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Message-ID: <20160308192640.GD6344@twins.programming.kicks-ass.net>
Date: Tue, 8 Mar 2016 20:26:40 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>,
Steve Muckle <steve.muckle@...aro.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Linux PM list <linux-pm@...r.kernel.org>,
Juri Lelli <juri.lelli@....com>,
ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
Viresh Kumar <viresh.kumar@...aro.org>,
Michael Turquette <mturquette@...libre.com>,
Ingo Molnar <mingo@...hat.com>
Subject: Re: [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@...radead.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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