From 089d84203ad42bc8fd6dbf41683e162ac6e848cd Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 27 Nov 2025 16:39:44 +0100 Subject: [PATCH] sched/fair: Fold the sched_avg update Nine (and a half) instances of the same pattern is just silly, fold the lot. Notably, the half instance in enqueue_load_avg() is right after setting cfs_rq->avg.load_sum to cfs_rq->avg.load_avg * get_pelt_divider(&cfs_rq->avg). Since get_pelt_divisor() >= PELT_MIN_DIVIDER, this ends up being a no-op change. Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Cc: Linus Torvalds Cc: Dietmar Eggemann Cc: Juri Lelli Cc: Linus Torvalds Cc: Mel Gorman Cc: Peter Zijlstra Cc: Shrikanth Hegde Cc: Valentin Schneider Cc: Vincent Guittot Link: https://patch.msgid.link/20251127154725.413564507@infradead.org --- kernel/sched/fair.c | 108 +++++++++++++------------------------------- 1 file changed, 32 insertions(+), 76 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index da46c31645378..aa033e45dce49 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3693,7 +3693,7 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se) */ #define add_positive(_ptr, _val) do { \ typeof(_ptr) ptr = (_ptr); \ - typeof(_val) val = (_val); \ + __signed_scalar_typeof(*ptr) val = (_val); \ typeof(*ptr) res, var = READ_ONCE(*ptr); \ \ res = var + val; \ @@ -3704,23 +3704,6 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se) WRITE_ONCE(*ptr, res); \ } while (0) -/* - * Unsigned subtract and clamp on underflow. - * - * Explicitly do a load-store to ensure the intermediate value never hits - * memory. This allows lockless observations without ever seeing the negative - * values. - */ -#define sub_positive(_ptr, _val) do { \ - typeof(_ptr) ptr = (_ptr); \ - typeof(*ptr) val = (_val); \ - typeof(*ptr) res, var = READ_ONCE(*ptr); \ - res = var - val; \ - if (res > var) \ - res = 0; \ - WRITE_ONCE(*ptr, res); \ -} while (0) - /* * Remove and clamp on negative, from a local variable. * @@ -3732,21 +3715,37 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se) *ptr -= min_t(typeof(*ptr), *ptr, _val); \ } while (0) + +/* + * 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 + */ +#define __update_sa(sa, name, delta_avg, delta_sum) do { \ + add_positive(&(sa)->name##_avg, delta_avg); \ + add_positive(&(sa)->name##_sum, delta_sum); \ + (sa)->name##_sum = max_t(typeof((sa)->name##_sum), \ + (sa)->name##_sum, \ + (sa)->name##_avg * PELT_MIN_DIVIDER); \ +} while (0) + static inline void enqueue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { - cfs_rq->avg.load_avg += se->avg.load_avg; - cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum; + __update_sa(&cfs_rq->avg, load, se->avg.load_avg, se->avg.load_sum); } static inline void dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { - sub_positive(&cfs_rq->avg.load_avg, se->avg.load_avg); - sub_positive(&cfs_rq->avg.load_sum, se_weight(se) * se->avg.load_sum); - /* See update_cfs_rq_load_avg() */ - cfs_rq->avg.load_sum = max_t(u32, cfs_rq->avg.load_sum, - cfs_rq->avg.load_avg * PELT_MIN_DIVIDER); + __update_sa(&cfs_rq->avg, load, -se->avg.load_avg, -se->avg.load_sum); } static void place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags); @@ -4242,7 +4241,6 @@ 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; new_sum = se->avg.util_avg * divider; @@ -4250,12 +4248,7 @@ update_tg_cfs_util(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq se->avg.util_sum = new_sum; /* Update parent cfs_rq utilization */ - add_positive(&cfs_rq->avg.util_avg, delta_avg); - add_positive(&cfs_rq->avg.util_sum, delta_sum); - - /* See update_cfs_rq_load_avg() */ - cfs_rq->avg.util_sum = max_t(u32, cfs_rq->avg.util_sum, - cfs_rq->avg.util_avg * PELT_MIN_DIVIDER); + __update_sa(&cfs_rq->avg, util, delta_avg, delta_sum); } static inline void @@ -4281,11 +4274,7 @@ update_tg_cfs_runnable(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cf se->avg.runnable_sum = new_sum; /* Update parent cfs_rq runnable */ - add_positive(&cfs_rq->avg.runnable_avg, delta_avg); - add_positive(&cfs_rq->avg.runnable_sum, delta_sum); - /* See update_cfs_rq_load_avg() */ - cfs_rq->avg.runnable_sum = max_t(u32, cfs_rq->avg.runnable_sum, - cfs_rq->avg.runnable_avg * PELT_MIN_DIVIDER); + __update_sa(&cfs_rq->avg, runnable, delta_avg, delta_sum); } static inline void @@ -4349,11 +4338,7 @@ update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq se->avg.load_sum = runnable_sum; se->avg.load_avg = load_avg; - add_positive(&cfs_rq->avg.load_avg, delta_avg); - add_positive(&cfs_rq->avg.load_sum, delta_sum); - /* See update_cfs_rq_load_avg() */ - cfs_rq->avg.load_sum = max_t(u32, cfs_rq->avg.load_sum, - cfs_rq->avg.load_avg * PELT_MIN_DIVIDER); + __update_sa(&cfs_rq->avg, load, delta_avg, delta_sum); } static inline void add_tg_cfs_propagate(struct cfs_rq *cfs_rq, long runnable_sum) @@ -4552,33 +4537,13 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq) raw_spin_unlock(&cfs_rq->removed.lock); r = removed_load; - sub_positive(&sa->load_avg, r); - sub_positive(&sa->load_sum, r * divider); - /* See sa->util_sum below */ - sa->load_sum = max_t(u32, sa->load_sum, sa->load_avg * PELT_MIN_DIVIDER); + __update_sa(sa, load, -r, -r*divider); r = removed_util; - sub_positive(&sa->util_avg, r); - 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); + __update_sa(sa, util, -r, -r*divider); r = removed_runnable; - sub_positive(&sa->runnable_avg, r); - sub_positive(&sa->runnable_sum, r * divider); - /* See sa->util_sum above */ - sa->runnable_sum = max_t(u32, sa->runnable_sum, - sa->runnable_avg * PELT_MIN_DIVIDER); + __update_sa(sa, runnable, -r, -r*divider); /* * removed_runnable is the unweighted version of removed_load so we @@ -4663,17 +4628,8 @@ static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { dequeue_load_avg(cfs_rq, se); - sub_positive(&cfs_rq->avg.util_avg, se->avg.util_avg); - sub_positive(&cfs_rq->avg.util_sum, se->avg.util_sum); - /* See update_cfs_rq_load_avg() */ - cfs_rq->avg.util_sum = max_t(u32, cfs_rq->avg.util_sum, - cfs_rq->avg.util_avg * PELT_MIN_DIVIDER); - - sub_positive(&cfs_rq->avg.runnable_avg, se->avg.runnable_avg); - sub_positive(&cfs_rq->avg.runnable_sum, se->avg.runnable_sum); - /* See update_cfs_rq_load_avg() */ - cfs_rq->avg.runnable_sum = max_t(u32, cfs_rq->avg.runnable_sum, - cfs_rq->avg.runnable_avg * PELT_MIN_DIVIDER); + __update_sa(&cfs_rq->avg, util, -se->avg.util_avg, -se->avg.util_sum); + __update_sa(&cfs_rq->avg, runnable, -se->avg.runnable_avg, -se->avg.runnable_sum); add_tg_cfs_propagate(cfs_rq, -se->avg.load_sum); -- 2.47.3