linux-kernel - Re: [RFC][PATCH 1/2] perf: Add 'perf kmem' tool

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Date:	Fri, 20 Nov 2009 15:52:28 -0200
From:	Arnaldo Carvalho de Melo <acme@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Li Zefan <lizf@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC][PATCH 1/2] perf: Add 'perf kmem' tool

Em Fri, Nov 20, 2009 at 05:41:10PM +0100, Ingo Molnar escreveu:
> > So we have a mechanism that is already present in several distros
> > (build-id), that is in the kernel build process since ~2.6.23, and that
> > avoids using mismatching DSOs when resolving symbols.
> 
> But what do we do if we have another box that runs say on a MIPS CPU, 
> uses some minimal distro - and copy that perf.data over to an x86 box.

There would be no problem, it would be just a matter of installing the
right -debuginfo packages, for MIPS.

Or the original, unstripped FS image sent to the machine with the MIPS
cpu, if there aren't -debuginfo packages.

Either one, the right DSOs would be found by the buildids.

There are other scenarios, like a binary that gets updated while a long
running perf record session runs, the way to differentiate between the
two DSOs wouldn't be the name, but the buildid.

> The idea is there to be some new mode of perf.data where all the 
> relevant DSO contents (symtabs but also sections with instructions for 
> perf annotate to work) are copied into perf.data, during or after data 
> capture - on the box that does the recording.
> 
> Once we have everything embedded in the perf.data, analysis passes only 
> have to work based on that particular perf.data - no external data.

Well, we can that, additionally, but think about stripped binaries, we
would lose potentially a lot because the symtabs on that small machine
would have poorer symtabs than the ones in an unstriped binary (or in a
-debuginfo package).

- Arnaldo
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