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Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler utilization data

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On 03/03/16 13:20, Peter Zijlstra wrote:
> On Wed, Mar 02, 2016 at 11:49:48PM +0100, Rafael J. Wysocki wrote:
> > >>> +       min_f = sg_policy->policy->cpuinfo.min_freq;
> > >>> +       max_f = sg_policy->policy->cpuinfo.max_freq;
> > >>> +       next_f = util > max ? max_f : min_f + util * (max_f - min_f) / max;
> 
> > In case a more formal derivation of this formula is needed, it is
> > based on the following 3 assumptions:
> > 
> > (1) Performance is a linear function of frequency.
> > (2) Required performance is a linear function of the utilization ratio
> > x = util/max as provided by the scheduler (0 <= x <= 1).
> 
> > (3) The minimum possible frequency (min_freq) corresponds to x = 0 and
> > the maximum possible frequency (max_freq) corresponds to x = 1.
> > 
> > (1) and (2) combined imply that
> > 
> > f = a * x + b
> > 
> > (f - frequency, a, b - constants to be determined) and then (3) quite
> > trivially leads to b = min_freq and a = max_freq - min_freq.
> 
> 3 is the problem, that just doesn't make sense and is probably the
> reason why you see very little selection of the min freq.
> 
> Suppose a machine with the following frequencies:
> 
> 	500, 750, 1000
> 
> And a utilization of 0.4, how does asking for 500 + 0.4 * (1000-500) =
> 700 make any sense? Per your point 1, it should should be asking for
> 0.4 * 1000 = 400.
> 
> Because, per 1, at 500 it runs exactly half as fast as at 1000, and we
> only need 0.4 times as much. Therefore 500 is more than sufficient.
> 

Oh, and that is probably also why the governor can reach max OPP with
freq invariance enabled (the point Vincent was making). When we run at
500 the util signal is capped at that capacity, but the formula makes us
requesting more, so we can jump to the next step and so on.

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[PATCH 0/6] cpufreq: schedutil governor "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-02 03:30 +0100
  [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler utilization data "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-02 03:30 +0100
    Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Vincent Guittot <vincent.guittot@linaro.org> - 2016-03-02 18:20 +0100
      Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data "Rafael J. Wysocki" <rafael@kernel.org> - 2016-03-02 19:00 +0100
        Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data "Rafael J. Wysocki" <rafael@kernel.org> - 2016-03-02 23:50 +0100
          Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Peter Zijlstra <peterz@infradead.org> - 2016-03-03 13:30 +0100
            Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Juri Lelli <juri.lelli@arm.com> - 2016-03-03 13:40 +0100
            Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data "Rafael J. Wysocki" <rafael@kernel.org> - 2016-03-03 17:30 +0100
              Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Peter Zijlstra <peterz@infradead.org> - 2016-03-03 17:40 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Peter Zijlstra <peterz@infradead.org> - 2016-03-03 17:50 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data "Rafael J. Wysocki" <rafael@kernel.org> - 2016-03-04 02:20 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Peter Zijlstra <peterz@infradead.org> - 2016-03-03 18:00 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Juri Lelli <juri.lelli@arm.com> - 2016-03-03 18:20 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Juri Lelli <juri.lelli@arm.com> - 2016-03-03 18:00 +0100
          Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Vincent Guittot <vincent.guittot@linaro.org> - 2016-03-03 15:10 +0100
            Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Peter Zijlstra <peterz@infradead.org> - 2016-03-03 16:40 +0100
              Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Peter Zijlstra <peterz@infradead.org> - 2016-03-03 17:30 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Peter Zijlstra <peterz@infradead.org> - 2016-03-03 17:50 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Dietmar Eggemann <dietmar.eggemann@arm.com> - 2016-03-03 18:30 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Peter Zijlstra <peterz@infradead.org> - 2016-03-03 19:30 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Dietmar Eggemann <dietmar.eggemann@arm.com> - 2016-03-03 20:20 +0100
                Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data "Rafael J. Wysocki" <rafael@kernel.org> - 2016-03-03 20:00 +0100
        Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Vincent Guittot <vincent.guittot@linaro.org> - 2016-03-03 14:10 +0100
          Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Steve Muckle <steve.muckle@linaro.org> - 2016-03-03 21:10 +0100
            Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data "Rafael J. Wysocki" <rafael@kernel.org> - 2016-03-03 21:30 +0100
              Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler  utilization data Steve Muckle <steve.muckle@linaro.org> - 2016-03-03 22:40 +0100
  [PATCH v2 10/10] cpufreq: schedutil: New governor based on scheduler utilization data "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
  [PATCH v2 6/10] cpufreq: Support for fast frequency switching "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
  [PATCH v2 0/10] cpufreq: schedutil governor "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
    [PATCH v2 1/10] cpufreq: Reduce cpufreq_update_util() overhead a bit "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
    [PATCH v2 5/10] cpufreq: Move governor attribute set headers to cpufreq.h "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
      Re: [PATCH v2 5/10] cpufreq: Move governor attribute set headers to  cpufreq.h Viresh Kumar <viresh.kumar@linaro.org> - 2016-03-04 07:00 +0100
    [PATCH v2 2/10][Resend] cpufreq: acpi-cpufreq: Make read and write operations more efficient "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
    [PATCH v2 3/10] cpufreq: governor: New data type for management part of dbs_data "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
      Re: [PATCH v2 3/10] cpufreq: governor: New data type for management  part of dbs_data Viresh Kumar <viresh.kumar@linaro.org> - 2016-03-04 07:00 +0100
    [PATCH v2 7/10] cpufreq: Rework the scheduler hooks for triggering updates "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
    [PATCH v2 4/10] cpufreq: governor: Move abstract gov_attr_set code to seperate file "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
      Re: [PATCH v2 4/10] cpufreq: governor: Move abstract gov_attr_set  code to seperate file Viresh Kumar <viresh.kumar@linaro.org> - 2016-03-04 07:00 +0100
    [PATCH v2 8/10] cpufreq: Move scheduler-related code to the sched directory "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100
    [PATCH v2 9/10] cpufreq: sched: Re-introduce cpufreq_update_util() "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-03-04 04:40 +0100

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