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Date: Thu, 10 Mar 2016 11:56:14 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Juri Lelli <juri.lelli@arm.com>, mturquette <mturquette@baylibre.com>,
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Subject: Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler
 utilization data
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On Thu, Mar 10, 2016 at 11:30:08AM +0100, Peter Zijlstra wrote:
> On Thu, Mar 10, 2016 at 05:23:54PM +0700, Vincent Guittot wrote:
> 
> > > 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.
> 
> Well, its bound to get more 'interesting' since I forse implementations
> not always actually doing the invariant thing.
> 
> Take for example the thing I send:
> 
>   lkml.kernel.org/r/20160303162829.GB6375@twins.programming.kicks-ass.net
> 
> it both shows why you cannot talk about current_freq but also that the
> above needs a little more help (for the !X86_FEATURE_APERFMPERF case).
> 
> But the !arch_scale_freq_capacity case should indeed be that simple.

Maybe something like:

#ifdef arch_scale_freq_capacity
#ifndef arch_scale_freq_invariant
#define arch_scale_freq_invariant()	(true)
#endif
#else /* arch_scale_freq_capacity */
#define arch_scale_freq_invariant()	(false)
#endif

	if (arch_scale_freq_invariant())

And have archs that have conditional arch_scale_freq_capacity()
implementation provide a arch_scale_freq_invariant implementation.