>

For general info and legal blurb, please look in README.

==============================================================

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 ]
- callhome		     [ S390 only ]
- cap_last_cap
- core_pattern
- core_pipe_limit
- core_uses_pid
- ctrl-alt-del
- dmesg_restrict
- domainname
- hostname
- hotplug
- hardlockup_all_cpu_backtrace
- hung_task_panic
- hung_task_check_count
- hung_task_timeout_secs
- hung_task_warnings
- 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_on_rcu_stall
- perf_cpu_time_max_percent
- perf_event_paranoid
- perf_event_max_stack
- 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
- 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
- stop-a                      [ SPARC only ]
- sysrq                       ==> Documentation/admin-guide/sysrq.rst
- sysctl_writes_strict
- tainted
- 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.txt 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.txt for additional information.

==============================================================

callhome:

Controls the kernel's callhome behavior in case of a kernel panic.

The s390 hardware allows an operating system to send a notification
to a service organization (callhome) in case of an operating system panic.

When the value in this file is 0 (which is the default behavior)
nothing happens in case of a kernel panic. If this value is set to "1"
the complete kernel oops message is send to the IBM customer service
organization in case the mainframe the Linux operating system is running
on has a service contract with IBM.

==============================================================

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 128 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.
==============================================================

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:

Check interval. 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_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.

==============================================================

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, there are no restrictions.

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.

==============================================================

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_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
>=0: Disallow raw tracepoint access by users without CAP_IOC_LOCK
>=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_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_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.

==============================================================

tainted:

Non-zero if the kernel has been tainted.  Numeric values, which
can be ORed together:

   1 - A module with a non-GPL license has been loaded, this
       includes modules with no license.
       Set by modutils >= 2.4.9 and module-init-tools.
   2 - A module was force loaded by insmod -f.
       Set by modutils >= 2.4.9 and module-init-tools.
   4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
   8 - A module was forcibly unloaded from the system by rmmod -f.
  16 - A hardware machine check error occurred on the system.
  32 - A bad page was discovered on the system.
  64 - The user has asked that the system be marked "tainted".  This
       could be because they are running software that directly modifies
       the hardware, or for other reasons.
 128 - The system has died.
 256 - The ACPI DSDT has been overridden with one supplied by the user
        instead of using the one provided by the hardware.
 512 - A kernel warning has occurred.
1024 - A module from drivers/staging was loaded.
2048 - The system is working around a severe firmware bug.
4096 - An out-of-tree module has been loaded.
8192 - An unsigned module has been loaded in a kernel supporting module
       signature.
16384 - A soft lockup has previously occurred on the system.
32768 - The kernel has been live patched.

==============================================================

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.

==============================================================