[RFC][PATCH 07/14] sched/fair: Rewrite cfs_rq->removed_*avg

[Peter Zijlstra]

Since on wakeup migration we don't hold the rq->lock for the old CPU
we cannot update its state. Instead we add the removed 'load' to an
atomic variable and have the next update on that CPU collect and
process it.

Currently we have 2 atomic variables; which already have the issue
that they can be read out-of-sync. Also, two atomic ops on a single
cacheline is already more expensive than an uncontended lock.

Since we want to add more, convert the thing over to an explicit
cacheline with a lock in.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/debug.c |    8 ++++----
 kernel/sched/fair.c  |   51 +++++++++++++++++++++++++++++++++++----------------
 kernel/sched/sched.h |   13 +++++++++----
 3 files changed, 48 insertions(+), 24 deletions(-)

--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -519,10 +519,10 @@ void print_cfs_rq(struct seq_file *m, in
 			cfs_rq->runnable_load_avg);
 	SEQ_printf(m, "  .%-30s: %lu\n", "util_avg",
 			cfs_rq->avg.util_avg);
-	SEQ_printf(m, "  .%-30s: %ld\n", "removed_load_avg",
-			atomic_long_read(&cfs_rq->removed_load_avg));
-	SEQ_printf(m, "  .%-30s: %ld\n", "removed_util_avg",
-			atomic_long_read(&cfs_rq->removed_util_avg));
+	SEQ_printf(m, "  .%-30s: %ld\n", "removed.load_avg",
+			cfs_rq->removed.load_avg);
+	SEQ_printf(m, "  .%-30s: %ld\n", "removed.util_avg",
+			cfs_rq->removed.util_avg);
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	SEQ_printf(m, "  .%-30s: %lu\n", "tg_load_avg_contrib",
 			cfs_rq->tg_load_avg_contrib);
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3386,36 +3386,47 @@ static inline void cfs_rq_util_change(st
 static inline int
 update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq)
 {
+	unsigned long removed_load = 0, removed_util = 0;
 	struct sched_avg *sa = &cfs_rq->avg;
-	int decayed, removed_load = 0, removed_util = 0;
+	int decayed = 0;
 
-	if (atomic_long_read(&cfs_rq->removed_load_avg)) {
-		s64 r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0);
+	if (cfs_rq->removed.nr) {
+		unsigned long r;
+
+		raw_spin_lock(&cfs_rq->removed.lock);
+		swap(cfs_rq->removed.util_avg, removed_util);
+		swap(cfs_rq->removed.load_avg, removed_load);
+		cfs_rq->removed.nr = 0;
+		raw_spin_unlock(&cfs_rq->removed.lock);
+
+		/*
+		 * The LOAD_AVG_MAX for _sum is a slight over-estimate,
+		 * which is safe due to sub_positive() clipping at 0.
+		 */
+		r = removed_load;
 		sub_positive(&sa->load_avg, r);
 		sub_positive(&sa->load_sum, r * LOAD_AVG_MAX);
-		removed_load = 1;
-		set_tg_cfs_propagate(cfs_rq);
-	}
 
-	if (atomic_long_read(&cfs_rq->removed_util_avg)) {
-		long r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0);
+		r = removed_util;
 		sub_positive(&sa->util_avg, r);
 		sub_positive(&sa->util_sum, r * LOAD_AVG_MAX);
-		removed_util = 1;
+
 		set_tg_cfs_propagate(cfs_rq);
+
+		decayed = 1;
 	}
 
-	decayed = __update_load_avg_cfs_rq(now, cpu_of(rq_of(cfs_rq)), cfs_rq);
+	decayed |= __update_load_avg_cfs_rq(now, cpu_of(rq_of(cfs_rq)), cfs_rq);
 
 #ifndef CONFIG_64BIT
 	smp_wmb();
 	cfs_rq->load_last_update_time_copy = sa->last_update_time;
 #endif
 
-	if (update_freq && (decayed || removed_util))
+	if (update_freq && decayed)
 		cfs_rq_util_change(cfs_rq);
 
-	return decayed || removed_load;
+	return decayed;
 }
 
 /**
@@ -3548,6 +3559,7 @@ void sync_entity_load_avg(struct sched_e
 void remove_entity_load_avg(struct sched_entity *se)
 {
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
+	unsigned long flags;
 
 	/*
 	 * tasks cannot exit without having gone through wake_up_new_task() ->
@@ -3557,11 +3569,19 @@ void remove_entity_load_avg(struct sched
 	 * Similarly for groups, they will have passed through
 	 * post_init_entity_util_avg() before unregister_sched_fair_group()
 	 * calls this.
+	 *
+	 * XXX in case entity_is_task(se) && task_of(se)->on_rq == MIGRATING
+	 * we could actually get the right time, since we're called with
+	 * rq->lock held, see detach_task().
 	 */
 
 	sync_entity_load_avg(se);
-	atomic_long_add(se->avg.load_avg, &cfs_rq->removed_load_avg);
-	atomic_long_add(se->avg.util_avg, &cfs_rq->removed_util_avg);
+
+	raw_spin_lock_irqsave(&cfs_rq->removed.lock, flags);
+	++cfs_rq->removed.nr;
+	cfs_rq->removed.util_avg	+= se->avg.util_avg;
+	cfs_rq->removed.load_avg	+= se->avg.load_avg;
+	raw_spin_unlock_irqrestore(&cfs_rq->removed.lock, flags);
 }
 
 static inline unsigned long cfs_rq_runnable_load_avg(struct cfs_rq *cfs_rq)
@@ -9350,8 +9370,7 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	cfs_rq->propagate_avg = 0;
 #endif
-	atomic_long_set(&cfs_rq->removed_load_avg, 0);
-	atomic_long_set(&cfs_rq->removed_util_avg, 0);
+	raw_spin_lock_init(&cfs_rq->removed.lock);
 #endif
 }
 
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -424,14 +424,19 @@ struct cfs_rq {
 	struct sched_avg avg;
 	u64 runnable_load_sum;
 	unsigned long runnable_load_avg;
+#ifndef CONFIG_64BIT
+	u64 load_last_update_time_copy;
+#endif
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	unsigned long tg_load_avg_contrib;
 	unsigned long propagate_avg;
 #endif
-	atomic_long_t removed_load_avg, removed_util_avg;
-#ifndef CONFIG_64BIT
-	u64 load_last_update_time_copy;
-#endif
+	struct {
+		raw_spinlock_t	lock ____cacheline_aligned;
+		int		nr;
+		unsigned long	load_avg;
+		unsigned long	util_avg;
+	} removed;
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	/*