linux-kernel - Re: [RFC PATCH v4 08/12] sched: Add over-utilization/tipping point indicator

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Message-ID: <20180706134003.GB17577@e108498-lin.cambridge.arm.com>
Date:   Fri, 6 Jul 2018 14:40:03 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, gregkh@...uxfoundation.org,
        mingo@...hat.com, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com,
        patrick.bellasi@....com, valentin.schneider@....com,
        vincent.guittot@...aro.org, thara.gopinath@...aro.org,
        viresh.kumar@...aro.org, tkjos@...gle.com, joel@...lfernandes.org,
        smuckle@...gle.com, adharmap@...cinc.com, skannan@...cinc.com,
        pkondeti@...eaurora.org, juri.lelli@...hat.com,
        edubezval@...il.com, srinivas.pandruvada@...ux.intel.com,
        currojerez@...eup.net, javi.merino@...nel.org
Subject: Re: [RFC PATCH v4 08/12] sched: Add over-utilization/tipping point
 indicator

On Friday 06 Jul 2018 at 15:24:28 (+0200), Peter Zijlstra wrote:
> On Fri, Jul 06, 2018 at 02:20:57PM +0100, Quentin Perret wrote:
> > So, what we try to do here is to _not_ set the overutilized flag based
> > on the utilization of a new task, because its utilization is 'wrong'.
> > That should help avoiding spurious transitions above the overutilized
> > threshold.
> 
> That most certainly deserves a comment,

Ok, will do.

> also didn't util_est fix some of that wrong-ness?

When a new task is enqueued, we don't have util_est samples yet, so the
task's util_est is equal to its util_avg. So util_est won't help here,
unfortunately.

But it does help in the wake-up path to predict what OPP will be
selected (see cpu_util_next() in patch 09/12).

Thanks,
Quentin