Re: [PATCH v2] sched/pelt: Fix task util_est update filtering
From: Vincent Guittot
Date: Fri Feb 26 2021 - 02:24:00 EST
On 2021年2月25日 at 17:58, Vincent Donnefort
<vincent.donnefort@xxxxxxx> wrote:
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Being called for each dequeue, util_est reduces the number of its updates
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by filtering out when the EWMA signal is different from the task util_avg
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by less than 1%. It is a problem for a sudden util_avg ramp-up. Due to the
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decay from a previous high util_avg, EWMA might now be close enough to
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the new util_avg. No update would then happen while it would leave
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ue.enqueued with an out-of-date value.
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Taking into consideration the two util_est members, EWMA and enqueued for
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the filtering, ensures, for both, an up-to-date value.
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This is for now an issue only for the trace probe that might return the
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stale value. Functional-wise, it isn't a problem, as the value is always
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accessed through max(enqueued, ewma).
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This problem has been observed using LISA's UtilConvergence:test_means on
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the sd845c board.
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No regression observed with Hackbench on sd845c and Perf-bench sched pipe
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on hikey/hikey960.
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Signed-off-by: Vincent Donnefort <vincent.donnefort@xxxxxxx>
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Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
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diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
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index 9e4104ae39ae..214e02862994 100644
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--- a/kernel/sched/fair.c
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+++ b/kernel/sched/fair.c
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@@ -3966,24 +3966,27 @@ static inline void util_est_dequeue(struct cfs_rq *cfs_rq,
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trace_sched_util_est_cfs_tp(cfs_rq);
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}
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+#define UTIL_EST_MARGIN (SCHED_CAPACITY_SCALE / 100)
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+
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/*
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- * Check if a (signed) value is within a specified (unsigned) margin,
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+ * Check if a (signed) value is within the (unsigned) util_est margin,
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* based on the observation that:
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*
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* abs(x) < y := (unsigned)(x + y - 1) < (2 * y - 1)
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*
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- * NOTE: this only works when value + maring < INT_MAX.
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+ * NOTE: this only works when value + UTIL_EST_MARGIN < INT_MAX.
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*/
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-static inline bool within_margin(int value, int margin)
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+static inline bool util_est_within_margin(int value)
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{
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- return ((unsigned int)(value + margin - 1) < (2 * margin - 1));
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+ return ((unsigned int)(value + UTIL_EST_MARGIN - 1) <
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+ (2 * UTIL_EST_MARGIN - 1));
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}
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static inline void util_est_update(struct cfs_rq *cfs_rq,
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struct task_struct *p,
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bool task_sleep)
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{
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- long last_ewma_diff;
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+ long last_ewma_diff, last_enqueued_diff;
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struct util_est ue;
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if (!sched_feat(UTIL_EST))
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@@ -4004,6 +4007,8 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
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if (ue.enqueued & UTIL_AVG_UNCHANGED)
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return;
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+ last_enqueued_diff = ue.enqueued;
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+
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/*
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* Reset EWMA on utilization increases, the moving average is used only
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* to smooth utilization decreases.
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@@ -4017,12 +4022,17 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
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}
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/*
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- * Skip update of task's estimated utilization when its EWMA is
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+ * Skip update of task's estimated utilization when its members are
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* already ~1% close to its last activation value.
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*/
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last_ewma_diff = ue.enqueued - ue.ewma;
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- if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100)))
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+ last_enqueued_diff -= ue.enqueued;
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+ if (util_est_within_margin(last_ewma_diff)) {
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+ if (!util_est_within_margin(last_enqueued_diff))
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+ goto done;
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+
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return;
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+ }
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/*
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* To avoid overestimation of actual task utilization, skip updates if
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--
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2.25.1
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