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

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Message-ID: <20170321140325.gf64gc7eaqu335t5@hirez.programming.kicks-ass.net>
Date:   Tue, 21 Mar 2017 15:03:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.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 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 :-)

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,