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

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Date:   Thu, 17 May 2018 12:10:22 -0700
From:   Saravana Kannan <skannan@...eaurora.org>
To:     Joel Fernandes <joel@...lfernandes.org>
CC:     Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        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 05/12/2018 10:19 PM, Joel Fernandes wrote:
> On Tue, May 08, 2018 at 10:42:37AM +0100, Quentin Perret wrote:
>> 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 ?
>
> The usecase I guess is, as Dietmar was saying, that it makes sense for
> kthread to update its own cluster and not disturb other clusters or random
> CPUs. I agree with this point.

I agree with Viresh. Also, why exactly did we make it deadline instead 
of RT? Was RT not getting scheduled quick enough? Is it because Android 
creates a lot of RT threads?

-Saravana


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