On 10 March 2016 at 17:07, Peter Zijlstra [off-list ref] wrote:

On Thu, Mar 10, 2016 at 10:44:21AM +0700, Vincent Guittot wrote:

quoted

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,

quoted

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.

quoted

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.

quoted

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.

quoted

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.