Messages in this thread Patch in this message | | | From | Patrick Bellasi <> | Subject | [PATCH v2] sched/fair: util_est: fast ramp-up EWMA on utilization increases | Date | Wed, 23 Oct 2019 21:56:30 +0100 |
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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.
Signed-off-by: Patrick Bellasi <patrick.bellasi@matbug.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> --- kernel/sched/fair.c | 14 +++++++++++++- kernel/sched/features.h | 1 + 2 files changed, 14 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index a81c36472822..a14487462b6c 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3768,11 +3768,22 @@ 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; @@ -3805,6 +3816,7 @@ 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); } diff --git a/kernel/sched/features.h b/kernel/sched/features.h index 2410db5e9a35..7481cd96f391 100644 --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -89,3 +89,4 @@ SCHED_FEAT(WA_BIAS, true) * UtilEstimation. Use estimated CPU utilization. */ SCHED_FEAT(UTIL_EST, true) +SCHED_FEAT(UTIL_EST_FASTUP, true) -- 2.17.1
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