[tip:perf/core] perf: Do not compute time values unnecessarily

[tip-bot for Peter Zijlstra <tipbot@zytor.com>]

Commit-ID:  5ec4c599a52362896c3e7c6a31ba6145dca9c6f5
Gitweb:     http://git.kernel.org/tip/5ec4c599a52362896c3e7c6a31ba6145dca9c6f5
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Fri, 2 Aug 2013 21:16:30 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 16 Aug 2013 17:55:52 +0200

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 <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20130802191630.GT27162@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/events/core.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2e675e8..928fae7 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3670,6 +3670,10 @@ void perf_event_update_userpage(struct perf_event *event)
 	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
@@ -3680,12 +3684,8 @@ void perf_event_update_userpage(struct perf_event *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.