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 No update would then happen while it would leave ue.enqueued with an out-of-date value.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 bbc78794224a..dfb65140eb2d 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3959,6 +3959,8 @@ 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: @@ -3976,7 +3978,7 @@ 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)) @@ -3997,6 +3999,8 @@ 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. @@ -4010,12 +4014,17 @@ 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