Re: [PATCH v2 4/5] perf record: add meta-data support for pipe-mode

[David Ahern <dsahern@gmail.com>]

On 5/24/12 10:19 AM, Arnaldo Carvalho de Melo wrote:
> Em Thu, May 24, 2012 at 09:36:35AM -0600, David Ahern escreveu:
>> On 5/22/12 11:51 AM, Stephane Eranian wrote:
>>> The problem is that the headers as they are written to the file need
>>> seeking in the file to update the offset table. That is NOT possible when
>>> you operate in pipe mode. As such you need to inject the header infos
>>> very much like kernel PERF_RECORD_*. That is also why you have
>>> perf inject -b. Buildids are added at the end of the run in file mode,
>>> and that's another seek to the offset table if I recall correctly.
>
>> Perhaps I am being too simple minded here, but why not dump the
>> features to the pipe as a series of structs when the perf session is
>> created?
>
>> struct pipe_data {
>> 	u32 length;
>> 	u32 type;
>> 	char data[0];		
>> }
>
>> It would require the features to be written to a buffer first to get
>> the length, but that's manageable without too much code change.
>> Endianness would need to be handled -- maybe a u8 flags at the
>> beginning.
>
> I guess this comes down to somebody prototyping it. The way its being
> done is just the easiest one, and one that at least Peter dislikes a lot
> ;-)
>
> - Arnaldo

I don't know about that 'easy' comment; that patch is fairly long. Hence 
my question about whether synthesized events are acceptable before 
spending time on it. ;-)

David