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 12:46:30 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
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

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?

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


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