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Fernando Luis Vázquez Cao9919cba2012-02-09 17:42:20 -05001===============================================================
2Softlockup detector and hardlockup detector (aka nmi_watchdog)
3===============================================================
4
5The Linux kernel can act as a watchdog to detect both soft and hard
6lockups.
7
8A 'softlockup' is defined as a bug that causes the kernel to loop in
9kernel mode for more than 20 seconds (see "Implementation" below for
10details), without giving other tasks a chance to run. The current
11stack trace is displayed upon detection and, by default, the system
12will stay locked up. Alternatively, the kernel can be configured to
13panic; a sysctl, "kernel.softlockup_panic", a kernel parameter,
Mauro Carvalho Chehab8c27ceff32016-10-18 10:12:27 -020014"softlockup_panic" (see "Documentation/admin-guide/kernel-parameters.rst" for
Harish Jenny K N8ae34ea2014-08-20 11:56:17 +053015details), and a compile option, "BOOTPARAM_SOFTLOCKUP_PANIC", are
Fernando Luis Vázquez Cao9919cba2012-02-09 17:42:20 -050016provided for this.
17
18A 'hardlockup' is defined as a bug that causes the CPU to loop in
19kernel mode for more than 10 seconds (see "Implementation" below for
20details), without letting other interrupts have a chance to run.
21Similarly to the softlockup case, the current stack trace is displayed
22upon detection and the system will stay locked up unless the default
Don Zickusac1f5912015-11-05 18:44:44 -080023behavior is changed, which can be done through a sysctl,
24'hardlockup_panic', a compile time knob, "BOOTPARAM_HARDLOCKUP_PANIC",
25and a kernel parameter, "nmi_watchdog"
Mauro Carvalho Chehab8c27ceff32016-10-18 10:12:27 -020026(see "Documentation/admin-guide/kernel-parameters.rst" for details).
Fernando Luis Vázquez Cao9919cba2012-02-09 17:42:20 -050027
28The panic option can be used in combination with panic_timeout (this
29timeout is set through the confusingly named "kernel.panic" sysctl),
30to cause the system to reboot automatically after a specified amount
31of time.
32
33=== Implementation ===
34
35The soft and hard lockup detectors are built on top of the hrtimer and
36perf subsystems, respectively. A direct consequence of this is that,
37in principle, they should work in any architecture where these
38subsystems are present.
39
40A periodic hrtimer runs to generate interrupts and kick the watchdog
41task. An NMI perf event is generated every "watchdog_thresh"
42(compile-time initialized to 10 and configurable through sysctl of the
43same name) seconds to check for hardlockups. If any CPU in the system
44does not receive any hrtimer interrupt during that time the
45'hardlockup detector' (the handler for the NMI perf event) will
46generate a kernel warning or call panic, depending on the
47configuration.
48
49The watchdog task is a high priority kernel thread that updates a
50timestamp every time it is scheduled. If that timestamp is not updated
51for 2*watchdog_thresh seconds (the softlockup threshold) the
52'softlockup detector' (coded inside the hrtimer callback function)
53will dump useful debug information to the system log, after which it
54will call panic if it was instructed to do so or resume execution of
55other kernel code.
56
57The period of the hrtimer is 2*watchdog_thresh/5, which means it has
58two or three chances to generate an interrupt before the hardlockup
59detector kicks in.
60
61As explained above, a kernel knob is provided that allows
62administrators to configure the period of the hrtimer and the perf
63event. The right value for a particular environment is a trade-off
64between fast response to lockups and detection overhead.
Chris Metcalffe4ba3c2015-06-24 16:55:45 -070065
66By default, the watchdog runs on all online cores. However, on a
67kernel configured with NO_HZ_FULL, by default the watchdog runs only
68on the housekeeping cores, not the cores specified in the "nohz_full"
69boot argument. If we allowed the watchdog to run by default on
70the "nohz_full" cores, we would have to run timer ticks to activate
71the scheduler, which would prevent the "nohz_full" functionality
72from protecting the user code on those cores from the kernel.
73Of course, disabling it by default on the nohz_full cores means that
74when those cores do enter the kernel, by default we will not be
75able to detect if they lock up. However, allowing the watchdog
76to continue to run on the housekeeping (non-tickless) cores means
77that we will continue to detect lockups properly on those cores.
78
79In either case, the set of cores excluded from running the watchdog
80may be adjusted via the kernel.watchdog_cpumask sysctl. For
81nohz_full cores, this may be useful for debugging a case where the
82kernel seems to be hanging on the nohz_full cores.