linux-kernel - Re: [BUG] schedutil governor produces regular max freq spikes because of lockup detector watchdog threads

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Message-ID: <20180110105451.GB16413@localhost.localdomain>
Date:   Wed, 10 Jan 2018 11:54:51 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Leonard Crestez <leonard.crestez@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Anson Huang <anson.huang@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [BUG] schedutil governor produces regular max freq spikes
 because of lockup detector watchdog threads

On 09/01/18 16:50, Rafael J. Wysocki wrote:
> On Tue, Jan 9, 2018 at 3:43 PM, Leonard Crestez <leonard.crestez@....com> wrote:

[...]

> > Every 4 seconds (really it's /proc/sys/kernel/watchdog_thresh * 2 / 5
> > and watchdog_thresh defaults to 10). There is a per-cpu hrtimer which
> > wakes the per-cpu thread in order to check that tasks can still
> > execute, this works very well against bugs like infinite loops in
> > softirq mode. The timers are synchronized initially but can get
> > staggered (for example by hotplug).
> >
> > My guess is that it's only marked RT so that it executes ahead of other
> > threads and the watchdog doesn't trigger simply when there are lots of
> > userspace tasks.
> 
> I think so too.
> 
> I see a couple of more-or-less hackish ways to avoid the issue, but
> nothing particularly attractive ATM.
> 
> I wouldn't change the general behavior with respect to RT tasks
> because of this, though, as we would quickly find a case in which that
> would turn out to be not desirable.

I agree we cannot generalize to all RT tasks, but what Patrick proposed
(clamping utilization of certain known tasks) might help here:

lkml.kernel.org/r/20170824180857.32103-1-patrick.bellasi@....com

Maybe with a per-task interface instead of using cgroups?

The other option would be to relax DL tasks affinity constraints, so
that a case like this might be handled. Daniel and Tommaso proposed
possible approaches, this might be a driving use case. Not sure how we
would come up with a proper runtime for the watchdog, though.

Best,

- Juri