sched/fair: Add some serialization to the sched_domain load-balance walk

Since the sched_domain walk is completely unserialized (!SD_SERIALIZE)
it is possible that multiple cpus in the group get elected to do the
next level. Avoid this by adding some serialization.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-vqh9ai6s0ewmeakjz80w4qz6@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched.h

@@ -927,6 +927,7 @@ struct sched_group_power {
 struct sched_group {
 	struct sched_group *next;	/* Must be a circular list */
 	atomic_t ref;
+	int balance_cpu;
 
 	unsigned int group_weight;
 	struct sched_group_power *sgp;

kernel/sched/core.c

@@ -6060,6 +6060,7 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu)
 
 		sg->sgp = *per_cpu_ptr(sdd->sgp, cpumask_first(sg_span));
 		atomic_inc(&sg->sgp->ref);
+		sg->balance_cpu = -1;
 
 		if (cpumask_test_cpu(cpu, sg_span))
 			groups = sg;
@@ -6135,6 +6136,7 @@ build_sched_groups(struct sched_domain *sd, int cpu)
 
 		cpumask_clear(sched_group_cpus(sg));
 		sg->sgp->power = 0;
+		sg->balance_cpu = -1;
 
 		for_each_cpu(j, span) {
 			if (get_group(j, sdd, NULL) != group)

kernel/sched/fair.c

@@ -3828,7 +3828,8 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
 	 */
 	if (local_group) {
 		if (idle != CPU_NEWLY_IDLE) {
-			if (balance_cpu != this_cpu) {
+			if (balance_cpu != this_cpu ||
+			    cmpxchg(&group->balance_cpu, -1, balance_cpu) != -1) {
 				*balance = 0;
 				return;
 			}
@@ -4929,7 +4930,7 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
 	int balance = 1;
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long interval;
-	struct sched_domain *sd;
+	struct sched_domain *sd, *last = NULL;
 	/* Earliest time when we have to do rebalance again */
 	unsigned long next_balance = jiffies + 60*HZ;
 	int update_next_balance = 0;
@@ -4939,6 +4940,7 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
 
 	rcu_read_lock();
 	for_each_domain(cpu, sd) {
+		last = sd;
 		if (!(sd->flags & SD_LOAD_BALANCE))
 			continue;
 
@@ -4983,6 +4985,9 @@ out:
 		if (!balance)
 			break;
 	}
+	for (sd = last; sd; sd = sd->child)
+		(void)cmpxchg(&sd->groups->balance_cpu, cpu, -1);
+
 	rcu_read_unlock();
 
 	/*