Re: [PATCH v10 04/11] sched: Make sched entity usage tracking scale-invariant

[Morten Rasmussen]

On Wed, Mar 25, 2015 at 06:08:42PM +0000, Vincent Guittot wrote:
> On 25 March 2015 at 18:33, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Tue, Mar 24, 2015 at 11:00:57AM +0100, Vincent Guittot wrote:
> >> On 23 March 2015 at 14:19, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >> > On Fri, Feb 27, 2015 at 04:54:07PM +0100, Vincent Guittot wrote:
> >> >
> >> >> +     unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);
> >> >
> >> >> +                     sa->running_avg_sum += delta_w * scale_freq
> >> >> +                             >> SCHED_CAPACITY_SHIFT;
> >> >
> >> > so the only thing that could be improved is somehow making this
> >> > multiplication go away when the arch doesn't implement the function.
> >> >
> >> > But I'm not sure how to do that without #ifdef.
> >> >
> >> > Maybe a little something like so then... that should make the compiler
> >> > get rid of those multiplications unless the arch needs them.
> >>
> >> yes, it removes useless multiplication when not used by an arch.
> >> It also adds a constraint on the arch side which have to define
> >> arch_scale_freq_capacity like below:
> >>
> >> #define arch_scale_freq_capacity xxx_arch_scale_freq_capacity
> >> with xxx_arch_scale_freq_capacity an architecture specific function
> >
> > Yeah, but it not being weak should make that a compile time warn/fail,
> > which should be pretty easy to deal with.
> >
> >> If it sounds acceptable i can update the patch with your proposal ?
> >
> > I'll stick it to the end, I just wanted to float to patch to see if
> > people had better solutions.
> 
> ok. all other methods that i have tried, was removing the optimization
> when default arch_scale_freq_capacity was used

Another potential solution is to stay with weak functions but move the
multiplication and shift into the arch_scale_*() functions by passing
the value we want to scale into the arch_scale_*() function. That way we
can completely avoid multiplication and shift in the default case (no
arch_scale*() implementations, which is better than what we have today.

The only downside is that for frequency invariance we need three
arch_scale_freq_capacity() calls instead of two.
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