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

[Stephane Eranian]

On Thu, Nov 25, 2010 at 3:02 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> On Thu, 2010-11-25 at 14:19 +0100, Stephane Eranian wrote:
>
>> 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.
>
>
>
> I'm not sure I get it, there's both PERF_FORMAT_ID and PERF_SAMPLE_ID,
> so afaict there's a working combination for what you want to do.
>
Ok, I had forgotten about PERF_SAMPLE_ID. But that means that if the
number of groups > 1, then if you use PERF_SAMPLE_READ, you MUST
also use PERF_SAMPLE_ID. Otherwise you cannot get back to the event
that generated the sample and thus attr.read_format.
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