events: Ensure that timers are updated without requiring read() call

The event tracing infrastructure exposes two timers which should be updated
each time the value of the counter is updated.  Currently, these counters are
only updated when userspace calls read() on the fd associated with an event.
This means that counters which are read via the mmap'd page exclusively never
have their timers updated.  This patch adds ensures that the timers are updated
each time the values in the mmap'd page are updated.

Signed-off-by: Eric B Munson <emunson@mgebm.net>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1308932786-5111-1-git-send-email-emunson@mgebm.net
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/events/core.c

@@ -3372,8 +3372,19 @@ void perf_event_update_userpage(struct perf_event *event)
 {
 	struct perf_event_mmap_page *userpg;
 	struct ring_buffer *rb;
+	u64 enabled, running;
 
 	rcu_read_lock();
+	/*
+	 * compute total_time_enabled, total_time_running
+	 * based on snapshot values taken when the event
+	 * was last scheduled in.
+	 *
+	 * we cannot simply called update_context_time()
+	 * because of locking issue as we can be called in
+	 * NMI context
+	 */
+	calc_timer_values(event, &enabled, &running);
 	rb = rcu_dereference(event->rb);
 	if (!rb)
 		goto unlock;
@@ -3392,10 +3403,10 @@ void perf_event_update_userpage(struct perf_event *event)
 	if (event->state == PERF_EVENT_STATE_ACTIVE)
 		userpg->offset -= local64_read(&event->hw.prev_count);
 
-	userpg->time_enabled = event->total_time_enabled +
+	userpg->time_enabled = enabled +
 			atomic64_read(&event->child_total_time_enabled);
 
-	userpg->time_running = event->total_time_running +
+	userpg->time_running = running +
 			atomic64_read(&event->child_total_time_running);
 
 	barrier();