linux-kernel - Re: [PATCH] sched/fair: check for idle core

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Date:   Wed, 21 Oct 2020 20:18:32 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Julia Lawall <julia.lawall@...ia.fr>, Mel Gorman <mgorman@...e.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel-janitors@...r.kernel.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>,
        Gilles Muller <Gilles.Muller@...ia.fr>,
        viresh.kumar@...aro.org, srinivas.pandruvada@...ux.intel.com
Subject: Re: [PATCH] sched/fair: check for idle core

On Wednesday, October 21, 2020 2:52:03 PM CEST Peter Zijlstra wrote:
> 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.

Probably, the "default governor" setting was inherited from the old config.

It didn't matter when the active mode was used by default, because it only
recognizes the "powersave" and "performance" settings (and "ondemand" causes
it to fall back to "powersave" IIRC).

Cheers!