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Date: Fri, 29 Apr 2022 20:51:42 +0800
From: Tao Zhou <tao.zhou@...ux.dev>
To: Vincent Guittot <vincent.guittot@...aro.org>,
Tao Zhou <tao.zhou@...ux.dev>
Cc: Vincent Donnefort <vincent.donnefort@....com>,
peterz@...radead.org, mingo@...hat.com,
linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
morten.rasmussen@....com, chris.redpath@....com, qperret@...gle.com
Subject: Re: [PATCH v7 2/7] sched/fair: Decay task PELT values during wakeup
migration
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@...ux.dev> 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().
> + 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
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