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sched/fair: Fast ramp-up EWMA on utilization increases

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The estimated utilization for a task:

   util_est = max(util_avg, est.enqueue, est.ewma)

is defined based on:
 - util_avg: the PELT defined utilization
 - est.enqueued: the util_avg at the end of the last activation
 - est.ewma:     a exponential moving average on the est.enqueued
                 samples

According to this definition, when a task suddenly change its bandwidth
requirements from small to big, the EWMA will need to collect multiple
samples before converging up to track the new big utilization.

This slow convergence towards bigger utilization values is not
aligned to the default scheduler behavior, which is to optimize for
performance. Moreover, the est.ewma component fails to compensate for
temporarely utilization drops which spans just few est.enqueued samples.

To let util_est do a better job in the scenario depicted above, change
its definition by making util_est directly follow upward motion and
only decay the est.ewma on downward.

https://lkml.org/lkml/2019/10/23/1071

Change-Id: I1837e322b02958faeaff65563127e6ab682c93e1
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@matbug.com>
Signed-off-by: Adam W. Willis <return.of.octobot@gmail.com>
Signed-off-by: Yaroslav Furman <yaro330@gmail.com>
Signed-off-by: Richard Raya <rdxzv.dev@gmail.com>
This commit is contained in:
Patrick Bellasi 2019-10-23 21:56:30 +01:00 committed by Richard Raya
parent 61e59264a4
commit d34e463b5b
2 changed files with 14 additions and 1 deletions
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14
kernel/sched/fair.c
View File

@ -3884,11 +3884,22 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
if (ue.enqueued & UTIL_AVG_UNCHANGED)
return;
/*
* Reset EWMA on utilization increases, the moving average is used only
* to smooth utilization decreases.
*/
ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED);
if (sched_feat(UTIL_EST_FASTUP)) {
if (ue.ewma < ue.enqueued) {
ue.ewma = ue.enqueued;
goto done;
}
}
/*
* Skip update of task's estimated utilization when its EWMA is
* already ~1% close to its last activation value.
*/
ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED);
last_ewma_diff = ue.enqueued - ue.ewma;
if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100)))
return;
@ -3913,6 +3924,7 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
ue.ewma <<= UTIL_EST_WEIGHT_SHIFT;
ue.ewma += last_ewma_diff;
ue.ewma >>= UTIL_EST_WEIGHT_SHIFT;
done:
WRITE_ONCE(p->se.avg.util_est, ue);
trace_sched_util_est_task(p, &p->se.avg);

1
kernel/sched/features.h
View File

@ -92,6 +92,7 @@
* UtilEstimation. Use estimated CPU utilization.
*/
#define SCHED_FEAT_UTIL_EST 1
#define SCHED_FEAT_UTIL_EST_FASTUP 1
/*
* Energy aware scheduling. Use platform energy model to guide scheduling