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

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Date:   Mon, 8 Jan 2018 15:20:22 +0200
From:   Leonard Crestez <leonard.crestez@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Patrick Bellasi <patrick.bellasi@....com>
CC:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Anson Huang <anson.huang@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        <vincent.guittot@...aro.org>
Subject: Re: [BUG] schedutil governor produces regular max freq spikes
 because of lockup detector watchdog threads

On Mon, 2018-01-08 at 09:31 +0530, Viresh Kumar wrote:
> On 05-01-18, 23:18, Rafael J. Wysocki wrote:
> > On Fri, Jan 5, 2018 at 9:37 PM, Leonard Crestez <leonard.crestez@....com> wrote:

> > > When using the schedutil governor together with the softlockup detector
> > > all CPUs go to their maximum frequency on a regular basis. This seems
> > > to be because the watchdog creates a RT thread on each CPU and this
> > > causes regular kicks with:
> > > 
> > >     cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
> > > 
> > > The schedutil governor responds to this by immediately setting the
> > > maximum cpu frequency, this is very undesirable.
> > > 
> > > The issue can be fixed by this patch from android:
> > > 
> > > The patch stalled in a long discussion about how it's difficult for
> > > cpufreq to deal with RT and how some RT users might just disable
> > > cpufreq. It is indeed hard but if the system experiences regular power
> > > kicks from a common debug feature they will end up disabling schedutil
> > > instead.

> > Patrick has a series of patches dealing with this problem area AFAICS,
> > but we are currently integrating material from Juri related to
> > deadline tasks.

> I am not sure if Patrick's patches would solve this problem at all as
> we still go to max for RT and the RT task is created from the
> softlockup detector somehow.

I assume you're talking about the series starting with
"[PATCH v3 0/6] cpufreq: schedutil: fixes for flags updates"

I checked and they have no effect on this particular issue (not
surprising).

--
Regards,
Leonard