linux-kernel - Re: [PATCH 3/3] perf tools: adjust symbol address

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Date:	Thu, 31 Dec 2009 08:29:38 -0200
From:	Arnaldo Carvalho de Melo <acme@...radead.org>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] perf tools: adjust symbol address

Em Thu, Dec 31, 2009 at 10:59:35AM +0800, Xiao Guangrong escreveu:
> 
> 
> Arnaldo Carvalho de Melo wrote:
> > Em Wed, Dec 30, 2009 at 11:18:55AM +0800, Xiao Guangrong escreveu:
> >> Using relocation offset adjust symbol address if we get
> >> kernel symbol name form elf file
> >>
> >> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> >> index 79ca6a0..5b58d34 100644
> >> --- a/tools/perf/util/symbol.c
> >> +++ b/tools/perf/util/symbol.c
> >> @@ -19,6 +19,18 @@
> >>  #define NT_GNU_BUILD_ID 3
> >>  #endif
> >>  
> >> +static s32 relocate_offset;
> >> +void update_relocate_offset(s32 offset)
> >> +{
> >> +	relocate_offset = offset;
> >> +}
> >> +
> >> +static inline void update_kernel_address(GElf_Sym *sym, bool kernel)
> >> +{
> >> +	if (kernel)
> >> +		sym->st_value += relocate_offset;
> >> +}

> > This should be done on the dso level, we may process multiple kernels,
> > some with relocation, some without.

> Sorry, what does the 'multiple kernels' means?

perf diff? I think you are combining the 'perf record' and 'perf report'
steps as a single action...

> vmlinux* not know it's relocation or not, and we just handle the _specific_
> kernel at the time which match the current system. 

'perf record' in one machine + 'perf report' in another should be
possible.
 
> And i think adjust symbol address on the dso level is not a good idea since
> we hardly know it's a module address or kernel address on dso level, i.e.
> hardly to know whether the address need adjust.

yeah, as I contininued below, its not the right level, for other
reasons.
 
> > Humm, even at the _map_ level, because then we can just use the normal
> > map_ip mechanism, this time the not using the identity map stuff, i.e.
> > we have to get an IP from some event, then subtract the relocate_offset
> > that will be in map->start.
> > 
> 
> The 'inject' event is the first event in perf.data since we inject it at the
> first counter file and inject 'relocation offset' before enable event, i.e.
> before getting symbol address we have got the 'relocation offset' already.

There is no need for kernel changes, as the discussion showed.

- Arnaldo
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