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doc: watchdog: futher improvements

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Make further additions and alterations to the watchdog documentation.

Link: https://lkml.kernel.org/r/acF3tXBxSr0KOP9b@pathway.suse.cz
Signed-off-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Li Huafei <lihuafei1@huawei.com>
Cc: Max Kellermann <max.kellermann@ionos.com>
Cc: Mayank Rungta <mrungta@google.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Stephane Erainan <eranian@google.com>
Cc: Wang Jinchao <wangjinchao600@gmail.com>
Cc: Yunhui Cui <cuiyunhui@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Petr Mladek 2026-03-23 18:21:38 +01:00 committed by Andrew Morton
parent cb8615f3cb
commit 4580900fe1
1 changed files with 24 additions and 20 deletions
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44
Documentation/admin-guide/lockup-watchdogs.rst
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@ -41,31 +41,35 @@ is a trade-off between fast response to lockups and detection overhead.
Implementation
==============
The soft lockup detector is built on top of the hrtimer subsystem.
The hard lockup detector is built on top of the perf subsystem
(on architectures that support it) or uses an SMP "buddy" system.
Softlockup Detector
-------------------
The watchdog job runs in a stop scheduling thread that updates a
timestamp every time it is scheduled. If that timestamp is not updated
for 2*watchdog_thresh seconds (the softlockup threshold) the
'softlockup detector' (coded inside the hrtimer callback function)
will dump useful debug information to the system log, after which it
will call panic if it was instructed to do so or resume execution of
other kernel code.
The soft and hard lockup detectors are built around a hrtimer.
In addition, the softlockup detector regularly schedules a job, and
the hard lockup detector might use Perf/NMI events on architectures
that support it.
Frequency and Heartbeats
------------------------
The hrtimer used by the softlockup detector serves a dual purpose:
it detects softlockups, and it also generates the interrupts
(heartbeats) that the hardlockup detectors use to verify CPU liveness.
The core of the detectors in a hrtimer. It servers multiple purpose:
The period of this hrtimer is 2*watchdog_thresh/5. This means the
hrtimer has two or three chances to generate an interrupt before the
NMI hardlockup detector kicks in.
- schedules watchdog job for the softlockup detector
- bumps the interrupt counter for hardlockup detectors (heartbeat)
- detects softlockups
- detects hardlockups in Buddy mode
The period of this hrtimer is 2*watchdog_thresh/5, which is 4 seconds
by default. The hrtimer has two or three chances to generate an interrupt
(heartbeat) before the hardlockup detector kicks in.
Softlockup Detector
-------------------
The watchdog job is scheduled by the hrtimer and runs in a stop scheduling
thread. It updates a timestamp every time it is scheduled. If that timestamp
is not updated for 2*watchdog_thresh seconds (the softlockup threshold) the
'softlockup detector' (coded inside the hrtimer callback function)
will dump useful debug information to the system log, after which it
will call panic if it was instructed to do so or resume execution of
other kernel code.
Hardlockup Detector (NMI/Perf)
------------------------------