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    SubjectRe: [PATCH 2/4] sched/fair: Drop out incomplete current period when sched averages accrue
    On 12 April 2016 at 23:09, Yuyang Du <yuyang.du@intel.com> wrote:
    >
    > Hi Vincent,
    >
    > On Tue, Apr 12, 2016 at 01:56:45PM +0200, Vincent Guittot wrote:
    > > Le Tuesday 12 Apr 2016 à 03:41:41 (+0800), Yuyang Du a écrit :
    > > > 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 with a HZ set to 1000 and a sched-slice in the same range, having a precision
    > > of 1ms instead of 1us makes the precision of load tracking of short task quite
    > > random IMHO.
    > >
    > > you can keep recording this partial period without using it in the load tracking.
    > > Something like below keep precision without sacrifying the optimization.
    >
    > The residue is accumulated and rolled over to next update every time. But its
    > state is runnable/not-runnable, or running/not-running?

    yes, this need to be sorted

    >
    >
    > > ---
    > > kernel/sched/fair.c | 16 ++++++++++++----
    > > 1 file changed, 12 insertions(+), 4 deletions(-)
    > >
    > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > > index 68273e8..b234169 100644
    > > --- a/kernel/sched/fair.c
    > > +++ b/kernel/sched/fair.c
    > > @@ -674,6 +674,12 @@ void init_entity_runnable_average(struct sched_entity *se)
    > > struct sched_avg *sa = &se->avg;
    > >
    > > sa->last_update_time = 0;
    > > + /*
    > > + * sched_avg's period_contrib should be strictly less then 1024 * 1024, so
    > > + * we give it 1023 * 1024 to make sure it is almost a period (1024us), and
    > > + * will definitely be updated (after enqueue).
    > > + */
    > > + sa->period_contrib = 1023*1024;
    > > sa->load_avg = scale_load_down(se->load.weight);
    > > sa->load_sum = sa->load_avg * LOAD_AVG_MAX;
    > > /*
    > > @@ -2698,10 +2704,6 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
    > > 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
    > > @@ -2712,6 +2714,9 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
    > > return 0;
    > > }
    > >
    > > + /* Add how much left for the current period */
    > > + delta += sa->period_contrib;
    > > +
    > > /*
    > > * Use 1024*1024ns as an approximation of 1ms period, pretty close.
    > > */
    > > @@ -2720,6 +2725,9 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
    > > return 0;
    > > sa->last_update_time = now;
    > >
    > > + /* Get how much left for the next period */
    > > + sa->period_contrib = delta & (u64)(0xFFFFF);
    > > +
    > > scale_freq = arch_scale_freq_capacity(NULL, cpu);
    > > scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
    > >
    > > > 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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