perf, x86: Fix hw_perf_enable() event assignment

What happens is that we schedule badly like:

<...>-1987  [019]   280.252808: x86_pmu_start: event-46/1300c0: idx: 0
<...>-1987  [019]   280.252811: x86_pmu_start: event-47/1300c0: idx: 1
<...>-1987  [019]   280.252812: x86_pmu_start: event-48/1300c0: idx: 2
<...>-1987  [019]   280.252813: x86_pmu_start: event-49/1300c0: idx: 3
<...>-1987  [019]   280.252814: x86_pmu_start: event-50/1300c0: idx: 32
<...>-1987  [019]   280.252825: x86_pmu_stop: event-46/1300c0: idx: 0
<...>-1987  [019]   280.252826: x86_pmu_stop: event-47/1300c0: idx: 1
<...>-1987  [019]   280.252827: x86_pmu_stop: event-48/1300c0: idx: 2
<...>-1987  [019]   280.252828: x86_pmu_stop: event-49/1300c0: idx: 3
<...>-1987  [019]   280.252829: x86_pmu_stop: event-50/1300c0: idx: 32
<...>-1987  [019]   280.252834: x86_pmu_start: event-47/1300c0: idx: 1
<...>-1987  [019]   280.252834: x86_pmu_start: event-48/1300c0: idx: 2
<...>-1987  [019]   280.252835: x86_pmu_start: event-49/1300c0: idx: 3
<...>-1987  [019]   280.252836: x86_pmu_start: event-50/1300c0: idx: 32
<...>-1987  [019]   280.252837: x86_pmu_start: event-51/1300c0: idx: 32 *FAIL*

This happens because we only iterate the n_running events in the first
pass, and reset their index to -1 if they don't match to force a
re-assignment.

Now, in our RR example, n_running == 0 because we fully unscheduled, so
event-50 will retain its idx==32, even though in scheduling it will have
gotten idx=0, and we don't trigger the re-assign path.

The easiest way to fix this is the below patch, which simply validates
the full assignment in the second pass.
Reported-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1268311069.5037.31.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/perf_event.c

@@ -811,7 +811,6 @@ void hw_perf_enable(void)
 		 * step2: reprogram moved events into new counters
 		 */
 		for (i = 0; i < n_running; i++) {
-
 			event = cpuc->event_list[i];
 			hwc = &event->hw;
 
@@ -826,21 +825,16 @@ void hw_perf_enable(void)
 				continue;
 
 			x86_pmu_stop(event);
-
-			hwc->idx = -1;
 		}
 
 		for (i = 0; i < cpuc->n_events; i++) {
-
 			event = cpuc->event_list[i];
 			hwc = &event->hw;
 
-			if (i < n_running &&
-			    match_prev_assignment(hwc, cpuc, i))
-				continue;
-
-			if (hwc->idx == -1)
+			if (!match_prev_assignment(hwc, cpuc, i))
 				x86_assign_hw_event(event, cpuc, i);
+			else if (i < n_running)
+				continue;
 
 			x86_pmu_start(event);
 		}