linux-kernel - Re: [PATCH 2/2] watchdog: Add a mechanism to detect stalls on guest vCPUs

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Date:   Tue, 5 Apr 2022 14:15:51 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Sebastian Ene <sebastianene@...gle.com>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-watchdog@...r.kernel.org,
        will@...nel.org, qperret@...gle.com, maz@...nel.org
Subject: Re: [PATCH 2/2] watchdog: Add a mechanism to detect stalls on guest
 vCPUs

Sebastian,

On Tue, Apr 05, 2022 at 02:19:55PM +0000, Sebastian Ene wrote:
> This patch adds support for a virtual watchdog which relies on the
> per-cpu hrtimers to pet at regular intervals.
> 

The watchdog subsystem is not intended to detect soft and hard lockups.
It is intended to detect userspace issues. A watchdog driver requires
a userspace compinent which needs to ping the watchdog on a regular basis
to prevent timeouts (and watchdog drivers are supposed to use the
watchdog kernel API).

What you have here is a CPU stall detection mechanism, similar to the
existing soft/hard lockup detection mechanism. This code does not
belong into the watchdog subsystem; it is similar to the existing
hard/softlockup detection code (kernel/watchdog.c) and should reside
at the same location.

Having said that, I could imagine a watchdog driver to be used in VMs,
but that would be similar to existing watchdog drivers. The easiest way
to get there would probably be to just instantiate one of the watchdog
devices already supported by qemu.

Guenter