linux-kernel - Re: [PATCH v5 10/14] sched/cpufreq: Refactor the utilization aggregation method

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Date:   Wed, 1 Aug 2018 11:40:30 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Saravana Kannan <skannan@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <chris.redpath@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Steve Muckle <smuckle@...gle.com>, adharmap@...cinc.com,
        skannan@...cinc.com, Pavan Kondeti <pkondeti@...eaurora.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Eduardo Valentin <edubezval@...il.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        currojerez@...eup.net, Javi Merino <javi.merino@...nel.org>,
        linux-pm-owner@...r.kernel.org
Subject: Re: [PATCH v5 10/14] sched/cpufreq: Refactor the utilization
 aggregation method

On Wed, Aug 1, 2018 at 11:23 AM, Quentin Perret <quentin.perret@....com> wrote:
> On Wednesday 01 Aug 2018 at 10:35:32 (+0200), Rafael J. Wysocki wrote:
>> On Wed, Aug 1, 2018 at 10:23 AM, Quentin Perret <quentin.perret@....com> wrote:
>> > On Wednesday 01 Aug 2018 at 09:32:49 (+0200), Rafael J. Wysocki wrote:
>> >> On Tue, Jul 31, 2018 at 9:31 PM,  <skannan@...eaurora.org> wrote:
>> >> >> On Monday 30 Jul 2018 at 12:35:27 (-0700), skannan@...eaurora.org wrote:
>> >> >>> If it's going to be a different aggregation from what's done for
>> >> >>> frequency
>> >> >>> guidance, I don't see the point of having this inside schedutil. Why not
>> >> >>> keep it inside the scheduler files?
>> >> >>
>> >> >> This code basically results from a discussion we had with Peter on v4.
>> >> >> Keeping everything centralized can make sense from a maintenance
>> >> >> perspective, I think. That makes it easy to see the impact of any change
>> >> >> to utilization signals for both EAS and schedutil.
>> >> >
>> >> > In that case, I'd argue it makes more sense to keep the code centralized in
>> >> > the scheduler. The scheduler can let schedutil know about the utilization
>> >> > after it aggregates them. There's no need for a cpufreq governor to know
>> >> > that there are scheduling classes or how many there are. And the scheduler
>> >> > can then choose to aggregate one way for task packing and another way for
>> >> > frequency guidance.
>> >>
>> >> Also the aggregate utilization may be used by cpuidle governors in
>> >> principle to decide how deep they can go with idle state selection.
>> >
>> > The only issue I see with this right now is that some of the things done
>> > in this function are policy decisions which really belong to the governor,
>> > I think.
>>
>> Well, the scheduler makes policy decisions too, in quite a few places. :-)
>
> That is true ... ;-) But not so much about frequency selection yet I guess
>
>> The really important consideration here is whether or not there may be
>> multiple governors making different policy decisions in that respect.
>> If not, then where exactly the single policy decision is made doesn't
>> particularly matter IMO.
>
> I think some users of the aggregated utilization signal do want to make
> slightly different decisions (I'm thinking about the RT-go-to-max thing
> again which makes perfect sense in sugov, but could possibly hurt EAS).

Fair enough.

> So the "hard" part of this work is to figure out what really is a
> governor-specific policy decision, and what is common between all users.
> I put "hard" between quotes because I only see the case of RT as truly
> sugov-specific for now.

OK

> If we also want a special case for DL, Peter's enum should work OK, and
> enable to add more special cases for new users (cpuidle ?) if needed.
> But maybe that is something for later ?

I agree.  That can be done later if need be.