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Subject: Re: [BUG] perf_event: sampling buffer format cannot handle
 multi-event sampling
From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>, mingo@elte.hu,
        Paul Mackerras <paulus@samba.org>,
        Robert Richter <robert.richter@amd.com>,
        Andi Kleen <andi@firstfloor.org>
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Date: Tue, 19 Apr 2011 21:22:15 +0200
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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?