linux-kernel - Re: [RFC/PATCH] perf tools: Introduce perf

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Date:	Tue, 24 Nov 2015 16:24:50 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
CC:	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jiri Olsa <jolsa@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC/PATCH] perf tools: Introduce perf_thread for backtrace

Hi Arnaldo and Masami,

On Fri, Nov 20, 2015 at 09:10:44AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Fri, Nov 20, 2015 at 09:05:23AM +0000, 平松雅巳 / HIRAMATU，MASAMI escreveu:
> > >From: Namhyung Kim [mailto:namhyung@...nel.org]
> > >
> > >Backtrace is a crucial info for debugging.  And upcoming refcnt
> > >tracking facility also wants to use it.
> > >
> > >So instead of relying on glibc's backtrace_symbols[_fd] which misses
> > >some (static) functions , use our own symbol searching mechanism.  To
> > >do that, add perf_thread global variable to keep its maps and symbols.
> > 
> > Hmm, I doubt that this can work for debugging situation, because
> > sometimes backtrace facilities has to debug itself by itself.
> 
> That is a valid point, possibly we can have both and when we think that
> the code we rely on for resolving symbols has issues, activate the
> other, more expensive, binutils/elfutils spawned command line utilities
> to do compare the results?

Yeah, that's a possible solution.  We can start by using our own, and
if there's a certain amount of failure in symbol resolving, then
fallback to glibc's backtrace_symbols + addr2line.


>  
> > For the some (static) functions, I'd rather like to use glibc's 
> > backtrace_symbols and addr2line or even with raw address for
> > reliability...

I also printed the raw addresses in case of doubts, so you could
verify its correctness. :)  And IMHO, if something is severely broken,
we might not rely on glibc too.

Having said that, I agree with your concern and it needs the fallback
method for possible malfunction.  But I guess it'd work quite well for
most cases so it's worth trying to convert using it.  I'll work on the
fallback method then..

Thanks,
Namhyung
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