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SubjectRe: [PATCH V5 09/12] perf: make events stream always parsable
On 12/07/13 17:55, Peter Zijlstra wrote:
> On Fri, Jul 12, 2013 at 03:56:01PM +0300, Adrian Hunter wrote:
>>> There's events where this isn't a possible location; take PERF_RECORD_MMAP for
>>> instance; the tail is the complete filename.
>> PERF_RECORD_MMAP falls in the category I have called non-sample events.
>> Those events are appended with an ID sample. perf tools parses the ID
>> sample backwards from header.size. So the ID is at the last position
>> relative to header.size
> But why? Why make it different per PERF_RECORD type?

There have always been two formats:

1. PERF_RECORD_SAMPLE as defined by perf_output_sample()

2. everything else as defined by __perf_event__output_id_sample()

The two formats are not the same, and there is no reason for them to be.

PERF_RECORD_SAMPLE is parsed forwards, so the ID is at the first position.

ID samples are parsed backwards, so the ID is at the last position (i.e. the
first position parsed).

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