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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Tue, 21 Mar 2017 18:00:59 +0100
Message-ID: <CAKfTPtBzeKZkYeyw2pp7g3fvhnY0gzViargmDpEbkPhA+df72g@mail.gmail.com>
Subject: Re: [RFC][PATCH v2 2/2] cpufreq: schedutil: Avoid decreasing
 frequency of busy CPUs
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Linux PM <linux-pm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Juri Lelli <juri.lelli@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Joel Fernandes <joelaf@google.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Ingo Molnar <mingo@redhat.com>
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On 21 March 2017 at 15:58, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Mar 21, 2017 at 03:16:19PM +0100, Vincent Guittot wrote:
> > On 21 March 2017 at 15:03, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > > On Tue, Mar 21, 2017 at 02:37:08PM +0100, Vincent Guittot wrote:
> > > > On 21 March 2017 at 14:22, Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > > For the not overloaded case, it makes sense to immediately update to
> > > > OPP to be aligned with the new utilization of the CPU even if it was
> > > > not idle in the past couple of ticks
> > >
> > > Yeah, but we cannot know. Also, who cares?
> > >
> >
> > embedded system that doesn't want to stay at higest OPP if significant part
> > of the utilzation has moved away as an example
> > AFAICT, schedutil tries to select the best OPP according to the current
> > utilization of the CPU so if the utilization decreases, the OPP should also
> > decrease
>
> Sure I get that; but given the lack of crystal ball instructions we
> cannot know if this is the case or not.

cfs_rq->avg.load_avg account the waiting time of CPU (in addition to
the weight of task) so i was wondering if we can't use it to detect if
we are in the overloaded  case or not even if utilization is not mac
capacity because we have just migrated a task (and its utilization)
out


>
> And if we really dropped below 100% utilization, we should hit idle
> fairly soon.