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

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Message-ID: <8cf21b1a-ca6e-fed7-43c5-94c66ff5986b@arm.com>
Date:   Tue, 8 May 2018 11:09:57 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     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>,
        Quentin Perret <quentin.perret@....com>
Subject: Re: [PATCH] Revert "cpufreq: schedutil: Don't restrict kthread to
 related_cpus unnecessarily"

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.