perf_event: fix broken calc_timer_values()

We detected a serious issue with PERF_SAMPLE_READ and timing information when events were being multiplexing.

Samples would have time_running > time_enabled. That was easy to reproduce with a libpfm4 example (ran 3 times to cause multiplexing on Core 2):

$ syst_smpl -e uops_retired:freq=1 &
$ syst_smpl -e uops_retired:freq=1 &
$ syst_smpl -e uops_retired:freq=1 &
IIP:0x0000000040062d ... PERIOD:2355332948 ENA=40144625315 RUN=60014875184
syst_smpl: WARNING: time_running > time_enabled
	63277537998 uops_retired:freq=1 , scaled

The bug was not present in kernel up to 3.0. It turns out the bug was introduced by the following commit:

commit c4794295917ebeda8013b6cb9c8d71ab4f74a1fa

Author: Eric B Munson <[email protected]>
Date:   Thu Jun 23 16:34:38 2011 -0400

events: Move lockless timer calculation into helper function

The parameters of the function got reversed yet the call sites were not updated to reflect the change. That lead to time_running and time_enabled being swapped. That had no effect when there was no multiplexing because in that case time_running = time_enabled but it would show up in any other scenario.

---

index adc3ef3..6bacaee 100644
@@ -3354,8 +3354,8 @@ static int perf_event_index(struct perf_event *event)
 }
 
 static void calc_timer_values(struct perf_event *event,
-				u64 *running,
-				u64 *enabled)
+				u64 *enabled,
+				u64 *running)
 {
 	u64 now, ctx_time;
 

Ah, up to /and including/ 3.0, since:

git describe --contains c4794295917ebeda8013b6cb9c8d71ab4f74a1fa --match "v*" v3.1-rc1~299^2~32

So only perf/urgent is needed.