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Subject[PATCH] sched/pelt: Fix task util_est update filtering
Date
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ue.enqueued with an out-of-date value.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
always accessed through max(enqueued, ewma).

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;
+ goto done;
+ }
+
return;
+ }

/*
* To avoid overestimation of actual task utilization, skip updates if
--
2.25.1
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