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Documentation / lockup-watchdogs.txt

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Based on kernel version 4.9. Page generated on 2016-12-21 14:35 EST.

1	===============================================================
2	Softlockup detector and hardlockup detector (aka nmi_watchdog)
3	===============================================================
5	The Linux kernel can act as a watchdog to detect both soft and hard
6	lockups.
8	A 'softlockup' is defined as a bug that causes the kernel to loop in
9	kernel mode for more than 20 seconds (see "Implementation" below for
10	details), without giving other tasks a chance to run. The current
11	stack trace is displayed upon detection and, by default, the system
12	will stay locked up. Alternatively, the kernel can be configured to
13	panic; a sysctl, "kernel.softlockup_panic", a kernel parameter,
14	"softlockup_panic" (see "Documentation/kernel-parameters.txt" for
15	details), and a compile option, "BOOTPARAM_SOFTLOCKUP_PANIC", are
16	provided for this.
18	A 'hardlockup' is defined as a bug that causes the CPU to loop in
19	kernel mode for more than 10 seconds (see "Implementation" below for
20	details), without letting other interrupts have a chance to run.
21	Similarly to the softlockup case, the current stack trace is displayed
22	upon detection and the system will stay locked up unless the default
23	behavior is changed, which can be done through a sysctl,
24	'hardlockup_panic', a compile time knob, "BOOTPARAM_HARDLOCKUP_PANIC",
25	and a kernel parameter, "nmi_watchdog"
26	(see "Documentation/kernel-parameters.txt" for details).
28	The panic option can be used in combination with panic_timeout (this
29	timeout is set through the confusingly named "kernel.panic" sysctl),
30	to cause the system to reboot automatically after a specified amount
31	of time.
33	=== Implementation ===
35	The soft and hard lockup detectors are built on top of the hrtimer and
36	perf subsystems, respectively. A direct consequence of this is that,
37	in principle, they should work in any architecture where these
38	subsystems are present.
40	A periodic hrtimer runs to generate interrupts and kick the watchdog
41	task. An NMI perf event is generated every "watchdog_thresh"
42	(compile-time initialized to 10 and configurable through sysctl of the
43	same name) seconds to check for hardlockups. If any CPU in the system
44	does not receive any hrtimer interrupt during that time the
45	'hardlockup detector' (the handler for the NMI perf event) will
46	generate a kernel warning or call panic, depending on the
47	configuration.
49	The watchdog task is a high priority kernel thread that updates a
50	timestamp every time it is scheduled. If that timestamp is not updated
51	for 2*watchdog_thresh seconds (the softlockup threshold) the
52	'softlockup detector' (coded inside the hrtimer callback function)
53	will dump useful debug information to the system log, after which it
54	will call panic if it was instructed to do so or resume execution of
55	other kernel code.
57	The period of the hrtimer is 2*watchdog_thresh/5, which means it has
58	two or three chances to generate an interrupt before the hardlockup
59	detector kicks in.
61	As explained above, a kernel knob is provided that allows
62	administrators to configure the period of the hrtimer and the perf
63	event. The right value for a particular environment is a trade-off
64	between fast response to lockups and detection overhead.
66	By default, the watchdog runs on all online cores.  However, on a
67	kernel configured with NO_HZ_FULL, by default the watchdog runs only
68	on the housekeeping cores, not the cores specified in the "nohz_full"
69	boot argument.  If we allowed the watchdog to run by default on
70	the "nohz_full" cores, we would have to run timer ticks to activate
71	the scheduler, which would prevent the "nohz_full" functionality
72	from protecting the user code on those cores from the kernel.
73	Of course, disabling it by default on the nohz_full cores means that
74	when those cores do enter the kernel, by default we will not be
75	able to detect if they lock up.  However, allowing the watchdog
76	to continue to run on the housekeeping (non-tickless) cores means
77	that we will continue to detect lockups properly on those cores.
79	In either case, the set of cores excluded from running the watchdog
80	may be adjusted via the kernel.watchdog_cpumask sysctl.  For
81	nohz_full cores, this may be useful for debugging a case where the
82	kernel seems to be hanging on the nohz_full cores.
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