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

From: Peter Zijlstra
Date: Tue Mar 08 2016 - 14:26:59 EST

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@xxxxxxxxxxxxx> 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.


> 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'?