linux-kernel - Re: [Question] perf header: Why the HEADER_EVENT

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Message-ID: <CABPqkBTkFBq+A0u1u98_RXORuLDVzJO1exMmHGWZ=hWvsg_MDw@mail.gmail.com>
Date:	Sat, 21 Jul 2012 19:16:11 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Question] perf header: Why the HEADER_EVENT_DESC is needed?

On Sat, Jul 21, 2012 at 1:11 PM, Namhyung Kim <namhyung@...nel.org> wrote:
> Hi Stephane,
>
> I wonder why we need to save the event descriptions in the feature area
> of file header. Since the perf_event_attr, its ids and the event name
> are already saved in the header, it seems a duplicate work to do that.
> For printing the information, we might reuse those info or evlist
> directly IMHO.
>
> Am I missing something?

Unfortunately, I think you are.

There event names are not saved in header.c except indirectly by write_cmdline.
But more importantly, event_desc makes it possible to connect an event name as
passed on the cmdline with the corresponding attr struct.

Furthermore, it is also present in pipe mode, with the patch I posted
months ago.

Finally, you cannot arbitrarily remove features like this. We do have
tools that do
rely on the presence of EVENT_DESC.
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