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From: Quentin Perret <quentin.perret@arm.com>
Subject: Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point
 indicator
Date: Thu, 2 Aug 2018 16:30:37 +0100
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To: Vincent Guittot <vincent.guittot@linaro.org>
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
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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 ...

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