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

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