Re: [PATCH v7 2/7] sched/fair: Decay task PELT values during wakeup migration

From: Vincent Guittot
Date: Fri Apr 29 2022 - 09:23:33 EST

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On Fri, 29 Apr 2022 at 14:51, Tao Zhou <tao.zhou@xxxxxxxxx> wrote:
>
> On Fri, Apr 29, 2022 at 10:20:00AM +0200, Vincent Guittot wrote:
>
> > Le vendredi 29 avril 2022 à 01:22:19 (+0800), Tao Zhou a écrit :
> > > Hi Vincent,
> > >
> > > On Thu, Apr 28, 2022 at 03:38:39PM +0200, Vincent Guittot wrote:
> > >
> > > > On Wed, 27 Apr 2022 at 19:37, Tao Zhou <tao.zhou@xxxxxxxxx> wrote:
> > > > >
> >
> > [..]
> >
> > > > > > + /* sync clock_pelt_idle with last update */
> > > > > > + if (cfs_rq->nr_running == 0)
> > > > > > + update_idle_cfs_rq_clock_pelt(cfs_rq);
> > > > >
> > > > > I think that if cfs_rq->nr_running == 0 then use cfs rq pelt_idle to update
> > > > > idle cfs rq.
> > > >
> > > > update_blocked_averages() updates all cfs rq to be aligned with now so
> > > > we don't need to calculate an estimated now. update_rq_clock(rq) is
> > > > called 1st to update the rq->clock and childs
> > > >
> > > > With only need to save when happened the last update which is done in
> > > > update_rq_clock_pelt(rq) for rq->clock_pelt and with
> > > > update_idle_cfs_rq_clock_pelt(cfs) for the cfs_rq_clock_pelt
> > >
> > > I missed this.
> >
> > I ended up with something a bit different:
> >
> > ---
> > kernel/sched/fair.c | 133 ++++++++++++++++++++++++++++++++++---------
> > kernel/sched/pelt.h | 66 ++++++++++++++++++---
> > kernel/sched/sched.h | 10 ++++
> > 3 files changed, 174 insertions(+), 35 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index abd1feeec0c2..63e4cf225292 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3335,27 +3335,12 @@ 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;
> > }
> >
> > @@ -3694,6 +3679,88 @@ 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)
> > +{
> > + struct cfs_rq *cfs_rq;
> > + struct rq *rq;
> > + bool is_idle;
> > + u64 now, throttled = 0;
> > +
> > + /* utilization is already fully decayed */
> > + 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: last update of the cfs_lock_pelt +
> > + * cfs_idle_lag: rq_clock_pelt delta bewteen last cfs update and last rq update +
>
> s/bewteen/between/
>
> > + * rq_idle_lag: rq_clock delta between last rq update and now
> > + *
> > + * with
> > + *
> > + * last_update_time == cfs_clock_pelt()
> > + * == rq_clock_pelt() - cfs->throttled_clock_pelt_time
> > + *
> > + * cfs_idle_lag: rq_clock_pelt() when rq is idle - rq_clock_pelt() when cfs is idle
> > + *
> > + * rq_idle_lag : sched_clock_cpu() - rq_clock() when rq is idle
> > + *
> > + * In fact, rq_clock_pelt() that is used for last_update_time and when
> > + * cfs is idle are the same because their last update happens atthe
>
> s/atthe/at the/
>
> > + * same time.
> > + *
> > + * We can optimize "now" to be:
> > + * rq_clock_pelt when rq is idle - cfs->throttled_clock_pelt_time when cfs is idle +
> > + * sched_clock_cpu() - rq_clock() when rq is idle
> > + *
> > + * when rq is idle
> > + * rq_clock_pelt() is saved in rq->clock_pelt_idle
> > + * rq_clock() is saved in rq->enter idle
> > + *
> > + * when cfs is idle
> > + * cfs->throttled_clock_pelt_time is saved in cfs_rq->throttled_pelt_idle
> > + *
> > + * When !CFS_BANDWIDTH, cfs->throttled_clock_pelt_time is null
> > + */
> > +
> > +#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 +4496,9 @@ 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 +7016,8 @@ 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 +7025,6 @@ 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 +7036,29 @@ 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 +8224,10 @@ 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..4143c6dc64dc 100644
> > --- a/kernel/sched/pelt.h
> > +++ b/kernel/sched/pelt.h
> > @@ -37,6 +37,29 @@ update_irq_load_avg(struct rq *rq, u64 running)
> > }
> > #endif
> >
> > +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;
> > +}
> > +
> > #define PELT_MIN_DIVIDER (LOAD_AVG_MAX - 1024)
> >
> > static inline u32 get_pelt_divider(struct sched_avg *avg)
> > @@ -61,6 +84,23 @@ 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 +116,7 @@ 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 +169,26 @@ 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.
> > + */
> > + smp_wmb();
>
> No smp_rmb() at read side and I feel it is in migrate_se_pelt_lag()
> before this line：
> ‘now = u64_u32_load(rq->clock_pelt_idle);’
> The load of cfs_rq->throttled_pelt_idle should be valid before the load
> of rq->clock_pelt_idle and rq->enter_idle. This is paired with the write
> side in update_idle_cfs_rq_clock_pelt().

Yes

>
> > + 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 +198,7 @@ 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 +253,7 @@ 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;
> >
> > --
> > 2.17.1

Next message: Steven Rostedt: "Re: [PATCH 17/30] tracing: Improve panic/die notifiers"
Previous message: Pali Rohár: "[PATCH v2 2/2] watchdog: max63xx_wdt: Add support for specifying WDI logic via GPIO"
In reply to: Tao Zhou: "Re: [PATCH v7 2/7] sched/fair: Decay task PELT values during wakeup migration"
Next in thread: Vincent Donnefort: "[PATCH v7 3/7] sched, drivers: Remove max param from effective_cpu_util()/sched_cpu_util()"
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