From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed, 21 Oct 2020 12:52:03 +0000
Subject: Re: [PATCH] sched/fair: check for idle core
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References: <1603211879-1064-1-git-send-email-Julia.Lawall@inria.fr>
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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

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?