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

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Message-ID: <20180508094237.GA3752@e108498-lin.cambridge.arm.com>
Date:   Tue, 8 May 2018 10:42:37 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        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>,
        Juri Lelli <juri.lelli@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH] Revert "cpufreq: schedutil: Don't restrict kthread to
 related_cpus unnecessarily"

On Tuesday 08 May 2018 at 11:09:57 (+0200), Dietmar Eggemann wrote:
> On 05/08/2018 10:22 AM, Viresh Kumar wrote:
> > On 08-05-18, 08:33, Dietmar Eggemann wrote:
> > > This reverts commit e2cabe48c20efb174ce0c01190f8b9c5f3ea1d13.
> > > 
> > > Lifting the restriction that the sugov kthread is bound to the
> > > policy->related_cpus for a system with a slow switching cpufreq driver,
> > > which is able to perform DVFS from any cpu (e.g. cpufreq-dt), is not
> > > only not beneficial it also harms Enery-Aware Scheduling (EAS) on
> > > systems with asymmetric cpu capacities (e.g. Arm big.LITTLE).
> > > 
> > > The sugov kthread which does the update for the little cpus could
> > > potentially run on a big cpu. It could prevent that the big cluster goes
> > > into deeper idle states although all the tasks are running on the little
> > > cluster.
> > 
> > I think the original patch did the right thing, but that doesn't suit
> > everybody as you explained.
> > 
> > I wouldn't really revert the patch but fix my platform's cpufreq
> > driver to set dvfs_possible_from_any_cpu = false, so that other
> > platforms can still benefit from the original commit.
> 
> This would make sure that the kthreads are bound to the correct set of cpus
> for platforms with those cpufreq drivers (cpufreq-dt (h960), scmi-cpufreq,
> scpi-cpufreq) but it will also change the logic (e.g.
> sugov_should_update_freq() -> cpufreq_can_do_remote_dvfs()).
> 
> I'm still struggling to understand when a driver/platform should set
> dvfs_possible_from_any_cpu to true and what the actual benefit would be.

I assume it might be beneficial to have the kthread moving around freely
in some cases, but since it is a SCHED_DEADLINE task now it can't really
migrate anywhere anyway. So I'm not sure either if this commits still makes
sense now. Or is there another use case for this ?

Thanks,
Quentin