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From: tip-bot for Peter Zijlstra on 11 Mar 2010 09:50
Commit-ID: 45e16a6834b6af098702e5ea6c9a40de42ff77d8
Gitweb: http://git.kernel.org/tip/45e16a6834b6af098702e5ea6c9a40de42ff77d8
Author: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
AuthorDate: Thu, 11 Mar 2010 13:40:30 +0100
Committer: Ingo Molnar <mingo(a)elte.hu>
CommitDate: Thu, 11 Mar 2010 15:21:28 +0100

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(a)google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
LKML-Reference: <1268311069.5037.31.camel(a)laptop>
Signed-off-by: Ingo Molnar <mingo(a)elte.hu>
---
arch/x86/kernel/cpu/perf_event.c | 12 +++---------
1 files changed, 3 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index c6bde7d..5fb490c 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/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);
}
--
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Next: [tip:perf/core] perf_events: Improve task_sched_in()