linux-kernel - Re: [PATCH] sched/fair: schedutil: update only with all info available

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Date:   Wed, 11 Apr 2018 14:34:54 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steve Muckle <smuckle@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH] sched/fair: schedutil: update only with all info available

Hi Vincent,

On Wed, Apr 11, 2018 at 4:56 AM, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
> On 11 April 2018 at 12:15, Patrick Bellasi <patrick.bellasi@....com> wrote:
>> On 11-Apr 08:57, Vincent Guittot wrote:
>>> On 10 April 2018 at 13:04, Patrick Bellasi <patrick.bellasi@....com> wrote:
>>> > On 09-Apr 10:51, Vincent Guittot wrote:
>>> >> On 6 April 2018 at 19:28, Patrick Bellasi <patrick.bellasi@....com> wrote:
>>> >> Peter,
>>> >> what was your goal with adding the condition "if
>>> >> (rq->cfs.h_nr_running)" for the aggragation of CFS utilization
>>> >
>>> > The original intent was to get rid of sched class flags, used to track
>>> > which class has tasks runnable from within schedutil. The reason was
>>> > to solve some misalignment between scheduler class status and
>>> > schedutil status.
>>>
>>> This was mainly for RT tasks but it was not the case for cfs task
>>> before commit 8f111bc357aa
>>
>> True, but with his solution Peter has actually come up with a unified
>> interface which is now (and can be IMO) based just on RUNNABLE
>> counters for each class.
>
> But do we really want to only take care of runnable counter for all class ?
>
>>
>>> > The solution, initially suggested by Viresh, and finally proposed by
>>> > Peter was to exploit RQ knowledges directly from within schedutil.
>>> >
>>> > The problem is that now schedutil updated depends on two information:
>>> > utilization changes and number of RT and CFS runnable tasks.
>>> >
>>> > Thus, using cfs_rq::h_nr_running is not the problem... it's actually
>>> > part of a much more clean solution of the code we used to have.
>>>
>>> So there are 2 problems there:
>>> - using cfs_rq::h_nr_running when aggregating cfs utilization which
>>> generates a lot of frequency drop
>>
>> You mean because we now completely disregard the blocked utilization
>> where a CPU is idle, right?
>
> yes
>
>>
>> Given how PELT works and the recent support for IDLE CPUs updated, we
>> should probably always add contributions for the CFS class.
>>
>>> - making sure that the nr-running are up-to-date when used in sched_util
>>
>> Right... but, if we always add the cfs_rq (to always account for
>> blocked utilization), we don't have anymore this last dependency,
>> isn't it?
>
> yes
>
>>
>> We still have to account for the util_est dependency.
>>
>> Should I add a patch to this series to disregard cfs_rq::h_nr_running
>> from schedutil as you suggested?
>
> It's probably better to have a separate patch as these are 2 different topics
> - when updating cfs_rq::h_nr_running and when calling cpufreq_update_util
> - should we use runnable or running utilization for CFS

By runnable you don't mean sched_avg::load_avg right? I got a bit
confused, since runnable means load_avg and running means util_avg.
But I didn't follow here why we are talking about load_avg for
schedutil at all. I am guessing by "runnable" you mean h_nr_running !=
0.

Also that aside, the "running util" is what was used to drive the CFS
util before Peter's patch (8f111bc357a). That was accounting the
blocked and decaying utilization but that patch changed the behavior.
It seems logical we should just use that not check for h_nr_running
for CFS so we don't miss on the decayed  utilization. What is the use
of checking h_nr_running or state of runqueue for CFS? I am sure to be
missing something here. :-(

thanks!

- Joel