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

[Vincent Guittot]

On 8 September 2015 at 14:26, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 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.

fair point

>
> 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?

Yes this should save 1 left shift and 1 right shift

 >>

>
> 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

In this case why not simply doing
#define SCHED_CAPACITY_SHIFT SCHED_LOAD_SHIFT
or the opposite ?

>
> somewhere near here.
>
>
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