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Date:   Wed, 21 Oct 2020 14:52:03 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Julia Lawall <julia.lawall@inria.fr>
Cc:     Mel Gorman <mgorman@suse.de>, Ingo Molnar <mingo@redhat.com>,
        kernel-janitors@vger.kernel.org,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel@vger.kernel.org,
        Valentin Schneider <valentin.schneider@arm.com>,
        Gilles Muller <Gilles.Muller@inria.fr>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>, viresh.kumar@linaro.org,
        srinivas.pandruvada@linux.intel.com
Subject: Re: [PATCH] sched/fair: check for idle core
Message-ID: <20201021125203.GG2628@hirez.programming.kicks-ass.net>
References: <1603211879-1064-1-git-send-email-Julia.Lawall@inria.fr>
 <20201021112038.GC32041@suse.de>
 <alpine.DEB.2.22.394.2010211336410.8475@hadrien>
 <20201021121950.GF2628@hirez.programming.kicks-ass.net>
 <alpine.DEB.2.22.394.2010211422230.8475@hadrien>
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On Wed, Oct 21, 2020 at 02:42:20PM +0200, Julia Lawall wrote:
> 
> 
> On Wed, 21 Oct 2020, Peter Zijlstra wrote:
> 
> > On Wed, Oct 21, 2020 at 01:56:55PM +0200, Julia Lawall wrote:
> > > Prior to 5.8, my machine was using intel_pstate and had few background
> > > tasks.  Thus the problem wasn't visible in practice.  Starting with 5.8
> > > the kernel decided that intel_cpufreq would be more appropriate, which
> > > introduced kworkers every 0.004 seconds on all cores.
> >
> > That still doesn't make any sense. Are you running the legacy on-demand
> > thing or something?
> >
> > Rafael, Srinivas, Viresh, how come it defaults to that?
> 
> The relevant commits are 33aa46f252c7, and 39a188b88332 that fixes a small
> bug.  I have a Intel(R) Xeon(R) CPU E7-8870 v4 @ 2.10GHz that does not
> have the HWP feature, even though the cores seemed to be able to change
> their frequencies at the hardware level.

That just makes intel_pstate not prefer active mode. With the clear
intent that it should then go use schedutil, but somehow it looks like
you landed on ondemand, which is absolutely atrocious.

What's:

  $ for i in /sys/devices/system/cpu/cpu0/cpufreq/scaling_*; do echo -n $i ": "; cat $i; done

say, for you? And if you do:

  $ for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor ; do echo schedutil > $i; done

Are the kworkers gone?