linux-kernel - Re: [tip:perf/core] perf ui annotate browser: Allow toggling addr offset view

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Date:	Sat, 14 Apr 2012 10:55:19 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, hpa@...or.com,
	paulus@...ba.org, eranian@...gle.com, linux-kernel@...r.kernel.org,
	efault@....de, peterz@...radead.org, namhyung@...il.com,
	fweisbec@...il.com, dsahern@...il.com,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf ui annotate browser: Allow toggling addr
 offset view

* Arnaldo Carvalho de Melo <acme@...hat.com> wrote:

> Em Fri, Apr 13, 2012 at 11:30:52AM -0700, Linus Torvalds escreveu:
> > On Fri, Apr 13, 2012 at 11:25 AM, Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:
> > >
> > > <kmem_cache_free>:
> > >    1.91 :          push   %rbp
> > 
> > Oh, btw, talking about kmem_cache_free: that one uses altinstructions,
> > and so perf report shows the hottest instruction wrong (and I'm not
> > talking about "ugly"):
> 
> Well, if we use Masami's disassembler we would use the actual 
> code as it is being used and not the original DSO that was 
> later patched by altinstructions.

Key would be to use the kernel's live RAM image of instructions. 

I.e. we should provide a live /proc/vmlinux image in essence: a 
'virtual' ELF binary image constructed out of the live kernel 
RAM image - with no extra RAM overhead. (Maybe with modules 
included in an intelligent way - although personally I don't use 
modules when I instrument the kernel)

That plus the always-available /proc/kallsyms would offer rather 
powerful annotation already: without *any* debug info - out of 
box, on any Linux installation. (This was always the main 
advantage of /proc/profile and readprofile btw: it worked 
everywhere while most other profiling solutions needed a 
debuginfo, etc.)

Doing /proc/vmlinux would be different from /dev/mem as it only 
shows the kernel RAM image, and only in a read-only fashion.

Default permissions of /proc/vmlinux should probably track 
console permissions: it should be possible to allow people 
sitting in front of the computer to read /proc/vmlinux, while 
people logged in over the network wouldn't.

Doing such a live kernel vmlinux would have other debugging and 
instrumentation advantages as well: various code patching 
effects could be checked and observed directly.

Thanks,

	Ingo
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