linux-kernel - Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator

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Date:   Thu, 9 Aug 2018 11:30:57 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <chris.redpath@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        viresh kumar <viresh.kumar@...aro.org>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        "Cc: Steve Muckle" <smuckle@...gle.com>, adharmap@...cinc.com,
        "Kannan, Saravana" <skannan@...cinc.com>, pkondeti@...eaurora.org,
        Juri Lelli <juri.lelli@...hat.com>,
        Eduardo Valentin <edubezval@...il.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        currojerez@...eup.net, Javi Merino <javi.merino@...nel.org>
Subject: Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator

On Tue, 24 Jul 2018 at 14:26, Quentin Perret <quentin.perret@....com> wrote:
>
> From: Morten Rasmussen <morten.rasmussen@....com>
>
> Energy-aware scheduling is only meant to be active while the system is
> _not_ over-utilized. That is, there are spare cycles available to shift
> tasks around based on their actual utilization to get a more
> energy-efficient task distribution without depriving any tasks. When
> above the tipping point task placement is done the traditional way based
> on load_avg, spreading the tasks across as many cpus as possible based
> on priority scaled load to preserve smp_nice. Below the tipping point we
> want to use util_avg instead. We need to define a criteria for when we
> make the switch.
>
> The util_avg for each cpu converges towards 100% (1024) regardless of

remove the "(1024)" because util_avg converges to max cpu capacity
which can be different from 1024

> how many task additional task we may put on it. If we define
> over-utilized as:
>
> sum_{cpus}(rq.cfs.avg.util_avg) + margin > sum_{cpus}(rq.capacity)
>
> some individual cpus may be over-utilized running multiple tasks even
> when the above condition is false. That should be okay as long as we try
> to spread the tasks out to avoid per-cpu over-utilization as much as
> possible and if all tasks have the _same_ priority. If the latter isn't
> true, we have to consider priority to preserve smp_nice.
>