linux-kernel - Re: [RFC][PATCH v2 2/2] cpufreq: schedutil: Avoid decreasing frequency of busy CPUs

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Date:   Tue, 21 Mar 2017 18:00:59 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Juri Lelli <juri.lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC][PATCH v2 2/2] cpufreq: schedutil: Avoid decreasing
 frequency of busy CPUs

On 21 March 2017 at 15:58, Peter Zijlstra <peterz@...radead.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@...radead.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@...radead.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.