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

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Message-ID: <20120524161929.GB6344@infradead.org>
Date:	Thu, 24 May 2012 13:19:29 -0300
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	David Ahern <dsahern@...il.com>
Cc:	Stephane Eranian <eranian@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCH v2 4/5] perf record: add meta-data support for pipe-mode

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
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