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

From: Vincent Guittot
Date: Thu Mar 10 2016 - 05:27:13 EST

On 10 March 2016 at 17:07, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.