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

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Message-ID: <7433376.gbrxC2yhp4@aspire.rjw.lan>
Date:   Mon, 08 Jan 2018 14:10:56 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Leonard Crestez <leonard.crestez@....com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Anson Huang <anson.huang@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        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 Monday, January 8, 2018 5:01:21 AM CET 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:
> > > Hello,
> > >
> > > 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:
> > >     https://patchwork.kernel.org/patch/9301909/
> > >
> > > 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.
> > 
> > They are basically free to use the other governors instead if they prefer them.
> > 
> > > No other governors behave this way,
> > 
> > Because they work differently overall.
> > 
> > > perhaps the current behavior should be considered a bug in schedutil.
> > >
> > > That patch now has conflicts with latest upstream. Perhaps a modified
> > > variant should be reconsidered for inclusion, or is there some other
> > > solution pending?
> > 
> > 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.
> 
> One way to fix that can be to use DL for the softlockup detector as
> after Juri's patches we don't always go to max for DL.
> 
> On the other side, AFAIR, Peter was very clear during the previous LPC
> that it doesn't make sense to use rt-avg as the above patch suggests.

Right.

Why does the softlockup watchdog use RT tasks in the first place?

Thanks,
Rafael