Messages in this thread Patch in this message | | | From | Vincent Guittot <> | Subject | [PATCH 1/3] sched/pelt: Don't sync hardly util_sum with uti_avg | Date | Tue, 21 Dec 2021 10:46:51 +0100 |
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Rick reported performance regressions in bugzilla because of cpu frequency being lower than before: https://bugzilla.kernel.org/show_bug.cgi?id=215045
He bisected the problem to: commit 1c35b07e6d39 ("sched/fair: Ensure _sum and _avg values stay consistent")
This commit forces util_sum to be synced with the new util_avg after removing the contribution of a task and before the next periodic sync. By doing so util_sum is rounded to its lower bound and might lost up to LOAD_AVG_MAX-1 of accumulated contribution which has not yet been reflected in util_avg.
Instead of always setting util_sum to the low bound of util_avg, which can significantly lower the utilization of root cfs_rq after propagating the change down into the hierarchy, we revert the commit and propagate the difference in util_sum.
In addition, we also check that cfs's util_sum always stays above the lower bound for a given util_avg as it has been observed that sched_entity's util_sum is sometimes above cfs one.
Fixes: 1c35b07e6d39 ("sched/fair: Ensure _sum and _avg values stay consistent") Reported-by: Rick Yiu <rickyiu@google.com> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> --- kernel/sched/fair.c | 41 ++++++++++++++++++++++++++++++++--------- 1 file changed, 32 insertions(+), 9 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 095b0aa378df..9ac28f0f3645 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3381,6 +3381,7 @@ void set_task_rq_fair(struct sched_entity *se, se->avg.last_update_time = n_last_update_time; } +#define MIN_DIVIDER (LOAD_AVG_MAX - 1024) /* * When on migration a sched_entity joins/leaves the PELT hierarchy, we need to @@ -3449,15 +3450,14 @@ void set_task_rq_fair(struct sched_entity *se, * XXX: only do this for the part of runnable > running ? * */ - static inline void update_tg_cfs_util(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq) { - long delta = gcfs_rq->avg.util_avg - se->avg.util_avg; - u32 divider; + long delta_sum, delta_avg = gcfs_rq->avg.util_avg - se->avg.util_avg; + u32 new_sum, divider; /* Nothing to update */ - if (!delta) + if (!delta_avg) return; /* @@ -3466,13 +3466,30 @@ update_tg_cfs_util(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq */ divider = get_pelt_divider(&cfs_rq->avg); + /* Set new sched_entity's utilization */ se->avg.util_avg = gcfs_rq->avg.util_avg; - se->avg.util_sum = se->avg.util_avg * divider; + new_sum = se->avg.util_avg * divider; + delta_sum = (long)new_sum - (long)se->avg.util_sum; + se->avg.util_sum = new_sum; /* Update parent cfs_rq utilization */ - add_positive(&cfs_rq->avg.util_avg, delta); - cfs_rq->avg.util_sum = cfs_rq->avg.util_avg * divider; + add_positive(&cfs_rq->avg.util_avg, delta_avg); + add_positive(&cfs_rq->avg.util_sum, delta_sum); + + /* + * Because of rounding, se->util_sum might ends up being +1 more than + * cfs->util_sum (XXX fix the rounding). Although this is not + * a problem by itself, detaching a lot of tasks with the rounding + * problem between 2 updates of util_avg (~1ms) can make cfs->util_sum + * becoming null whereas cfs_util_avg is not. + * Check that util_sum is still above its lower bound for the new + * util_avg. Given that period_contrib might have moved since the last + * sync, we are only sure that util_sum must be above or equal to + * util_avg * minimum possible divider + */ + cfs_rq->avg.util_sum = max_t(u32, cfs_rq->avg.util_sum, + cfs_rq->avg.util_avg * MIN_DIVIDER); } static inline void @@ -3681,7 +3698,9 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq) r = removed_util; sub_positive(&sa->util_avg, r); - sa->util_sum = sa->util_avg * divider; + sub_positive(&sa->util_sum, r * divider); + /* See update_tg_cfs_util() */ + sa->util_sum = max_t(u32, sa->util_sum, sa->util_avg * MIN_DIVIDER); r = removed_runnable; sub_positive(&sa->runnable_avg, r); @@ -3780,7 +3799,11 @@ static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s dequeue_load_avg(cfs_rq, se); sub_positive(&cfs_rq->avg.util_avg, se->avg.util_avg); - cfs_rq->avg.util_sum = cfs_rq->avg.util_avg * divider; + sub_positive(&cfs_rq->avg.util_sum, se->avg.util_sum); + /* See update_tg_cfs_util() */ + cfs_rq->avg.util_sum = max_t(u32, cfs_rq->avg.util_sum, + cfs_rq->avg.util_avg * MIN_DIVIDER); + sub_positive(&cfs_rq->avg.runnable_avg, se->avg.runnable_avg); cfs_rq->avg.runnable_sum = cfs_rq->avg.runnable_avg * divider; -- 2.17.1
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