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

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Message-ID: <CAKfTPtAV0yxBQu7CSA-3GZY+ibs8=rkP13ZoBDMJfH3jV6SX=Q@mail.gmail.com>
Date:	Thu, 10 Mar 2016 17:26:34 +0700
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Steve Muckle <steve.muckle@...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 10 March 2016 at 17:07, Peter Zijlstra <peterz@...radead.org> wrote:
> 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

selecting formula at compilation is clearly better. I wrongly thought
that it can't be accepted as a solution.