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

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Message-ID: <alpine.DEB.2.22.394.2010212147230.8475@hadrien>
Date:   Wed, 21 Oct 2020 21:47:51 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
cc:     Julia Lawall <julia.lawall@...ia.fr>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Wed, 21 Oct 2020, Rafael J. Wysocki wrote:

> On Wednesday, October 21, 2020 2:42:20 PM CEST 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's in the range of "turbo" P-states (if a P-state above a certain threshold
> is requested by the governor, the processor has a license to choose P-states
> in the range above this threshold by itself).

Sorry, but I don't understand this answer at all.

thanks,
julia