From: Vincent Guittot vincent.guittot@linaro.org
[ Upstream commit 98b0d890220d45418cfbc5157b3382e6da5a12ab ]
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 change of util_sum and propagate the difference.
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 Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com Tested-by: Sachin Sant sachinp@linux.ibm.com Link: https://lkml.kernel.org/r/20220111134659.24961-2-vincent.guittot@linaro.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/fair.c | 16 +++++++++++++--- kernel/sched/pelt.h | 4 +++- 2 files changed, 16 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 8d2f238fdd2ac..acd9833b8ec22 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3379,7 +3379,6 @@ void set_task_rq_fair(struct sched_entity *se, se->avg.last_update_time = n_last_update_time; }
- /* * When on migration a sched_entity joins/leaves the PELT hierarchy, we need to * propagate its contribution. The key to this propagation is the invariant @@ -3447,7 +3446,6 @@ 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) { @@ -3676,7 +3674,19 @@ 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); + /* + * Because of rounding, se->util_sum might ends up being +1 more than + * cfs->util_sum. 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 + */ + sa->util_sum = max_t(u32, sa->util_sum, sa->util_avg * PELT_MIN_DIVIDER);
r = removed_runnable; sub_positive(&sa->runnable_avg, r); diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h index 0b9aeebb9c325..45bf08e22207c 100644 --- a/kernel/sched/pelt.h +++ b/kernel/sched/pelt.h @@ -37,9 +37,11 @@ update_irq_load_avg(struct rq *rq, u64 running) } #endif
+#define PELT_MIN_DIVIDER (LOAD_AVG_MAX - 1024) + static inline u32 get_pelt_divider(struct sched_avg *avg) { - return LOAD_AVG_MAX - 1024 + avg->period_contrib; + return PELT_MIN_DIVIDER + avg->period_contrib; }
static inline void cfs_se_util_change(struct sched_avg *avg)