linux-kernel - Re: perf_events: question about __perf_event

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Date:	Fri, 21 Jan 2011 19:25:43 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, mingo@...e.hu,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: perf_events: question about __perf_event_read()

On Fri, Jan 21, 2011 at 7:18 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, 2011-01-21 at 19:06 +0100, Stephane Eranian wrote:
>> Hi,
>>
>> I think the code below still has a problem in case of a per-cpu event.
>>
>> If you issue a read() on a different CPU, then you IPI to the event's cpu.
>> By the time you get there, the event may be de-scheduled in which
>> case you don't want to issue event->pmu_read() nor update context
>> timings. The function has a test but it seems to be checking the per-cpu
>> case only.
>>
>> I have seen panics on P4 with this code because it goes all the way
>> down to rdmsrl() with a bogus counter index (like -1).
>>
>> Am I missing something here?
>>
>> static void __perf_event_read(void *info)
>> {
>>         struct perf_event *event = info;
>>         struct perf_event_context *ctx = event->ctx;
>>         struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
>>
>>         /*
>>          * If this is a task context, we need to check whether it is
>>          * the current task context of this cpu.  If not it has been
>>          * scheduled out before the smp call arrived.  In that case
>>          * event->count would have been updated to a recent sample
>>          * when the event was scheduled out.
>>          */
>>         if (ctx->task && cpuctx->task_ctx != ctx)
>>                 return;
>>
>>         raw_spin_lock(&ctx->lock);
>
> Shouldn't we re-check event->state once we hold this lock?
>
I remember checking this about a month ago and tip-x86 had
some checks in this routine, but now the're gone.

I think you need something like:

+	if (ctx->is_active)
+		update_context_time(ctx);

+	if (event->state == PERF_EVENT_STATE_ACTIVE)
+		event->pmu->read(event);

>>         update_context_time(ctx);
>>         update_event_times(event);
>>         raw_spin_unlock(&ctx->lock);
>>
>>         event->pmu->read(event);
>> }
>
>
>
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