Commit b89997aa authored by Vincent Donnefort's avatar Vincent Donnefort Committed by Ingo Molnar
Browse files

sched/pelt: Fix task util_est update filtering 



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ue.enqueued with an out-of-date value.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: default avatarVincent Donnefort <vincent.donnefort@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Reviewed-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210225165820.1377125-1-vincent.donnefort@arm.com
parent 39a2a6eb

kernel/sched/fair.c

+12 −3
Original line number Diff line number Diff line
@@ -3941,6 +3941,8 @@ 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:
@@ -3958,7 +3960,7 @@ 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))
@@ -3979,6 +3981,8 @@ 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 +3996,17 @@ 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