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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 9 Aug 2018 11:30:57 +0200
Message-ID: <CAKfTPtBxWheWa4igKtwUmQR3x=7Zojnb65uFpeFUqxHmSkOh5w@mail.gmail.com>
Subject: Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator
To:     Quentin Perret <quentin.perret@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "open list:THERMAL" <linux-pm@vger.kernel.org>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Chris Redpath <chris.redpath@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Thara Gopinath <thara.gopinath@linaro.org>,
        viresh kumar <viresh.kumar@linaro.org>,
        Todd Kjos <tkjos@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        "Cc: Steve Muckle" <smuckle@google.com>, adharmap@quicinc.com,
        "Kannan, Saravana" <skannan@quicinc.com>, pkondeti@codeaurora.org,
        Juri Lelli <juri.lelli@redhat.com>,
        Eduardo Valentin <edubezval@gmail.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        currojerez@riseup.net, Javi Merino <javi.merino@kernel.org>
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On Tue, 24 Jul 2018 at 14:26, Quentin Perret <quentin.perret@arm.com> wrote:
>
> From: Morten Rasmussen <morten.rasmussen@arm.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.
>