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>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2e675e830976f8fe5f0ee33967a776a11b179c98..928fae7ca8c78e280ad4d925f32b2fd5fbc94366 100644 (file)
--- 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.