sched: fix inconsistency when redistribute per-cpu tg->cfs_rq shares

Impact: make load-balancing more consistent

In the update_shares() path leading to tg_shares_up(), the calculation of
per-cpu cfs_rq shares is rather erratic even under moderate task wake up
rate.  The problem is that the per-cpu tg->cfs_rq load weight used in the
sd_rq_weight aggregation and actual redistribution of the cfs_rq->shares
are collected at different time.  Under moderate system load, we've seen
quite a bit of variation on the cfs_rq->shares and ultimately wildly
affects sched_entity's load weight.

This patch caches the result of initial per-cpu load weight when doing the
sum calculation, and then pass it down to update_group_shares_cpu() for
redistributing per-cpu cfs_rq shares.  This allows consistent total cfs_rq
shares across all CPUs. It also simplifies the rounding and zero load
weight check.

Signed-off-by: Ken Chen <kenchen@google.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -1453,27 +1453,13 @@ static void
 update_group_shares_cpu(struct task_group *tg, int cpu,
 			unsigned long sd_shares, unsigned long sd_rq_weight)
 {
-	int boost = 0;
 	unsigned long shares;
 	unsigned long rq_weight;
 
 	if (!tg->se[cpu])
 		return;
 
-	rq_weight = tg->cfs_rq[cpu]->load.weight;
-
-	/*
-	 * If there are currently no tasks on the cpu pretend there is one of
-	 * average load so that when a new task gets to run here it will not
-	 * get delayed by group starvation.
-	 */
-	if (!rq_weight) {
-		boost = 1;
-		rq_weight = NICE_0_LOAD;
-	}
-
-	if (unlikely(rq_weight > sd_rq_weight))
-		rq_weight = sd_rq_weight;
+	rq_weight = tg->cfs_rq[cpu]->rq_weight;
 
 	/*
 	 *           \Sum shares * rq_weight
@@ -1481,7 +1467,7 @@ update_group_shares_cpu(struct task_group *tg, int cpu,
 	 *               \Sum rq_weight
 	 *
 	 */
-	shares = (sd_shares * rq_weight) / (sd_rq_weight + 1);
+	shares = (sd_shares * rq_weight) / sd_rq_weight;
 	shares = clamp_t(unsigned long, shares, MIN_SHARES, MAX_SHARES);
 
 	if (abs(shares - tg->se[cpu]->load.weight) >
@@ -1490,11 +1476,7 @@ update_group_shares_cpu(struct task_group *tg, int cpu,
 		unsigned long flags;
 
 		spin_lock_irqsave(&rq->lock, flags);
-		/*
-		 * record the actual number of shares, not the boosted amount.
-		 */
-		tg->cfs_rq[cpu]->shares = boost ? 0 : shares;
-		tg->cfs_rq[cpu]->rq_weight = rq_weight;
+		tg->cfs_rq[cpu]->shares = shares;
 
 		__set_se_shares(tg->se[cpu], shares);
 		spin_unlock_irqrestore(&rq->lock, flags);
@@ -1508,13 +1490,23 @@ update_group_shares_cpu(struct task_group *tg, int cpu,
  */
 static int tg_shares_up(struct task_group *tg, void *data)
 {
-	unsigned long rq_weight = 0;
+	unsigned long weight, rq_weight = 0;
 	unsigned long shares = 0;
 	struct sched_domain *sd = data;
 	int i;
 
 	for_each_cpu_mask(i, sd->span) {
-		rq_weight += tg->cfs_rq[i]->load.weight;
+		/*
+		 * If there are currently no tasks on the cpu pretend there
+		 * is one of average load so that when a new task gets to
+		 * run here it will not get delayed by group starvation.
+		 */
+		weight = tg->cfs_rq[i]->load.weight;
+		if (!weight)
+			weight = NICE_0_LOAD;
+
+		tg->cfs_rq[i]->rq_weight = weight;
+		rq_weight += weight;
 		shares += tg->cfs_rq[i]->shares;
 	}
 
@@ -1524,9 +1516,6 @@ static int tg_shares_up(struct task_group *tg, void *data)
 	if (!sd->parent || !(sd->parent->flags & SD_LOAD_BALANCE))
 		shares = tg->shares;
 
-	if (!rq_weight)
-		rq_weight = cpus_weight(sd->span) * NICE_0_LOAD;
-
 	for_each_cpu_mask(i, sd->span)
 		update_group_shares_cpu(tg, i, shares, rq_weight);