On 10/07/17 10:42, Viresh Kumar wrote: > On 10-07-17, 11:30, Peter Zijlstra wrote: >> On Sat, Jul 08, 2017 at 02:09:37PM +0200, Rafael J. Wysocki wrote: >>> Anyway, if everyone agrees that doing it in the core is the way to go >>> (Peter?), >>> why don't you introduce a __weak function for setting policy->cur and >>> override it from your arch so as to call arch_set_freq_scale() from there? >>> >> >> So I'm terminally backlogged and my recent break didn't help any with >> that. >> >> I'm at a total loss as to what is proposed here and why we need it. I >> tried reading both the Changelog and patch but came up empty. > > Dietmar is proposing the implementation of arch_set_freq_scale() for ARM > (32/64) > platforms here with following equation in drivers/base/arch_topology.c: > > scale = (cur_freq << SCHED_CAPACITY_SHIFT) / max_freq > > The only variable part here is "cur_freq" and he is looking for sane ways to > get > that value in the arch_topology.c file, so he can use that in the above > equation. He tried to use cpufreq transition notifiers earlier but they block > us > from using fast switching. > > What he is proposing now is a function: > > void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq, > unsigned long max_freq); > > which has to be called by someone after the frequency of the CPU is changed. > > Dietmar proposed that this be called by cpufreq core and Rafael was wondering > if > the cpufreq drivers should call it. Dietmar's argument is that it will be used > for the entire ARM architecture this way and wouldn't lead to redundant core > across drivers. > > Hope I didn't confuse you more with this :) >
Perfect summary, thanks Viresh! This is required for architectures (like arm/arm64) which do not have any other way to know about the current CPU frequency. X86 can do the frequency invariance support based on APERF/MPERF already today so it does not need the support from cpufreq.
