Date: Tue, 8 Mar 2016 20:26:40 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Steve Muckle <steve.muckle@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Linux PM list <linux-pm@vger.kernel.org>,
        Juri Lelli <juri.lelli@arm.com>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler
 utilization data
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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'?