perf: Update shadow timestamp before add event

Update the shadow timestamp before start event, because .add might
use the timestamp.

Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Mackerras <paulus@samba.org>
Link: http://lkml.kernel.org/r/9cd0276d6a047cb7c2885994f25e3a1f7c8c28af.1423180257.git.shli@fb.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -1881,17 +1881,17 @@ event_sched_in(struct perf_event *event,
 
 	perf_pmu_disable(event->pmu);
 
+	event->tstamp_running += tstamp - event->tstamp_stopped;
+
+	perf_set_shadow_time(event, ctx, tstamp);
+
 	if (event->pmu->add(event, PERF_EF_START)) {
 		event->state = PERF_EVENT_STATE_INACTIVE;
 		event->oncpu = -1;
 		ret = -EAGAIN;
 		goto out;
 	}
 
-	event->tstamp_running += tstamp - event->tstamp_stopped;
-
-	perf_set_shadow_time(event, ctx, tstamp);
-
 	if (!is_software_event(event))
 		cpuctx->active_oncpu++;
 	if (!ctx->nr_active++)