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 14:25:29 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        "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>,
        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-Mar 15:03, Peter Zijlstra 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?
> 
> > > does exactly that. Note that the lack of idle time is an exact
> > > equivalent of 100% utilized.
> > >
> > > So even while we cannot currently detect the 100% utilized state through
> > > the running state tracking; because averages etc.. we can detect the
> > > lack of idle time.
> > 
> > But after how much lack of idle time do we consider that we are overloaded ?
> 
> 0 :-)

If we should use "utilization" this time can be non 0 and it depends
for example on how long PELT takes to build up a utilization value
which marks the CPU as "overutilized"... thus we already have a
suitable time at least for CFS tasks.

> Note that utilization is an absolute metric, not a windowed one. That
> is, there is no actual time associated with it. Now, for practical
> purposes we end up using windowed things in many places,
> 

-- 
#include <best/regards.h>

Patrick Bellasi