linux-kernel - Re: [PATCH v5 00/10] track CPU utilization

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Date:   Wed, 6 Jun 2018 12:02:35 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Quentin Perret <quentin.perret@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        viresh kumar <viresh.kumar@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH v5 00/10] track CPU utilization

On 6 June 2018 at 11:59, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> On 6 June 2018 at 11:44, Quentin Perret <quentin.perret@....com> wrote:
>> On Tuesday 05 Jun 2018 at 16:18:09 (+0200), Peter Zijlstra wrote:

[snip]

>>>
>>> > As you mentioned, scale_rt_capacity give the remaining capacity for
>>> > cfs and it will behave like cfs util_avg now that it uses PELT. So as
>>> > long as cfs util_avg <  scale_rt_capacity(we probably need a margin)
>>> > we keep using dl bandwidth + cfs util_avg + rt util_avg for selecting
>>> > OPP because we have remaining spare capacity but if  cfs util_avg ==
>>> > scale_rt_capacity, we make sure to use max OPP.
>>>
>>> Good point, when cfs-util < cfs-cap then there is idle time and the util
>>> number is 'right', when cfs-util == cfs-cap we're overcommitted and
>>> should go max.
>>>
>>> Since the util and cap values are aligned that should track nicely.

I have re run my tests and and the results seem to be ok so far.

I'm going to clean up a bit the code used for the test and sent a new
version of the proposal

>>
>> So Vincent proposed to have a margin between cfs util and cfs cap to be
>> sure there is a little bit of idle time. This is _exactly_ what the
>> overutilized flag in EAS does. That would actually make a lot of sense
>> to use that flag in schedutil. The idea is basically to say, if there
>> isn't enough idle time on all CPUs, the util signal are kinda wrong, so
>> let's not make any decisions (task placement or OPP selection) based on
>> that. If overutilized, go to max freq. Does that make sense ?
>
> Yes it's similar to the overutilized except that
> - this is done per cpu and whereas overutilization is for the whole system
> - the test is done at every freq update and not only during some cfs
> event and it uses the last up to date value and not a periodically
> updated snapshot of the value
> - this is done also without EAS
>
> Then for the margin, it has to be discussed if it is really needed or not
>
>>
>> Thanks,
>> Quentin