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

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