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

From: Vincent Guittot <vincent.guittot@linaro.org>
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>
Subject: Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator
Date: Thu, 2 Aug 2018 17:55:24 +0200	[thread overview]
Message-ID: <CAKfTPtCGGqykN027kxK+chvKvCpNu4bYYdvfZJ+3_7YzT3Utfg@mail.gmail.com> (raw)
In-Reply-To: <20180802153035.vjtmqwdwujvt7ojs@queper01-lin>

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

>
> 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