Date:   Fri, 29 Apr 2022 10:20:00 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     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

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 +
+	 *   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
+	 * 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();
+	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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