linux-kernel - Re: [PATCH] sched/fair: util_est: fast ramp-up EWMA on utilization increases

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Date:   Fri, 28 Jun 2019 14:38:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Douglas Raillard <douglas.raillard@....com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH] sched/fair: util_est: fast ramp-up EWMA on utilization
 increases

On Fri, Jun 28, 2019 at 11:08:14AM +0100, Patrick Bellasi wrote:
> On 26-Jun 13:40, Vincent Guittot wrote:
> > Hi Patrick,
> > 
> > On Thu, 20 Jun 2019 at 17:06, Patrick Bellasi <patrick.bellasi@....com> wrote:
> > >
> > > The estimated utilization for a task is currently defined based on:
> > >  - enqueued: the utilization value at the end of the last activation
> > >  - ewma:     an exponential moving average which samples are the enqueued values
> > >
> > > According to this definition, when a task suddenly change it's bandwidth
> > > requirements from small to big, the EWMA will need to collect multiple
> > > samples before converging up to track the new big utilization.
> > >
> > > Moreover, after the PELT scale invariance update [1], in the above scenario we
> > > can see that the utilization of the task has a significant drop from the first
> > > big activation to the following one. That's implied by the new "time-scaling"
> > 
> > Could you give us more details about this? I'm not sure to understand
> > what changes between the 1st big activation and the following one ?
> 
> We are after a solution for the problem Douglas Raillard discussed at
> OSPM, specifically the "Task util drop after 1st idle" highlighted in
> slide 6 of his presentation:
> 
>   http://retis.sssup.it/ospm-summit/Downloads/02_05-Douglas_Raillard-How_can_we_make_schedutil_even_more_effective.pdf
> 

So I see the problem, and I don't hate the patch, but I'm still
struggling to understand how exactly it related to the time-scaling
stuff. Afaict the fundamental problem here is layering two averages. The
second (EWMA in our case) will always lag/delay the input of the first
(PELT).

The time-scaling thing might make matters worse, because that helps PELT
ramp up faster, but that is not the primary issue.

Or am I missing something?