sched: optimize update_curr()

Impact: micro-optimization

Skip the hard work when there is none.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -492,6 +492,8 @@ static void update_curr(struct cfs_rq *cfs_rq)
 	 * overflow on 32 bits):
 	 */
 	delta_exec = (unsigned long)(now - curr->exec_start);
+	if (!delta_exec)
+		return;
 
 	__update_curr(cfs_rq, curr, delta_exec);
 	curr->exec_start = now;