linux-kernel - Re: [PATCH] sched: watchdog: Touch kernel watchdog in sched code

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Message-ID: <20200305075742.GR2596@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Mar 2020 08:57:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Xi Wang <xii@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Don <joshdon@...gle.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH] sched: watchdog: Touch kernel watchdog in sched code

On Wed, Mar 04, 2020 at 01:39:41PM -0800, Xi Wang wrote:
> The main purpose of kernel watchdog is to test whether scheduler can
> still schedule tasks on a cpu. In order to reduce latency from
> periodically invoking watchdog reset in thread context, we can simply
> touch watchdog from pick_next_task in scheduler. Compared to actually
> resetting watchdog from cpu stop / migration threads, we lose coverage
> on: a migration thread actually get picked and we actually context
> switch to the migration thread. Both steps are heavily protected by
> kernel locks and unlikely to silently fail. Thus the change would
> provide the same level of protection with less overhead.
> 
> The new way vs the old way to touch the watchdogs is configurable
> from:
> 
> /proc/sys/kernel/watchdog_touch_in_thread_interval
> 
> The value means:
> 0: Always touch watchdog from pick_next_task
> 1: Always touch watchdog from migration thread
> N (N>0): Touch watchdog from migration thread once in every N
>          invocations, and touch watchdog from pick_next_task for
>          other invocations.
> 

This is configurable madness. What are we really trying to do here?