The following commit has been merged in the linux branch: commit 03541f8b69c058162e4cf9675ec9181e6a204d55 Author: Paul Mackerras paulus@samba.org Date: Wed Oct 14 16:58:03 2009 +1100
perf_event: Adjust frequency and unthrottle for non-group-leader events
The loop in perf_ctx_adjust_freq checks the frequency of sampling event counters, and adjusts the event interval and unthrottles the event if required, and resets the interrupt count for the event. However, at present it only looks at group leaders.
This means that a sampling event that is not a group leader will eventually get throttled, once its interrupt count reaches sysctl_perf_event_sample_rate/HZ --- and that is guaranteed to happen, if the event is active for long enough, since the interrupt count never gets reset. Once it is throttled it never gets unthrottled, so it basically just stops working at that point.
This fixes it by making perf_ctx_adjust_freq use ctx->event_list rather than ctx->group_list. The existing spin_lock/spin_unlock around the loop makes it unnecessary to put rcu_read_lock/ rcu_read_unlock around the list_for_each_entry_rcu().
Reported-by: Mark W. Krentel krentel@cs.rice.edu Signed-off-by: Paul Mackerras paulus@samba.org Cc: Corey Ashford cjashfor@linux.vnet.ibm.com Cc: Peter Zijlstra a.p.zijlstra@chello.nl LKML-Reference: 19157.26731.855609.165622@cargo.ozlabs.ibm.com Signed-off-by: Ingo Molnar mingo@elte.hu
diff --git a/kernel/perf_event.c b/kernel/perf_event.c index 9d0b5c6..afb7ef3 100644 --- a/kernel/perf_event.c +++ b/kernel/perf_event.c @@ -1355,7 +1355,7 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx) u64 interrupts, freq;
spin_lock(&ctx->lock); - list_for_each_entry(event, &ctx->group_list, group_entry) { + list_for_each_entry_rcu(event, &ctx->event_list, event_entry) { if (event->state != PERF_EVENT_STATE_ACTIVE) continue;