Re: [PATCH V5 09/12] perf: make events stream always parsable

[Adrian Hunter <adrian.hunter@intel.com>]

On 15/07/13 14:53, Stephane Eranian wrote:
> On Mon, Jul 15, 2013 at 8:14 AM, Adrian Hunter <adrian.hunter@intel.com> wrote:
>> 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).
>>
> I am missing something here.
> Why do we need an event ID for RECORD_MMAP records?
> I understand those are requested by events, but do we care which one?
> The information is global to the monitored process and not specific to an event.

The ID sample has, for example, the time, so you still have to parse it -
which means you need the sample_type which means you need the id.