linux-kernel - Re: [PATCH] Revert "cpufreq: schedutil: Don't restrict kthread to related

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Message-ID: <20180508121735.GD19168@localhost.localdomain>
Date:   Tue, 8 May 2018 14:17:35 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-pm@...r.kernel.org,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Quentin Perret <quentin.perret@....com>
Subject: Re: [PATCH] Revert "cpufreq: schedutil: Don't restrict kthread to
 related_cpus unnecessarily"

On 08/05/18 16:23, Viresh Kumar wrote:
> On 08-05-18, 12:36, Dietmar Eggemann wrote:
> > That's true but where is the benefit by doing so? (Multiple) per-cluster or
> > per-cpu frequency domains, why should the sugov kthread run on a foreign
> > cpu?
> 
> I am not sure I know the answer, but I have a question which you can
> answer :)
> 
> Is it possible for a CPU (which already has high priority deadline
> activity going on) to be busy enough to be not able to run the
> schedutil kthread for sometime ? That would be the only case I believe
> where it would be better to let some other CPU go and change the
> frequency for this one as we better run faster while we have the high
> load going on.

Shouldn't happen. This kthreads are "special" and will preempt any other
DL task (stop class win of course).