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Documentation / lockup-watchdogs.txt




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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	===============================================================
2	Softlockup detector and hardlockup detector (aka nmi_watchdog)
3	===============================================================
4	
5	The Linux kernel can act as a watchdog to detect both soft and hard
6	lockups.
7	
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/admin-guide/kernel-parameters.rst" for
15	details), and a compile option, "BOOTPARAM_SOFTLOCKUP_PANIC", are
16	provided for this.
17	
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/admin-guide/kernel-parameters.rst" for details).
27	
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.
32	
33	Implementation
34	==============
35	
36	The soft and hard lockup detectors are built on top of the hrtimer and
37	perf subsystems, respectively. A direct consequence of this is that,
38	in principle, they should work in any architecture where these
39	subsystems are present.
40	
41	A periodic hrtimer runs to generate interrupts and kick the watchdog
42	task. An NMI perf event is generated every "watchdog_thresh"
43	(compile-time initialized to 10 and configurable through sysctl of the
44	same name) seconds to check for hardlockups. If any CPU in the system
45	does not receive any hrtimer interrupt during that time the
46	'hardlockup detector' (the handler for the NMI perf event) will
47	generate a kernel warning or call panic, depending on the
48	configuration.
49	
50	The watchdog task is a high priority kernel thread that updates a
51	timestamp every time it is scheduled. If that timestamp is not updated
52	for 2*watchdog_thresh seconds (the softlockup threshold) the
53	'softlockup detector' (coded inside the hrtimer callback function)
54	will dump useful debug information to the system log, after which it
55	will call panic if it was instructed to do so or resume execution of
56	other kernel code.
57	
58	The period of the hrtimer is 2*watchdog_thresh/5, which means it has
59	two or three chances to generate an interrupt before the hardlockup
60	detector kicks in.
61	
62	As explained above, a kernel knob is provided that allows
63	administrators to configure the period of the hrtimer and the perf
64	event. The right value for a particular environment is a trade-off
65	between fast response to lockups and detection overhead.
66	
67	By default, the watchdog runs on all online cores.  However, on a
68	kernel configured with NO_HZ_FULL, by default the watchdog runs only
69	on the housekeeping cores, not the cores specified in the "nohz_full"
70	boot argument.  If we allowed the watchdog to run by default on
71	the "nohz_full" cores, we would have to run timer ticks to activate
72	the scheduler, which would prevent the "nohz_full" functionality
73	from protecting the user code on those cores from the kernel.
74	Of course, disabling it by default on the nohz_full cores means that
75	when those cores do enter the kernel, by default we will not be
76	able to detect if they lock up.  However, allowing the watchdog
77	to continue to run on the housekeeping (non-tickless) cores means
78	that we will continue to detect lockups properly on those cores.
79	
80	In either case, the set of cores excluded from running the watchdog
81	may be adjusted via the kernel.watchdog_cpumask sysctl.  For
82	nohz_full cores, this may be useful for debugging a case where the
83	kernel seems to be hanging on the nohz_full cores.
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