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

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Date:   Thu, 2 Aug 2018 16:30:37 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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 Thursday 02 Aug 2018 at 17:14:15 (+0200), Vincent Guittot wrote:
> On Thu, 2 Aug 2018 at 16:14, Quentin Perret <quentin.perret@....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 ...

Sorry if that wasn't clear

> Then, select_task_rq_fair is called for a new task but util_avg is
> still 0 at that time in the current code so you will have consistent
> util_avg of the new task before and after calling
> find_energy_efficient_cpu

New tasks don't go in find_energy_efficient_cpu(), because, as you said,
they have no consistent util_avg yet when select_task_rq_fair() is called
for the first time.

Thanks,
Quentin