linux-kernel - Re: [BUG] perf_event: sampling buffer format cannot handle multi-event sampling

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Message-ID: <BANLkTi=GX3JGw-Dq+ejsCguV0A+9w2eCjQ@mail.gmail.com>
Date:	Tue, 19 Apr 2011 21:16:27 +0000
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>, mingo@...e.hu,
	Paul Mackerras <paulus@...ba.org>,
	Robert Richter <robert.richter@....com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [BUG] perf_event: sampling buffer format cannot handle
 multi-event sampling

On Tue, Apr 19, 2011 at 7:22 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, 2011-04-19 at 19:10 +0000, Stephane Eranian wrote:
>> Hi,
>>
>> I was looking at the sampling buffer format and I think there is an
>> issue in case
>> one samples on more than one event at a time and each event as a different
>> sample_type value (i.e., what it wants to record in each sample).
>>
>> The sample_type is exported per-event by the API. In order to decode a sample
>> (PERF_RECORD_SAMPLE) one has to know which event caused it. To identify
>> the event, one has to include PERF_SAMPLE_ID in the sample_type. Each such
>> id uniquely identifies an event. Assuming the tool keeps a mapping of event to
>> ID, we can then reconstruct. This is what perf does.
>>
>> The problem is that to extract the event ID, one has to already parse the sample
>> itself. That means, you need sample_type to extract the event ID.
>>
>> Thus, we have a catch 22 situation.
>>
>> Looking at the perf tool, it sort of works today simply because the
>> same sample_type
>> is applied to all events.
>>
>> To solve this, we could either:
>>    - add the event ID to the header for PERF_RECORD_SAMPLE
>>    - enforce event ID is systematically saved at the beginning on
>> PERF_RECORD_SAMPLE
>>
>> None of those options is transparent, I am afraid, unless we introduce
>> new record types.
>
> Or simply don't mix different sample_types in the same buffer?
>
Does the kernel disallow this when you merge the output via that ioctl()?
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