When a pinned group cannot be scheduled it goes into error state.
Normally a group cannot go out of error state without being explicitly
re-enabled or disabled. There was a bug in per-thread mode, whereby
upon termination of the thread, the group would transition from error
to off leading to bogus counts and timing information returned by
read().
It is important to realize that the current perf_events implementation
assigns higher priority to system-wide events over per-thread events
and that regardless of the fact that per-thread events may be pinned.
It is not clear to me whether this is per design of the API or just a
side effect of the implementation. I believe it is desirable that a
system-wide tool gets priority access to the PMU but then this causes
issues with per-thread events and especially when they request pinning.
A per-thread pinned event can be evicted until there is enough PMU
resource freed by system-wide events. Although, with this patch it is
now possible to detect this when counting, it remains unclear how this
situation could be detected when sampling, as it incurs potientially
large blind spots and thus bias degrading the quality of the data
collected.
The API is missing a clear definition of what it means to be pinned
for a per-thread event vs. system-wide event. Just like it does not
clearly state that system-wide event have higher priority than per
thread events.
Signed-off-by: Stephane Eranian <[email protected]>
---
perf_event.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 0b0d5f7..7a8bb5b 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -333,7 +333,16 @@ list_del_event(struct perf_event *event, struct
perf_event_context *ctx)
event->group_leader->nr_siblings--;
update_event_times(event);
- event->state = PERF_EVENT_STATE_OFF;
+
+ /*
+ * If event was in error state, then keep it
+ * that way, otherwise bogus counts will be
+ * returned on read(). The only way to get out
+ * of error state is by explicit re-enabling
+ * of the event
+ */
+ if (event->state > PERF_EVENT_STATE_OFF)
+ event->state = PERF_EVENT_STATE_OFF;
/*
* If this was a group event with sibling events then
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