Re: [PATCH v2 5/5] doc: watchdog: Document buddy detector

[Doug Anderson]

Hi,

On Mon, Mar 23, 2026 at 10:26 AM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> From f1cfdc330cfbc68568dfe6bf2513bde9373c89d7 Mon Sep 17 00:00:00 2001
> From: Petr Mladek <pmladek@xxxxxxxx>
> Date: Mon, 23 Mar 2026 18:21:38 +0100
> Subject: [PATCH] doc: watchdog: Futher improvements
>
> Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
> ---
>  .../admin-guide/lockup-watchdogs.rst          | 44 ++++++++++---------
>  1 file changed, 24 insertions(+), 20 deletions(-)
>
> diff --git a/Documentation/admin-guide/lockup-watchdogs.rst b/Documentation/admin-guide/lockup-watchdogs.rst
> index 7ae7ce3abd2c..d0773edf3396 100644
> --- a/Documentation/admin-guide/lockup-watchdogs.rst
> +++ b/Documentation/admin-guide/lockup-watchdogs.rst
> @@ -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.

I'm happy with Petr's further improvements.

Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>

I think Andrew can just pick it up atop Mayank's. Andrew: If you need
any reposting, please yell.

Petr: thank you very much for your review of these patches! I'm super
happy you found the bug in Mayank's V1 that I missed and I think
things look nice now. :-)

-Doug