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

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Message-ID: <20120524164409.GC2336@infradead.org>
Date:	Thu, 24 May 2012 13:44:09 -0300
From:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
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 10:22:04AM -0600, David Ahern escreveu:
> 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
> >;-)
> 
> I don't know about that 'easy' comment; that patch is fairly long.

Easy in the sense that we didn't need to come up with any new mechanism
to insert that meta data.

> Hence my question about whether synthesized events are acceptable
> before spending time on it. ;-)

I think that we need to follow Peter suggestion, which looks to me
similar to what you're suggesting, i.e. before the event stream, that
will become multichannel anyway.

- Arnaldo
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