Messages in this thread | | | Date | Sat, 11 Jun 2022 21:56:15 +0800 | From | Tao Zhou <> | Subject | Re: [PATCH v10 2/7] sched/fair: Decay task PELT values during wakeup migration |
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On Tue, Jun 07, 2022 at 01:32:49PM +0100, Vincent Donnefort wrote:
> From: Vincent Donnefort <vincent.donnefort@arm.com> > > 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 the clock is having the biggest risk of being outdated.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_lag part of the missing time is however an estimation that > doesn't take into account IRQ and Paravirt time. > > [1] https://lkml.kernel.org/r/20190709115759.10451-1-chris.redpath@arm.com > > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com> > Signed-off-by: Vincent Donnefort <vdonnefort@google.com> > Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 05614d9b919c..77f3eb1bf624 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -3310,6 +3310,29 @@ static inline void cfs_rq_util_change(struct cfs_rq *cfs_rq, int flags) > } > > #ifdef CONFIG_SMP > +static inline bool load_avg_is_decayed(struct sched_avg *sa) > +{ > + if (sa->load_sum) > + return false; > + > + if (sa->util_sum) > + return false; > + > + if (sa->runnable_sum) > + return false; > + > + /* > + * _avg must be null when _sum are null because _avg = _sum / divider > + * Make sure that rounding and/or propagation of PELT values never > + * break this. > + */ > + SCHED_WARN_ON(sa->load_avg || > + sa->util_avg || > + sa->runnable_avg); > + > + return true; > +} > + > static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq) > { > return u64_u32_load_copy(cfs_rq->avg.last_update_time, > @@ -3347,27 +3370,12 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) > if (cfs_rq->load.weight) > return false; > > - if (cfs_rq->avg.load_sum) > - return false; > - > - if (cfs_rq->avg.util_sum) > - return false; > - > - if (cfs_rq->avg.runnable_sum) > + if (!load_avg_is_decayed(&cfs_rq->avg)) > return false; > > if (child_cfs_rq_on_list(cfs_rq)) > return false; > > - /* > - * _avg must be null when _sum are null because _avg = _sum / divider > - * 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); > - > return true; > } > > @@ -3706,6 +3714,89 @@ 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, lut; > + 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 + cfs_idle_lag + rq_idle_lag, where: > + * > + * last_update_time (the cfs_rq's last_update_time) > + * = cfs_rq_clock_pelt()@cfs_rq_idle > + * = rq_clock_pelt()@cfs_rq_idle > + * - cfs->throttled_clock_pelt_time@cfs_rq_idle > + * > + * cfs_idle_lag (delta between cfs_rq's update and rq's update) > + * = rq_clock_pelt()@rq_idle - rq_clock_pelt()@cfs_rq_idle > + * > + * rq_idle_lag (delta between rq's update and now) > + * = sched_clock_cpu() - rq_clock()@rq_idle > + * > + * We can then 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->clock_idle > + * cfs->throttled_clock_pelt_time@cfs_rq_idle 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); > + /* > + * Paired with _update_idle_rq_clock_pelt(). It ensures at the worst case > + * is observed the old clock_pelt_idle value and the new clock_idle, > + * which lead to an underestimation. The opposite would lead to an > + * overestimation. > + */
The sentence of 'The opposite would lead to an overestimation' means that clock_pelt_idle is new clock_idle is old if I am not wrong. If clock_pelt_idle is new then clock_idle is new too. The later is the updated state of both, IMHO this sentence does not stand.
> + smp_rmb(); > + lut = cfs_rq_last_update_time(cfs_rq); > + > + now -= throttled; > + if (now < lut) > + /* > + * cfs_rq->avg.last_update_time is more recent than our > + * estimation, let's use it. > + */ > + now = lut; > + else > + now += sched_clock_cpu(cpu_of(rq)) - u64_u32_load(rq->clock_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() > @@ -4437,6 +4528,9 @@ 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); > } > > /* > @@ -6911,6 +7005,8 @@ 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 > @@ -6918,7 +7014,6 @@ 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); > @@ -6930,25 +7025,29 @@ 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); > } > @@ -8114,6 +8213,9 @@ 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); > > + 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..3a0e0dc28721 100644 > --- a/kernel/sched/pelt.h > +++ b/kernel/sched/pelt.h > @@ -61,6 +61,25 @@ 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->clock_idle, rq_clock(rq)); > + /* Paired with smp_rmb in migrate_se_pelt_lag() */ > + smp_wmb(); > + 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 +95,7 @@ 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 +148,23 @@ 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); > + u64 throttled; > > - return rq->clock_pelt - rq->lost_idle_time; > + if (unlikely(cfs_rq->throttle_count)) > + throttled = U64_MAX; > + else > + throttled = cfs_rq->throttled_clock_pelt_time; > + > + u64_u32_store(cfs_rq->throttled_pelt_idle, throttled); > } > > -#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 +174,7 @@ 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 +229,7 @@ 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 bf4a0ec98678..de5c99bea7be 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -648,6 +648,10 @@ 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; > @@ -1020,6 +1024,12 @@ struct rq { > u64 clock_task ____cacheline_aligned; > u64 clock_pelt; > unsigned long lost_idle_time; > + u64 clock_pelt_idle; > + u64 clock_idle; > +#ifndef CONFIG_64BIT > + u64 clock_pelt_idle_copy; > + u64 clock_idle_copy; > +#endif > > atomic_t nr_iowait; > > -- > 2.36.1.255.ge46751e96f-goog
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