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. -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
