Re: [PATCH 2/4] sched/fair: Drop out incomplete current period when sched averages accrue

[Yuyang Du]

Hi Vincent,

On Mon, Apr 11, 2016 at 11:08:04AM +0200, Vincent Guittot wrote:
> > @@ -2704,11 +2694,14 @@ static __always_inline int
> >  __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
> >                   unsigned long weight, int running, struct cfs_rq *cfs_rq)
> >  {
> > -       u64 delta, scaled_delta, periods;
> > -       u32 contrib;
> > -       unsigned int delta_w, scaled_delta_w, decayed = 0;
> > +       u64 delta;
> > +       u32 contrib, periods;
> >         unsigned long scale_freq, scale_cpu;
> >
> > +       /*
> > +        * now rolls down to a period boundary
> > +        */
> > +       now = now && (u64)(~0xFFFFF);
> >         delta = now - sa->last_update_time;
> >         /*
> >          * This should only happen when time goes backwards, which it
> > @@ -2720,89 +2713,56 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
> >         }
> >
> >         /*
> > -        * Use 1024ns as the unit of measurement since it's a reasonable
> > -        * approximation of 1us and fast to compute.
> > +        * Use 1024*1024ns as an approximation of 1ms period, pretty close.
> >          */
> > -       delta >>= 10;
> > -       if (!delta)
> > +       periods = delta >> 20;
> > +       if (!periods)
> >                 return 0;
> >         sa->last_update_time = now;
> 
> The optimization looks quite interesting but I see one potential issue
> with migration as we will lose the part of the ongoing period that is
> now not saved anymore. This lost part can be quite significant for a
> short task that ping pongs between CPUs.

Yes, basically, it is we lose precision (~1ms scale in contrast with ~1us scale).
But as I wrote, we may either lose a sub-1ms, or gain a sub-1ms, statistically,
they should even out, given the load/util updates are quite a large number of
samples, and we do want a lot of samples for the metrics, this is the point of
the entire average thing. Plus, as you also said, the incomplete current period
also plays a (somewhat) negative role here.

In addition, excluding the flat hierarchical util patch, we gain quite some
efficiency.