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

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Message-ID: <20581608.8Dxr8OdOFj@kreacher>
Date:   Wed, 21 Oct 2020 20:15:47 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Julia Lawall <julia.lawall@...ia.fr>
Cc:     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 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).

Cheers!