linux-kernel - Re: [PATCH v4 1/3] sched/fair: add util

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Date:   Tue, 6 Feb 2018 20:15:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH v4 1/3] sched/fair: add util_est on top of PELT

On Tue, Feb 06, 2018 at 08:09:00PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 06, 2018 at 06:33:15PM +0000, Patrick Bellasi wrote:
> 
> > Good point, I was actually expecting this question and I should have
> > added it to the cover letter, sorry.
> > 
> > The reasoning was: the task's estimated utilization is defined as the
> > max between PELT and the "estimation". Where "estimation" is
> > the max between EWMA and the last ENQUEUED utilization.
> > 
> > Thus I was envisioning these two calls:
> > 
> >    _task_util_est := max(EWMA, ENQUEUED)
> >     task_util_est := max(util_avg, _task_util_est)
> > 
> > but since now we have clients only for the first API, I've not added
> > the second one. Still I would prefer to keep the "_" to make it clear
> > that's and util_est's internal signal, not the actual task's estimated
> > utilization.
> > 
> > Does it make sense?
> > 
> > Do you prefer I just remove the "_" and we will refactor it once we
> > should add a customer for the proper task's util_est?
> 
> Hurm... I was thinking:
> 
>   task_util_est := max(util_avg, EWMA)
> 
> But the above mixes ENQUEUED into it.. *confused*.

So mixing in ENQUEUED seems to give it an upward BIAS if the very last
activation was 'high'. Thereby improving ramp-up.

That seems to be what we want.. might be nice to have that in a comment
;-)

I'm thinking we want a different name for max(EWMA, ENQUEUED) though,
but I really can't come up with a sensible suggestion, which I suppose,
is why you stuck an underscore on it and went on with things.