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    SubjectRe: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler utilization data
    On Tue, Mar 08, 2016 at 07:00:57PM +0100, Rafael J. Wysocki wrote:
    > On Tue, Mar 8, 2016 at 12:27 PM, Peter Zijlstra <peterz@infradead.org> wrote:

    > > Seeing how frequency invariance is an arch feature, and cpufreq drivers
    > > are also typically arch specific, do we really need a flag at this
    > > level?
    >
    > The next frequency is selected by the governor and that's why. The
    > driver gets a frequency to set only.
    >
    > Now, the governor needs to work with different platforms, so it needs
    > to know how to deal with the given one.

    Ah, indeed. In any case, the availability of arch_sched_scale_freq() is
    a compile time thingy, so we can, at compile time, know what to use.

    > > In any case, I think the only difference between the two formula should
    > > be the addition of (1) for the platforms that do not already implement
    > > frequency invariance.
    >
    > OK
    >
    > So I'm reading this as a statement that linear is a better
    > approximation for frequency invariant utilization.

    Well, (1) is what the scheduler does with frequency invariance, except
    that allows a more flexible definition of 'current frequency' by asking
    for it every time we update the util stats.

    But if a platform doesn't need this, ie. it has a fixed frequency, or
    simply doesn't provide anything like this, assuming we run at the
    frequency we asked for is a reasonable assumption no?

    > This means that on platforms where the utilization is frequency
    > invariant we should use
    >
    > next_freq = a * x
    >
    > (where x is given by (2) above) and for platforms where the
    > utilization is not frequency invariant
    >
    > next_freq = a * x * current_freq / max_freq
    >
    > and all boils down to finding a.

    Right.

    > Now, it seems reasonable for a to be something like (1 + 1/n) *
    > max_freq, so for non-frequency invariant we get
    >
    > nex_freq = (1 + 1/n) * current_freq * x

    This seems like a big leap; where does:

    (1 + 1/n) * max_freq

    come from? And what is 'n'?

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