Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 08, 2015 at 09:22:05AM +0200, Vincent Guittot wrote:
> No, but
> sa->util_avg = (sa->util_sum << SCHED_CAPACITY_SHIFT) / LOAD_AVG_MAX;
> will fix the unit issue.

Tricky that, LOAD_AVG_MAX very much relies on the unit being 1<<10.

And where load_sum already gets a factor 1024 from the weight
multiplication, util_sum does not get such a factor, and all the scaling
we do on it loose bits.

So at the moment we go compute the util_avg value, we need to inflate
util_sum with an extra factor 1024 in order to make it work.

And seeing that we do the shift up on sa->util_sum without consideration
of overflow, would it not make sense to add that factor before the
scaling and into the addition?

Now, given all that, units are a complete mess here, and I'd not mind
something like:

#if (SCHED_LOAD_SHIFT - SCHED_LOAD_RESOLUTION) != SCHED_CAPACITY_SHIFT
#error "something usefull"
#endif

somewhere near here.