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

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Message-ID: <4FBE5583.8070405@gmail.com>
Date:	Thu, 24 May 2012 09:36:35 -0600
From:	David Ahern <dsahern@...il.com>
To:	Stephane Eranian <eranian@...gle.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCH v2 4/5] perf record: add meta-data support for pipe-mode

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.

David
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