sched: Reduce overestimating rq->avg_idle

When updating avg_idle, if the delta exceeds some max value, then avg_idle
gets set to the max, regardless of what the previous avg was. This can cause
avg_idle to often be overestimated.

This patch modifies the way we update avg_idle by always updating it with the
function call to update_avg() first. Then, if avg_idle exceeds the max, we set
it to the max.
Signed-off-by: Jason Low <jason.low2@hp.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1379096813-3032-2-git-send-email-jason.low2@hp.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -1332,10 +1332,11 @@ ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
 		u64 delta = rq_clock(rq) - rq->idle_stamp;
 		u64 max = 2*sysctl_sched_migration_cost;
 
-		if (delta > max)
-			rq->avg_idle = max;
-		else
 		update_avg(&rq->avg_idle, delta);
+
+		if (rq->avg_idle > max)
+			rq->avg_idle = max;
+
 		rq->idle_stamp = 0;
 	}
 #endif