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

[Morten Rasmussen]

On Tue, Sep 08, 2015 at 04:06:36PM +0200, Vincent Guittot wrote:
> On 8 September 2015 at 14:52, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Tue, Sep 08, 2015 at 02:26:06PM +0200, Peter Zijlstra 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.
> >>
> >> 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.
> >
> > Something like teh below..
> >
> > Another thing to ponder; the downside of scaled_delta_w is that its
> > fairly likely delta is small and you loose all bits, whereas the weight
> > is likely to be large can could loose a fwe bits without issue.
> >
> > That is, in fixed point scaling like this, you want to start with the
> > biggest numbers, not the smallest, otherwise you loose too much.
> >
> > The flip side is of course that now you can share a multiplcation.
> >
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -682,7 +682,7 @@ void init_entity_runnable_average(struct
> >         sa->load_avg = scale_load_down(se->load.weight);
> >         sa->load_sum = sa->load_avg * LOAD_AVG_MAX;
> >         sa->util_avg = scale_load_down(SCHED_LOAD_SCALE);
> > -       sa->util_sum = LOAD_AVG_MAX;
> > +       sa->util_sum = sa->util_avg * LOAD_AVG_MAX;
> >         /* when this task enqueue'ed, it will contribute to its cfs_rq's load_avg */
> >  }
> >
> > @@ -2515,6 +2515,10 @@ static u32 __compute_runnable_contrib(u6
> >         return contrib + runnable_avg_yN_sum[n];
> >  }
> >
> > +#if (SCHED_LOAD_SHIFT - SCHED_LOAD_RESOLUTION) != 10 || SCHED_CAPACITY_SHIFT != 10
> > +#error "load tracking assumes 2^10 as unit"
> > +#endif
> 
> so why don't we set SCHED_CAPACITY_SHIFT to SCHED_LOAD_SHIFT ?

Don't you mean:

#define SCHED_LOAD_SHIFT (SCHED_CAPACITY_SHIFT + SCHED_LOAD_RESOLUTION)

?

Or do you want to increase the capacity resolution as well if you
increase the load resolution?
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