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From:   Xi Wang <xii@google.com>
Date:   Fri, 6 Mar 2020 14:34:20 -0800
Message-ID: <CAOBoifiWWcodi9HddxVsKUahTSdAS5OiQOcapDJ-4p+HufRzeQ@mail.gmail.com>
Subject: Re: [PATCH] sched: watchdog: Touch kernel watchdog in sched code
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Paul Turner <pjt@google.com>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Don <joshdon@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org
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On Fri, Mar 6, 2020 at 12:40 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Mar 05, 2020 at 02:11:49PM -0800, Paul Turner wrote:
> > The goal is to improve jitter since we're constantly periodically
> > preempting other classes to run the watchdog.   Even on a single CPU
> > this is measurable as jitter in the us range.  But, what increases the
> > motivation is this disruption has been recently magnified by CPU
> > "gifts" which require evicting the whole core when one of the siblings
> > schedules one of these watchdog threads.
> >
> > The majority outcome being asserted here is that we could actually
> > exercise pick_next_task if required -- there are other potential
> > things this will catch, but they are much more braindead generally
> > speaking (e.g. a bug in pick_next_task itself).
>
> I still utterly hate what the patch does though; there is no way I'll
> have watchdog code hook in the scheduler like this. That's just asking
> for trouble.
>
> Why isn't it sufficient to sample the existing context switch counters
> from the watchdog? And why can't we fix that?

We could go to pick next and repick the same task. There won't be a
context switch but we still want to hold the watchdog. I assume such a
counter also needs to be per cpu and inside the rq lock. There doesn't
seem to be an existing one that fits this purpose.