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?
> ---
> 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.
