linux-kernel - Re: [PATCH] perf: Fix symbol resolution on old ppc64 ABI

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Message-ID: <20110725204859.51e50611@kryten>
Date:	Mon, 25 Jul 2011 20:48:59 +1000
From:	Anton Blanchard <anton@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	emunson@...bm.net, linux-kernel@...r.kernel.org, amodra@...il.com
Subject: Re: [PATCH] perf: Fix symbol resolution on old ppc64 ABI

Hi Ingo,

> > The synthetic symbol creation code has an issue with the old ppc64 
> > ABI. We end up with duplicate symbols of different sizes that 
> > overlap.
> > 
> > To fix this, walk all of the symbols and remove any duplicates that 
> > are the length of a function descriptor.
> 
> > I'd prefer not to add a ppc64 specific hack here, but I'm not sure 
> > how we can fix this in a simpler way.
> 
> Symbol space problems are common on other architectures as well.
> 
> We could use a heuristic: when symbols are overlapping then we could 
> throw away the one that is smaller. This would implicitly cover the 
> ppc64 case, right?
> 
> I'd also suggest we warn somewhere that a symbol has been thrown 
> away, if verbosity is turned off. (i.e. don't warn by default.)

It would cover most cases. There would be an issue with functions that
are less than 24 bytes in length (ie 6 instructions). Tiny functions
are probably rare enough that we can live with it.

Anton
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