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From: Tao Zhou To: Vincent Donnefort Cc: peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org, linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com, morten.rasmussen@arm.com, chris.redpath@arm.com, qperret@google.com, Tao Zhou Subject: Re: [PATCH v8 2/7] sched/fair: Decay task PELT values during wakeup migration Message-ID: References: <20220429141148.181816-1-vincent.donnefort@arm.com> <20220429141148.181816-3-vincent.donnefort@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220429141148.181816-3-vincent.donnefort@arm.com> X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 29, 2022 at 03:11:43PM +0100, Vincent Donnefort wrote: > Before being migrated to a new CPU, a task sees its PELT values > synchronized with rq last_update_time. Once done, that same task will also > have its sched_avg last_update_time reset. This means the time between > the migration and the last clock update will not be accounted for in > util_avg and a discontinuity will appear. This issue is amplified by the > PELT clock scaling. It takes currently one tick after the CPU being idle > to let clock_pelt catching up clock_task. > > This is especially problematic for asymmetric CPU capacity systems which > need stable util_avg signals for task placement and energy estimation. > > Ideally, this problem would be solved by updating the runqueue clocks > before the migration. But that would require taking the runqueue lock > which is quite expensive [1]. Instead estimate the missing time and update > the task util_avg with that value. > > To that end, we need sched_clock_cpu() but it is a costly function. Limit > the usage to the case where the source CPU is idle as we know this is when > the clock is having the biggest risk of being outdated. In this such case, > let's call it cfs_idle_lag the delta time between the rq_clock_pelt value > at rq idle and cfs_rq idle. And rq_idle_lag the delta between "now" and > the rq_clock_pelt at rq idle. > > The estimated PELT clock is then: > > last_update_time + (the cfs_rq's last_update_time) > cfs_idle_lag + (delta between cfs_rq's update and rq's update) > rq_idle_lag (delta between rq's update and now) > > last_update_time = cfs_rq_clock_pelt() > = rq_clock_pelt() - cfs->throttled_clock_pelt_time > > cfs_idle_lag = rq_clock_pelt()@rq_idle - > rq_clock_pelt()@cfs_rq_idle > > rq_idle_lag = sched_clock_cpu() - rq_clock()@rq_idle > > The rq_clock_pelt() from last_update_time being the same as > rq_clock_pelt()@cfs_rq_idle, we can write: > > estimation = rq_clock_pelt()@rq_idle - cfs->throttled_clock_pelt_time + > sched_clock_cpu() - rq_clock()@rq_idle > > The clocks being not accessible without the rq lock taken, some timestamps > are created: > > rq_clock_pelt()@rq_idle is rq->clock_pelt_idle > rq_clock()@rq_idle is rq->enter_idle > cfs->throttled_clock_pelt_time is cfs_rq->throttled_pelt_idle > > The rq_idle_lqg part of the missing time is however an estimation that s/rq_idle_lqg/rq_idle_lag/ > doesn't take into account IRQ and Paravirt time. > > [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@arm.com/ > > Signed-off-by: Vincent Donnefort > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index abd1feeec0c2..4e9e0d453a00 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -3330,21 +3330,15 @@ static inline bool child_cfs_rq_on_list(struct cfs_rq *cfs_rq) > return (prev_cfs_rq->tg->parent == cfs_rq->tg); > } > > -static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) > +static inline bool load_avg_is_decayed(struct sched_avg *sa) > { > - if (cfs_rq->load.weight) > - return false; > - > - if (cfs_rq->avg.load_sum) > + if (sa->load_sum) > return false; > > - if (cfs_rq->avg.util_sum) > + if (sa->util_sum) > return false; > > - if (cfs_rq->avg.runnable_sum) > - return false; > - > - if (child_cfs_rq_on_list(cfs_rq)) > + if (sa->runnable_sum) > return false; > > /* > @@ -3352,9 +3346,23 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) > * Make sure that rounding and/or propagation of PELT values never > * break this. > */ > - SCHED_WARN_ON(cfs_rq->avg.load_avg || > - cfs_rq->avg.util_avg || > - cfs_rq->avg.runnable_avg); > + SCHED_WARN_ON(sa->load_avg || > + sa->util_avg || > + sa->runnable_avg); > + > + return true; > +} > + > +static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) > +{ > + if (cfs_rq->load.weight) > + return false; > + > + if (!load_avg_is_decayed(&cfs_rq->avg)) > + return false; > + > + if (child_cfs_rq_on_list(cfs_rq)) > + return false; > > return true; > } > @@ -3694,6 +3702,79 @@ static inline void add_tg_cfs_propagate(struct cfs_rq *cfs_rq, long runnable_sum > > #endif /* CONFIG_FAIR_GROUP_SCHED */ > > +#ifdef CONFIG_NO_HZ_COMMON > +static inline void migrate_se_pelt_lag(struct sched_entity *se) > +{ > + u64 throttled = 0, now; > + struct cfs_rq *cfs_rq; > + struct rq *rq; > + bool is_idle; > + > + if (load_avg_is_decayed(&se->avg)) > + return; > + > + cfs_rq = cfs_rq_of(se); > + rq = rq_of(cfs_rq); > + > + rcu_read_lock(); > + is_idle = is_idle_task(rcu_dereference(rq->curr)); > + rcu_read_unlock(); > + > + /* > + * The lag estimation comes with a cost we don't want to pay all the > + * time. Hence, limiting to the case where the source CPU is idle and > + * we know we are at the greatest risk to have an outdated clock. > + */ > + if (!is_idle) > + return; > + > + /* > + * estimated "now" is: > + * last_update_time + (the cfs_rq's last_update_time) > + * cfs_idle_lag + (delta between cfs_rq's update and rq's update) > + * rq_idle_lag (delta between rq's update and now) > + * > + * last_update_time = cfs_rq_clock_pelt() > + * = rq_clock_pelt() - cfs->throttled_clock_pelt_time > + * > + * cfs_idle_lag = rq_clock_pelt()@rq_idle - > + * rq_clock_pelt()@cfs_rq_idle > + * > + * rq_idle_lag = sched_clock_cpu() - rq_clock()@rq_idle > + * > + * The rq_clock_pelt() from last_update_time being the same as > + * rq_clock_pelt()@cfs_rq_idle, we can write: > + * > + * now = rq_clock_pelt()@rq_idle - cfs->throttled_clock_pelt_time + > + * sched_clock_cpu() - rq_clock()@rq_idle > + * > + * Where: > + * rq_clock_pelt()@rq_idle is rq->clock_pelt_idle > + * rq_clock()@rq_idle is rq->enter_idle > + * cfs->throttled_clock_pelt_time is cfs_rq->throttled_pelt_idle > + */ > + > +#ifdef CONFIG_CFS_BANDWIDTH > + throttled = u64_u32_load(cfs_rq->throttled_pelt_idle); > + /* The clock has been stopped for throttling */ > + if (throttled == U64_MAX) > + return; > +#endif > + now = u64_u32_load(rq->clock_pelt_idle); > + now -= throttled; > + > + /* An update happened while computing lag */ > + if (now < cfs_rq_last_update_time(cfs_rq)) > + return; > + > + now += sched_clock_cpu(cpu_of(rq)) - u64_u32_load(rq->enter_idle); > + > + __update_load_avg_blocked_se(now, se); > +} > +#else > +static void migrate_se_pelt_lag(struct sched_entity *se) {} > +#endif > + > /** > * update_cfs_rq_load_avg - update the cfs_rq's load/util averages > * @now: current time, as per cfs_rq_clock_pelt() > @@ -4429,6 +4510,9 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > */ > if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE) > update_min_vruntime(cfs_rq); > + > + if (cfs_rq->nr_running == 0) > + update_idle_cfs_rq_clock_pelt(cfs_rq); > } > > /* > @@ -6946,6 +7030,8 @@ static void detach_entity_cfs_rq(struct