linux-kernel - Re: [BUG(?)] perf_events: combining multiple tracepoint events into a group produces no counts on member events

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Date:	Wed, 01 Dec 2010 11:22:12 -0800
From:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG(?)] perf_events: combining multiple tracepoint events into
 a group produces no counts on member events

Thanks for your reply, Peter.

On 12/01/2010 03:46 AM, Peter Zijlstra wrote:
> On Tue, 2010-11-30 at 17:00 -0800, Corey Ashford wrote:
>> Hi,
>>
>> I'm not sure that what I'm seeing is a bug, or was something intentional.
>>
>> If I place multiple tracepoint events into a group and measure counts of
>> these events on a process, I get no counts for the tracepoint events
>> other than the group leader.
>>
>> Is this expected behavior?
>>
>> It's not clear to me why this should be the case; grouping shouldn't
>> have any ill effects on tracepoint events, from my understanding.
>>
>> I noticed this because my private version of the perf tool has the event
>> group patch https://lkml.org/lkml/2010/11/24/584 as well as the patch
>> which fixes the parsing of multiple tracepoint events in the same -e
>> switch: https://lkml.org/lkml/2010/11/30/460
>>
>> When I dig into the code a bit, I find that each event opens
>> successfully, so that's not the problem.  If I disable the grouping,
>> then I get counts for all of the tracepoint events.
>
> Hrm,.. definitely not expected. I'll try and look into it, but I'm a bit
> over-committed atm.
>
> Also, I've started a rewrite of the whole tracepoint<->  perf
> interaction:
>
>    http://lkml.org/lkml/2010/11/23/147
>
> Could you see if that cures your problem?

I've had some trouble getting recent kernels to boot on my Power5 
machine, but I might be able to try it on my laptop.

>
> Another thing to test, does the same hold true for regular software
> events? tracepoints and software events share a lot of infrastructure.

I just tried "perf stat -e context-switches,faults ..." on both my 
laptop (running 2.6.35), and on my Power5 machine (running 2.6.33) and I 
get the same behavior when software events are grouped, e.g.:
% ./perf stat -e context-switches,faults ~/load 1000

  Performance counter stats for '/home/corey/load 1000':

                 240  context-switches
       <not counted>  page-faults

         2.382022393  seconds time elapsed

So I suppose that points to a common flaw in the tracepoint and software 
event logic.

With that in mind, would it still make sense to try out your tracepoint 
patch?

- Corey
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