linux-kernel - Re: [RFC PATCHv3] perf tools: add event grouping capability to "perf stat"

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Date:	Thu, 25 Nov 2010 14:19:48 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Julia Lawall <julia@...u.dk>, Tom Zanussi <tzanussi@...il.com>,
	linux-kernel@...r.kernel.org, Matt Fleming <matt@...sole-pimps.org>
Subject: Re: [RFC PATCHv3] perf tools: add event grouping capability to "perf stat"

On Thu, Nov 25, 2010 at 7:32 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Wed, 2010-11-24 at 17:54 -0800, Corey Ashford wrote:
>> Add the ability to create multiple event groups, each with their own leader
>> using the existing "-e <event>[,<event> ...] [-e <event>[,<event>]]"
>> syntax.  Each additional -e switch creates a new group, and each event
>> listed within a -e switch is within that group.
>>
>> Changes since v1:
>> - Because of a flub, v2 did not contain the changes I had intended to make,
>> and instead, v2 had the same patch contents as v1.
>> - When perf stat is not supplied any events on the command line, put
>> each default event in its own group.
>
> I like this, but could you also extend this to perf-record? its a bit
> odd to diverge between the two.
>
> Using Stephane's latest syntax changes you could actually do something
> like:
>
> perf record -e task-clock:freq=1000,cycles:period=0
>
> Which would create a group with 1 sampling counter and a counting
> counter (at which point we should probably start flipping
> PERF_SAMPLE_READ).
>

I think using PERF_SAMPLE_READ may expose a problem in the
perf.data format. To correctly parse a sample created with SAMPLE_READ,
you need to know the attr.read_format. But for that you need to know the
event which caused the sample, but for that you need the SAMPLE_ID,
and you don't know if it's there or not. In other words, there is a chicken
and egg problem.

I think the issue is that PERF_RECORD_SAMPLE is missing a mandatory
piece of information: overflow event ID. This must a mandatory field, not
optional as it is today. It is okay when you have only one group, but we'd
like to go beyond that.
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