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doc: watchdog: futher improvements

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>

authored by

Petr Mladek and committed by
Andrew Morton 4580900f cb8615f3

+24 -20
+24 -20
Documentation/admin-guide/lockup-watchdogs.rst
··· 41 41 Implementation 42 42 ============== 43 43 44 - The soft lockup detector is built on top of the hrtimer subsystem. 45 - The hard lockup detector is built on top of the perf subsystem 46 - (on architectures that support it) or uses an SMP "buddy" system. 47 - 48 - Softlockup Detector 49 - ------------------- 50 - 51 - The watchdog job runs in a stop scheduling thread that updates a 52 - timestamp every time it is scheduled. If that timestamp is not updated 53 - for 2*watchdog_thresh seconds (the softlockup threshold) the 54 - 'softlockup detector' (coded inside the hrtimer callback function) 55 - will dump useful debug information to the system log, after which it 56 - will call panic if it was instructed to do so or resume execution of 57 - other kernel code. 44 + The soft and hard lockup detectors are built around a hrtimer. 45 + In addition, the softlockup detector regularly schedules a job, and 46 + the hard lockup detector might use Perf/NMI events on architectures 47 + that support it. 58 48 59 49 Frequency and Heartbeats 60 50 ------------------------ 61 51 62 - The hrtimer used by the softlockup detector serves a dual purpose: 63 - it detects softlockups, and it also generates the interrupts 64 - (heartbeats) that the hardlockup detectors use to verify CPU liveness. 52 + The core of the detectors in a hrtimer. It servers multiple purpose: 65 53 66 - The period of this hrtimer is 2*watchdog_thresh/5. This means the 67 - hrtimer has two or three chances to generate an interrupt before the 68 - NMI hardlockup detector kicks in. 54 + - schedules watchdog job for the softlockup detector 55 + - bumps the interrupt counter for hardlockup detectors (heartbeat) 56 + - detects softlockups 57 + - detects hardlockups in Buddy mode 58 + 59 + The period of this hrtimer is 2*watchdog_thresh/5, which is 4 seconds 60 + by default. The hrtimer has two or three chances to generate an interrupt 61 + (heartbeat) before the hardlockup detector kicks in. 62 + 63 + Softlockup Detector 64 + ------------------- 65 + 66 + The watchdog job is scheduled by the hrtimer and runs in a stop scheduling 67 + thread. It updates a timestamp every time it is scheduled. If that timestamp 68 + is not updated for 2*watchdog_thresh seconds (the softlockup threshold) the 69 + 'softlockup detector' (coded inside the hrtimer callback function) 70 + will dump useful debug information to the system log, after which it 71 + will call panic if it was instructed to do so or resume execution of 72 + other kernel code. 69 73 70 74 Hardlockup Detector (NMI/Perf) 71 75 ------------------------------