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

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Date:	Sat, 14 Apr 2012 13:28:51 -0300
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
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

Em Sat, Apr 14, 2012 at 10:55:19AM +0200, Ingo Molnar escreveu:
> 
> * 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. 

Yeah, that is what I meant by "if we use Masami's disassembler" :-)
 
> 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.

I would say that even for userspace we would love to have such virtual
ELF files, as code patching is become more common...

- Arnaldo
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