From d519329f72a6f36bc4f2b85452640cfe583b4f81 Mon Sep 17 00:00:00 2001 From: Patrick Bellasi Date: Fri, 9 Mar 2018 09:52:45 +0000 Subject: [PATCH] sched/fair: Update util_est only on util_avg updates The estimated utilization of a task is currently updated every time the task is dequeued. However, to keep overheads under control, PELT signals are effectively updated at maximum once every 1ms. Thus, for really short running tasks, it can happen that their util_avg value has not been updates since their last enqueue. If such tasks are also frequently running tasks (e.g. the kind of workload generated by hackbench) it can also happen that their util_avg is updated only every few activations. This means that updating util_est at every dequeue potentially introduces not necessary overheads and it's also conceptually wrong if the util_avg signal has never been updated during a task activation. Let's introduce a throttling mechanism on task's util_est updates to sync them with util_avg updates. To make the solution memory efficient, both in terms of space and load/store operations, we encode a synchronization flag into the LSB of util_est.enqueued. This makes util_est an even values only metric, which is still considered good enough for its purpose. The synchronization bit is (re)set by __update_load_avg_se() once the PELT signal of a task has been updated during its last activation. Such a throttling mechanism allows to keep under control util_est overheads in the wakeup hot path, thus making it a suitable mechanism which can be enabled also on high-intensity workload systems. Thus, this now switches on by default the estimation utilization scheduler feature. Suggested-by: Chris Redpath Signed-off-by: Patrick Bellasi Signed-off-by: Peter Zijlstra (Intel) Cc: Dietmar Eggemann Cc: Joel Fernandes Cc: Juri Lelli Cc: Linus Torvalds Cc: Morten Rasmussen Cc: Paul Turner Cc: Peter Zijlstra Cc: Rafael J . Wysocki Cc: Steve Muckle Cc: Thomas Gleixner Cc: Todd Kjos Cc: Vincent Guittot Cc: Viresh Kumar Link: http://lkml.kernel.org/r/20180309095245.11071-5-patrick.bellasi@arm.com Signed-off-by: Ingo Molnar --- kernel/sched/fair.c | 42 +++++++++++++++++++++++++++++++++++++---- kernel/sched/features.h | 2 +- 2 files changed, 39 insertions(+), 5 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 570b8d056282..0951d1c58d2f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3242,6 +3242,32 @@ ___update_load_avg(struct sched_avg *sa, unsigned long load, unsigned long runna sa->util_avg = sa->util_sum / divider; } +/* + * When a task is dequeued, its estimated utilization should not be update if + * its util_avg has not been updated at least once. + * This flag is used to synchronize util_avg updates with util_est updates. + * We map this information into the LSB bit of the utilization saved at + * dequeue time (i.e. util_est.dequeued). + */ +#define UTIL_AVG_UNCHANGED 0x1 + +static inline void cfs_se_util_change(struct sched_avg *avg) +{ + unsigned int enqueued; + + if (!sched_feat(UTIL_EST)) + return; + + /* Avoid store if the flag has been already set */ + enqueued = avg->util_est.enqueued; + if (!(enqueued & UTIL_AVG_UNCHANGED)) + return; + + /* Reset flag to report util_avg has been updated */ + enqueued &= ~UTIL_AVG_UNCHANGED; + WRITE_ONCE(avg->util_est.enqueued, enqueued); +} + /* * sched_entity: * @@ -3293,6 +3319,7 @@ __update_load_avg_se(u64 now, int cpu, struct cfs_rq *cfs_rq, struct sched_entit cfs_rq->curr == se)) { ___update_load_avg(&se->avg, se_weight(se), se_runnable(se)); + cfs_se_util_change(&se->avg); return 1; } @@ -3900,7 +3927,7 @@ static inline void util_est_enqueue(struct cfs_rq *cfs_rq, /* Update root cfs_rq's estimated utilization */ enqueued = cfs_rq->avg.util_est.enqueued; - enqueued += _task_util_est(p); + enqueued += (_task_util_est(p) | UTIL_AVG_UNCHANGED); WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued); } @@ -3936,7 +3963,7 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep) if (cfs_rq->nr_running) { ue.enqueued = cfs_rq->avg.util_est.enqueued; ue.enqueued -= min_t(unsigned int, ue.enqueued, - _task_util_est(p)); + (_task_util_est(p) | UTIL_AVG_UNCHANGED)); } WRITE_ONCE(cfs_rq->avg.util_est.enqueued, ue.enqueued); @@ -3947,12 +3974,19 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep) if (!task_sleep) return; + /* + * If the PELT values haven't changed since enqueue time, + * skip the util_est update. + */ + ue = p->se.avg.util_est; + if (ue.enqueued & UTIL_AVG_UNCHANGED) + return; + /* * Skip update of task's estimated utilization when its EWMA is * already ~1% close to its last activation value. */ - ue = p->se.avg.util_est; - ue.enqueued = task_util(p); + ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED); last_ewma_diff = ue.enqueued - ue.ewma; if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100))) return; diff --git a/kernel/sched/features.h b/kernel/sched/features.h index c459a4b61544..85ae8488039c 100644 --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -89,4 +89,4 @@ SCHED_FEAT(WA_BIAS, true) /* * UtilEstimation. Use estimated CPU utilization. */ -SCHED_FEAT(UTIL_EST, false) +SCHED_FEAT(UTIL_EST, true) -- GitLab