linux-kernel - Re: [PATCH] perf top: Make -g refer to callchains

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Date:	Tue, 19 Nov 2013 10:26:42 +0100
From:	Jean Pihet <jean.pihet@...aro.org>
To:	Jan Kratochvil <jan.kratochvil@...hat.com>
Cc:	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	David Ahern <dsahern@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Petr Machata <pmachata@...hat.com>
Subject: Re: [PATCH] perf top: Make -g refer to callchains

Hi,

On 18 November 2013 21:16, Jan Kratochvil <jan.kratochvil@...hat.com> wrote:
> On Mon, 18 Nov 2013 18:49:45 +0100, Jiri Olsa wrote:
>> I think it's an option.. but it'll simpler to try the libdw
>> interface first and see if it's good/fast enough..
>
> The elfutils libdw unwinder is being upstreamed these weeks, the x86* unwinder
> itself is already upstream now.
Do you know about the support of AARCH64, both in 64-bit and 32-bit
(compat) mode?
I would be glad to give it a try.

>> also I recall discussing the speed with libdw developer
>
> My tests with perf using elfutils unwinder were 10x faster than libunwind;
> this is by some simple caching of ELF files.  Sure a similar cache could be
> implemented also for libunwind.  But the cache is a wrong solution.
>
> The problem is that currently perf loads the ELF files again and again for
> every process as the ELF file always gets automatically relocated for the
> address where it was loaded.  The right way is to load the ELF file only once
> and access always the same copy with process-specific displacement only.
>
> I did not investigate how much feasible it is with libunwind.  For elfutils
> there is pmachata/sharing unfinished branch from 2008 to implement that.
> I have not checked it more before the unwinder gets fully upstreamed.
>
>
> Jan

Jean

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