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Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required RT CPU capacity

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On Wednesday, August 31, 2016 06:40:09 PM Peter Zijlstra wrote:
> On Wed, Aug 31, 2016 at 06:28:10PM +0200, Thomas Gleixner wrote:
> > > That is the way it's been with cpufreq and many systems (including all
> > > mobile devices) rely on that to not destroy power. RT + variable cpufreq
> > > is not deterministic.
> > > 
> > > Given we don't have good constraints on RT tasks I don't think we should
> > > try to strengthen the semantics there. Folks should either move to DL if
> > > they want determinism *and* not-sucky power, or continue disabling
> > > cpufreq if they are able to do so.
> > 
> > RT deterministic behaviour is all about meeting the deadlines. If your
> > deadline is relaxed enough that you can meet it even with the lowest cpu
> > frequency then it's perfectly fine to enable cpufreq. The same logic applies
> > to C-States.
> > 
> > There are a lot of RT systems out there which enable both. If cpufreq or
> > c-states cause a deadline violation because the constraints of the system are
> > tight, then people will disable it and we need a knob for both.
> > 
> > Realtime is not as fast as possible. It's as fast as specified.
> 
> Sure, problem is of course that RR/FIFO doesn't specify anything so the
> users are left to prod knobs.
> 
> Another problem is that we have many semi related knobs; we have the
> global RT runtime limit knob, but that doesn't affect cpufreq (maybe it
> should) and cpufreq has knobs to set f_min and f_max, which again are
> unaware of RT anything.
> 
> So before we go do anything, I'd like input on what is needed and how
> things should tie together to make most sense.

I totally agree.

We need to know where we want to get to before deciding on which way to go.

Thanks,
Rafael

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[PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required RT CPU capacity Steve Muckle <steve.muckle@linaro.org> - 2016-08-26 20:50 +0200
  Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required RT CPU capacity "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-08-31 03:30 +0200
    Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Steve Muckle <steve.muckle@linaro.org> - 2016-08-31 16:50 +0200
  Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Peter Zijlstra <peterz@infradead.org> - 2016-08-31 16:40 +0200
    Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Steve Muckle <steve.muckle@linaro.org> - 2016-08-31 17:10 +0200
      Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Peter Zijlstra <peterz@infradead.org> - 2016-08-31 18:50 +0200
        Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Juri Lelli <Juri.Lelli@arm.com> - 2016-08-31 19:00 +0200
          Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Peter Zijlstra <peterz@infradead.org> - 2016-09-01 09:20 +0200
          Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Steve Muckle <steve.muckle@linaro.org> - 2016-09-01 23:50 +0200
            Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Juri Lelli <Juri.Lelli@arm.com> - 2016-09-02 11:40 +0200
              Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Thomas Gleixner <tglx@linutronix.de> - 2016-09-02 14:20 +0200
        Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required RT CPU capacity "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2016-09-01 00:50 +0200
        Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Thomas Gleixner <tglx@linutronix.de> - 2016-09-02 10:20 +0200
      Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required  RT CPU capacity Thomas Gleixner <tglx@linutronix.de> - 2016-08-31 19:10 +0200

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