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

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Message-ID: <20180411101517.GL14248@e110439-lin>
Date:   Wed, 11 Apr 2018 11:15:17 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     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>,
        Joel Fernandes <joelaf@...gle.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

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.

> > 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?

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?

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?

> > The problem, IMO is that we now depend on other information which
> > needs to be in sync before calling schedutil... and the patch I
> > proposed is meant to make it less likely that all the information
> > required are not aligned (also in the future).
> >
> > --
> > #include <best/regards.h>
> >
> > Patrick Bellasi

-- 
#include <best/regards.h>

Patrick Bellasi