perf: Do not compute time values unnecessarily

We should not be calling calc_timer_values() for events that do not actually
have an mmap()'ed userpage.

Signed-off-by: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/events/core.c

···
 	u64 enabled, running, now;
 
 	rcu_read_lock();
+	rb = rcu_dereference(event->rb);
+	if (!rb)
+		goto unlock;
+
 	/*
 	 * compute total_time_enabled, total_time_running
 	 * based on snapshot values taken when the event
···
 	 * NMI context
 	 */
 	calc_timer_values(event, &now, &enabled, &running);
-	rb = rcu_dereference(event->rb);
-	if (!rb)
-		goto unlock;
 
 	userpg = rb->user_page;
-
 	/*
 	 * Disable preemption so as to not let the corresponding user-space
 	 * spin too long if we get preempted.