sched_entity *se); > */ > static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > { > + struct sched_entity *se = &p->se; > + > /* > * As blocked tasks retain absolute vruntime the migration needs to > * deal with this by subtracting the old and adding the new > @@ -6953,7 +7039,6 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > * the task on the new runqueue. > */ > if (READ_ONCE(p->__state) == TASK_WAKING) { > - struct sched_entity *se = &p->se; > struct cfs_rq *cfs_rq = cfs_rq_of(se); > > se->vruntime -= u64_u32_load(cfs_rq->min_vruntime); > @@ -6965,25 +7050,29 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > * rq->lock and can modify state directly. > */ > lockdep_assert_rq_held(task_rq(p)); > - detach_entity_cfs_rq(&p->se); > + detach_entity_cfs_rq(se); > > } else { > + remove_entity_load_avg(se); > + > /* > - * We are supposed to update the task to "current" time, then > - * its up to date and ready to go to new CPU/cfs_rq. But we > - * have difficulty in getting what current time is, so simply > - * throw away the out-of-date time. This will result in the > - * wakee task is less decayed, but giving the wakee more load > - * sounds not bad. > + * Here, the task's PELT values have been updated according to > + * the current rq's clock. But if that clock hasn't been > + * updated in a while, a substantial idle time will be missed, > + * leading to an inflation after wake-up on the new rq. > + * > + * Estimate the missing time from the cfs_rq last_update_time > + * and update sched_avg to improve the PELT continuity after > + * migration. > */ > - remove_entity_load_avg(&p->se); > + migrate_se_pelt_lag(se); > } > > /* Tell new CPU we are migrated */ > - p->se.avg.last_update_time = 0; > + se->avg.last_update_time = 0; > > /* We have migrated, no longer consider this task hot */ > - p->se.exec_start = 0; > + se->exec_start = 0; > > update_scan_period(p, new_cpu); > } > @@ -8149,6 +8238,10 @@ static bool __update_blocked_fair(struct rq *rq, bool *done) > if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq)) { > update_tg_load_avg(cfs_rq); > > + /* sync clock_pelt_idle with last update */ > + if (cfs_rq->nr_running == 0) > + update_idle_cfs_rq_clock_pelt(cfs_rq); > + > if (cfs_rq == &rq->cfs) > decayed = true; > } > diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h > index 4ff2ed4f8fa1..9aed92262bd9 100644 > --- a/kernel/sched/pelt.h > +++ b/kernel/sched/pelt.h > @@ -61,6 +61,23 @@ static inline void cfs_se_util_change(struct sched_avg *avg) > WRITE_ONCE(avg->util_est.enqueued, enqueued); > } > > +static inline u64 rq_clock_pelt(struct rq *rq) > +{ > + lockdep_assert_rq_held(rq); > + assert_clock_updated(rq); > + > + return rq->clock_pelt - rq->lost_idle_time; > +} > + > +/* The rq is idle, we can sync to clock_task */ > +static inline void _update_idle_rq_clock_pelt(struct rq *rq) > +{ > + rq->clock_pelt = rq_clock_task(rq); > + > + u64_u32_store(rq->enter_idle, rq_clock(rq)); > + u64_u32_store(rq->clock_pelt_idle, rq_clock_pelt(rq)); > +} > + > /* > * The clock_pelt scales the time to reflect the effective amount of > * computation done during the running delta time but then sync back to > @@ -76,8 +93,7 @@ static inline void cfs_se_util_change(struct sched_avg *avg) > static inline void update_rq_clock_pelt(struct rq *rq, s64 delta) > { > if (unlikely(is_idle_task(rq->curr))) { > - /* The rq is idle, we can sync to clock_task */ > - rq->clock_pelt = rq_clock_task(rq); > + _update_idle_rq_clock_pelt(rq); > return; > } > > @@ -130,17 +146,26 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq) > */ > if (util_sum >= divider) > rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; > + > + _update_idle_rq_clock_pelt(rq); > } > > -static inline u64 rq_clock_pelt(struct rq *rq) > +#ifdef CONFIG_CFS_BANDWIDTH > +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) > { > - lockdep_assert_rq_held(rq); > - assert_clock_updated(rq); > - > - return rq->clock_pelt - rq->lost_idle_time; > + /* > + * Make sure that pending update of rq->clock_pelt_idle and > + * rq->enter_idle are visible during update_blocked_average() before > + * updating cfs_rq->throttled_pelt_idle. > + */ Two places to call update_idle_cfs_rq_clock_pelt(): 1 dequeue_entity() (no pending update before. and this is fast path) update_idle_cfs_rq_clock_pelt 2 update_blocked_averages() update_clock_rq() -> pending update here. __update_blocked_fair() update_idle_cfs_rq_clock_pelt Another way will be to move the smp_wmb() to _update_idle_rq_clock_pelt() static inline void _update_idle_rq_clock_pelt(struct rq *rq) { rq->clock_pelt = rq_clock_task(rq); u64_u32_store(rq->enter_idle, rq_clock(rq)); u64_u32_store(rq->clock_pelt_idle, rq_clock_pelt(rq)); smp_wmb(); } But does this function called more often enough than dequeue_entity(), pick_next_task_fair() (rq will be idle) update_idle_rq_clock_pelt() update_rq_clock_pelt() (curr is idle) _update_idle_rq_clock_pelt() The condition is they are all idle. And the migrate_se_pelt_lag() is for idle also. If smp_wmb() is here like the patch, smp_rmb() place in migrate_se_pelt_lag() here: #ifdef CONFIG_CFS_BANDWIDTH throttled = u64_u32_load(cfs_rq->throttled_pelt_idle); smp_rmb(); /* The clock has been stopped for throttling */ if (throttled == U64_MAX) return; #endif If smp_wmb() is in _update_idle_rq_clock_pelt(), smp_rmb() place in migrate_se_pelt_lag() here: #ifdef CONFIG_CFS_BANDWIDTH throttled = u64_u32_load(cfs_rq->throttled_pelt_idle); /* The clock has been stopped for throttling */ if (throttled == U64_MAX) return; #endif smp_rmb(); now = u64_u32_load(rq->clock_pelt_idle); now -= throttled; Sorry for these noise words. > + smp_wmb(); > + if (unlikely(cfs_rq->throttle_count)) > + u64_u32_store(cfs_rq->throttled_pelt_idle, U64_MAX); > + else > + u64_u32_store(cfs_rq->throttled_pelt_idle, > + cfs_rq->throttled_clock_pelt_time); > } > > -#ifdef CONFIG_CFS_BANDWIDTH > /* rq->task_clock normalized against any time this cfs_rq has spent throttled */ > static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) > { > @@ -150,6 +175,7 @@ static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) > return rq_clock_pelt(rq_of(cfs_rq)) - cfs_rq->throttled_clock_pelt_time; > } > #else > +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) { } > static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) > { > return rq_clock_pelt(rq_of(cfs_rq)); > @@ -204,6 +230,7 @@ update_rq_clock_pelt(struct rq *rq, s64 delta) { } > static inline void > update_idle_rq_clock_pelt(struct rq *rq) { } > > +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) { } > #endif > > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index e2cf6e48b165..ea9365e1a24e 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -641,6 +641,10 @@ struct cfs_rq { > int runtime_enabled; > s64 runtime_remaining; > > + u64 throttled_pelt_idle; > +#ifndef CONFIG_64BIT > + u64 throttled_pelt_idle_copy; > +#endif > u64 throttled_clock; > u64 throttled_clock_pelt; > u64 throttled_clock_pelt_time; > @@ -1013,6 +1017,12 @@ struct rq { > u64 clock_task ____cacheline_aligned; > u64 clock_pelt; > unsigned long lost_idle_time; > + u64 clock_pelt_idle; > + u64 enter_idle; > +#ifndef CONFIG_64BIT > + u64 clock_pelt_idle_copy; > + u64 enter_idle_copy; > +#endif > > atomic_t nr_iowait; > > -- > 2.25.1