sched: update aggregate when holding the RQs

It was observed that in __update_group_shares_cpu()

  rq_weight > aggregate()->rq_weight

This is caused by forks/wakeups in between the initial aggregate pass and
locking of the RQs for load balance. To avoid this situation partially re-do
the aggregation once we have the RQs locked (which avoids new tasks from
appearing).
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -1721,6 +1721,11 @@ aggregate_get_up(struct task_group *tg, int cpu, struct sched_domain *sd)
 	aggregate_group_set_shares(tg, cpu, sd);
 }
 
+static void
+aggregate_get_nop(struct task_group *tg, int cpu, struct sched_domain *sd)
+{
+}
+
 static DEFINE_PER_CPU(spinlock_t, aggregate_lock);
 
 static void __init init_aggregate(void)
@@ -1740,6 +1745,11 @@ static int get_aggregate(int cpu, struct sched_domain *sd)
 	return 1;
 }
 
+static void update_aggregate(int cpu, struct sched_domain *sd)
+{
+	aggregate_walk_tree(aggregate_get_down, aggregate_get_nop, cpu, sd);
+}
+
 static void put_aggregate(int cpu, struct sched_domain *sd)
 {
 	spin_unlock(&per_cpu(aggregate_lock, cpu));
@@ -1761,6 +1771,10 @@ static inline int get_aggregate(int cpu, struct sched_domain *sd)
 	return 0;
 }
 
+static inline void update_aggregate(int cpu, struct sched_domain *sd)
+{
+}
+
 static inline void put_aggregate(int cpu, struct sched_domain *sd)
 {
 }
@@ -2192,6 +2206,12 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
 	int load_idx = sd->forkexec_idx;
 	int imbalance = 100 + (sd->imbalance_pct-100)/2;
 
+	/*
+	 * now that we have both rqs locked the rq weight won't change
+	 * anymore - so update the stats.
+	 */
+	update_aggregate(this_cpu, sd);
+
 	do {
 		unsigned long load, avg_load;
 		int local_group;