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

From: Sebastian Ene <sebastianene@google.com>
To: Guenter Roeck <linux@roeck-us.net>
Cc: Wim Van Sebroeck <wim@linux-watchdog.org>,
	Rob Herring <robh+dt@kernel.org>,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-watchdog@vger.kernel.org, will@kernel.org,
	qperret@google.com, maz@kernel.org
Subject: Re: [PATCH 2/2] watchdog: Add a mechanism to detect stalls on guest vCPUs
Date: Wed, 6 Apr 2022 16:31:02 +0000	[thread overview]
Message-ID: <Yk3ARqLLPssVIM2/@google.com> (raw)
In-Reply-To: <20220405211551.GB2121947@roeck-us.net>

On Tue, Apr 05, 2022 at 02:15:51PM -0700, Guenter Roeck wrote:
> Sebastian,
> 

Hello Guenter,

> 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).
> 

Thanks for getting back ! I wanted to create a mechanism to detect
stalls on vCPUs and I am not sure if the current watchdog subsystem has a way
to create per-CPU binded watchdogs (in the same way as Power PC has
kernel/watchdog.c). 
The per-CPU watchdog is needed to account for time that the guest is not
running(either scheduled out or waiting for an event) to prevent spurious
reset events caused by the watchdog.

> 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.
>

I agree that this doesn't belong to the watchdog subsytem but the current
stall detection mechanism calls through MMIO into a virtual device
'qemu,virt-watchdog'. Calling a device from (kernel/watchdog.c) isn't
something that we should avoid ?

> 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.
>

I am looking forward for your response,

> Guenter

Cheers,
Sebastian