[PATCH AUTOSEL 5.10 014/100] sched/pelt: Fix task util_est update filtering

Sasha Levin
2021-05-03T16:47:20Z

From: Vincent Donnefort <vincent.donnefort@arm.com>

[ Upstream commit b89997aa88f0b07d8a6414c908af75062103b8c9 ]

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ue.enqueued with an out-of-date value.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 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.

No regression observed with Hackbench on sd845c and Perf-bench sched pipe
on hikey/hikey960.

Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210225165820.1377125-1-vincent.donnefort@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/sched/fair.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 348605306027..8f5bbc1469ed 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3948,6 +3948,8 @@ 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,
* based on the observation that:
@@ -3965,7 +3967,7 @@ 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))
@@ -3986,6 +3988,8 @@ 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.
@@ -3999,12 +4003,17 @@ 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 (within_margin(last_ewma_diff, UTIL_EST_MARGIN)) {
+ if (!within_margin(last_enqueued_diff, UTIL_EST_MARGIN))
+ goto done;
+
return;
+ }

/*
* To avoid overestimation of actual task utilization, skip updates if
--
2.30.2