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

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Message-ID: <87obpf209z.fsf@sejong.aot.lge.com>
Date:	Wed, 23 May 2012 09:45:12 +0900
From:	Namhyung Kim <namhyung.kim@....com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	David Ahern <dsahern@...il.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu
Subject: Re: [PATCH v2 4/5] perf record: add meta-data support for pipe-mode

Hi,

On Tue, 22 May 2012 19:51:15 +0200, Stephane Eranian wrote:
> On Tue, May 22, 2012 at 7:33 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Fri, 2012-05-18 at 14:19 -0300, Arnaldo Carvalho de Melo wrote:
>>
>>> PeterZ was the one objecting to adding more userspace only events,
>>
>> Ah, yes I was ;-)
>>
>> So uhm the argument was something like perf_event_type is a kernel enum
>> and userspace stealing space there is going to get us into trouble
>> eventually since userspace doesn't register its types in our enum.
>>
> Yes, I did not start this but I understand why it was done that way.
>
> 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.
>

IIUC, to add build-ids we should check all dso's whether hits or not,
but it's not same as the case of meta-data since we can know it before
processing. Am I missing something?

Thanks,
Namhyung
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