Date: Thu, 10 Mar 2016 11:07:28 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Steve Muckle <steve.muckle@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 Thu, Mar 10, 2016 at 10:44:21AM +0700, Vincent Guittot wrote:
> We have the arch_scale_freq_capacity function that is arch dependent
> and can be used to merge the 2 formula that were described by peter
> above.
> By default, arch_scale_freq_capacity return SCHED_CAPACITY_SCALE which
> is max capacity
> but when arch_scale_freq_capacity is defined by an architecture,

> arch_scale_freq_capacity returns current_freq * max_capacity/max_freq

However, current_freq is a very fluid thing, it might (and will) change
very rapidly on some platforms.

This is the same point I made earlier, you cannot try and divide out
current_freq from the invariant measure.

> so can't we use arch_scale_freq in your formula ? Taking your formula
> above it becomes:
> next_freq = 1.25 * current_freq * util / arch_scale_freq_capacity()

No, that cannot work, nor makes any sense, per the above.

> With invariance feature, we have:
>
>   next_freq = 1.25 * current_freq * util / (current_freq*max_capacity/max_freq)
>             = 1.25 * util * max_freq / max
>
> which is the formula that has to be used with frequency invariant
> utilization.

Wrong, you cannot talk about current_freq in the invariant case.

> May be we can pass arch_scale_freq_capacity value instead of max one
> as a parameter of update_util function prototype

No, since its a compile time thing, we can simply do:

#ifdef arch_scale_freq_capacity
	next_freq = (1 + 1/n) * max_freq * (util / max)
#else
	next_freq = (1 + 1/n) * current_freq * (util_raw / max)
#endif