Messages in this thread | | | Date | Thu, 25 Feb 2021 16:07:08 +0000 | From | Vincent Donnefort <> | Subject | Re: [PATCH] sched/pelt: Fix task util_est update filtering |
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On Thu, Feb 25, 2021 at 04:26:50PM +0100, Vincent Guittot wrote: > On Mon, 22 Feb 2021 at 10:24, Vincent Donnefort > <vincent.donnefort@arm.com> wrote: > > > > On Fri, Feb 19, 2021 at 11:48:28AM +0100, Vincent Guittot wrote: > > > On Tue, 16 Feb 2021 at 17:39, <vincent.donnefort@arm.com> wrote: > > > > > > > > From: Vincent Donnefort <vincent.donnefort@arm.com> > > > > > > > > Being called for each dequeue, util_est reduces the number of its updates > > > > by filtering out when the EWMA signal is different from the task util_avg > > > > by less than 1%. It is a problem for a sudden util_avg ramp-up. Due to the > > > > decay from a previous high util_avg, EWMA might now be close enough to > > > > the new util_avg. No update would then happen while it would leave > > > > ue.enqueued with an out-of-date value. > > > > > > > > Taking into consideration the two util_est members, EWMA and enqueued for > > > > the filtering, ensures, for both, an up-to-date value. > > > > > > > > This is for now an issue only for the trace probe that might return the > > > > stale value. Functional-wise, it isn't (yet) a problem, as the value is > > > > > > What do you mean by "it isn't (yet) a problem" ? How could this become > > > a problem ? > > > > I wrote "yet" as nothing prevents anyone from using the ue.enqueued signal. > > Hmm.. you are not supposed to use it outside the helper functions so > this is irrelevant IMO which means that only the trace probe is > impacted
I'll remove it.
> > > > > > > > > > always accessed through max(enqueued, ewma). > > > > > > > > > > This adds more tests and or update of struct avg.util_est. It would > > > be good to have an idea of the perf impact. Especially because this > > > only fixes a tracing problem > > > > I ran hackbench on the big cores of a SD845C board. After 100 iterations of > > 100 loops runs, the geometric mean of the hackbench test is 0.1% lower > > with this patch applied (2.0833s vs 2.0858s). The p-value, computed with > > the ks_2samp [1] is 0.37. We can't conclude that the two distributions are > > different. This patch, in this scenario seems completely harmless. > > For such kind of change, perf bench sched pipe is better to highlight > any perf regression. I have done a quick test and i haven't seen > noticeable difference
Thanks. I'll add your results to the commit message.
> > > > > Shall I include those results in the commit message? > > > > [1] https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.ks_2samp.html > > > > > > > > > > > > This problem has been observed using LISA's UtilConvergence:test_means on > > > > the sd845c board. > > > > > > > > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com> > > > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > > index 794c2cb945f8..9008e0c42def 100644 > > > > --- a/kernel/sched/fair.c > > > > +++ b/kernel/sched/fair.c > > > > @@ -3941,24 +3941,27 @@ static inline void util_est_dequeue(struct cfs_rq *cfs_rq, > > > > trace_sched_util_est_cfs_tp(cfs_rq); > > > > } > > > > > > > > +#define UTIL_EST_MARGIN (SCHED_CAPACITY_SCALE / 100) > > > > + > > > > /* > > > > - * Check if a (signed) value is within a specified (unsigned) margin, > > > > + * Check if a (signed) value is within the (unsigned) util_est margin, > > > > * based on the observation that: > > > > * > > > > * abs(x) < y := (unsigned)(x + y - 1) < (2 * y - 1) > > > > * > > > > - * NOTE: this only works when value + maring < INT_MAX. > > > > + * NOTE: this only works when value + UTIL_EST_MARGIN < INT_MAX. > > > > */ > > > > -static inline bool within_margin(int value, int margin) > > > > +static inline bool util_est_within_margin(int value) > > > > { > > > > - return ((unsigned int)(value + margin - 1) < (2 * margin - 1)); > > > > + return ((unsigned int)(value + UTIL_EST_MARGIN - 1) < > > > > + (2 * UTIL_EST_MARGIN - 1)); > > > > } > > > > > > > > static inline void util_est_update(struct cfs_rq *cfs_rq, > > > > struct task_struct *p, > > > > bool task_sleep) > > > > { > > > > - long last_ewma_diff; > > > > + long last_ewma_diff, last_enqueued_diff; > > > > struct util_est ue; > > > > > > > > if (!sched_feat(UTIL_EST)) > > > > @@ -3979,6 +3982,8 @@ static inline void util_est_update(struct cfs_rq *cfs_rq, > > > > if (ue.enqueued & UTIL_AVG_UNCHANGED) > > > > return; > > > > > > > > + last_enqueued_diff = ue.enqueued; > > > > + > > > > /* > > > > * Reset EWMA on utilization increases, the moving average is used only > > > > * to smooth utilization decreases. > > > > @@ -3992,12 +3997,19 @@ static inline void util_est_update(struct cfs_rq *cfs_rq, > > > > } > > > > > > > > /* > > > > - * Skip update of task's estimated utilization when its EWMA is > > > > + * Skip update of task's estimated utilization when its members are > > > > * already ~1% close to its last activation value. > > > > */ > > > > last_ewma_diff = ue.enqueued - ue.ewma; > > > > - if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100))) > > > > + last_enqueued_diff -= ue.enqueued; > > > > + if (util_est_within_margin(last_ewma_diff)) { > > > > + if (!util_est_within_margin(last_enqueued_diff)) { > > > > + ue.ewma = ue.enqueued; > > why do you set ewma directly with latest enqueued value ?
The idea was to align both ewma and enqueued, as the diff is < 1% anyway.
I'll remove that in v2.
> > > > > + goto done; > > > > + } > > > > + > > > > return; > > > > + } > > > > > > > > /* > > > > * To avoid overestimation of actual task utilization, skip updates if > > > > -- > > > > 2.25.1 > > > >
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