From: Vincent Donnefort <[email protected]> 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 No update would then happen while it would leave ue.enqueued with an out-of-date value.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 <[email protected]> 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
