Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler utilization data
From: Peter Zijlstra
Date: Thu Mar 10 2016 - 05:07:42 EST
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
> 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
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:
next_freq = (1 + 1/n) * max_freq * (util / max)
next_freq = (1 + 1/n) * current_freq * (util_raw / max)