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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 2 Aug 2018 18:38:01 +0200
Message-ID: <CAKfTPtDtaF9Qs4NWkKWUSr8Ryi-ScYNmPyacocPQUPxT00XAYg@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 Thu, 2 Aug 2018 at 18:10, Quentin Perret <quentin.perret@arm.com> wrote:
>
> On Thursday 02 Aug 2018 at 18:07:49 (+0200), Vincent Guittot wrote:
> > On Thu, 2 Aug 2018 at 18:00, Quentin Perret <quentin.perret@arm.com> wrote:
> > >
> > > On Thursday 02 Aug 2018 at 17:55:24 (+0200), Vincent Guittot wrote:
> > > > On Thu, 2 Aug 2018 at 17:30, Quentin Perret <quentin.perret@arm.com> wrote:
> > > > >
> > > > > On Thursday 02 Aug 2018 at 17:14:15 (+0200), Vincent Guittot wrote:
> > > > > > On Thu, 2 Aug 2018 at 16:14, Quentin Perret <quentin.perret@arm.com> wrote:
> > > > > > > Good point, setting the util_avg to 0 for new tasks should help
> > > > > > > filtering out those tiny tasks too. And that would match with the idea
> > > > > > > of letting tasks build their history before looking at their util_avg ...
> > > > > > >
> > > > > > > But there is one difference w.r.t frequency selection. The current code
> > > > > > > won't mark the system overutilized, but will let sugov raise the
> > > > > > > frequency when a new task is enqueued. So in case of a fork bomb, we
> > > > > >
> > > > > > If the initial value of util_avg is 0, we should not have any impact
> > > > > > on the util_avg of the cfs rq on which the task is attached, isn't it
> > > > > > ? so this should not impact both the over utilization state and the
> > > > > > frequency selected by sugov or I'm missing something ?
> > > > >
> > > > > What I tried to say is that setting util_avg to 0 for new tasks will
> > > > > prevent schedutil from raising the frequency in case of a fork bomb, and
> > > > > I think that could be an issue. And I think this isn't an issue with the
> > > > > patch as-is ...
> > > >
> > > > ok. So you also want to deal with fork bomb
> > > > Not sure that you don't have some problem with current proposal too
> > > > because select_task_rq_fair will always return prev_cpu because
> > > > util_avg and util_est are 0 at that time
> > >
> > > But find_idlest_cpu() should select a CPU using load in case of a forkee
> > > no ?
> >
> > So you have to wait for the next tick that will set the overutilized
> > and disable the want_energy. Until this point, all new tasks will be
> > put on the current cpu
>
> want_energy should always be false for forkees, because we set it only
> for SD_BALANCE_WAKE.

Ah yes I forgot that point.
But doesn't this break the EAS policy ? I mean each time a new task is
created, we use the load to select the best CPU