linux-kernel - Re: [PATCH] perf tools: Fix JIT profiling on heap

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Date:	Tue, 21 Jan 2014 10:55:52 -0300
From:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Gaurav Jain <gjain@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jiri Olsa <jolsa@...hat.com>, David Ahern <dsahern@...il.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH] perf tools: Fix JIT profiling on heap

Em Tue, Jan 21, 2014 at 02:54:32PM +0900, Namhyung Kim escreveu:
> On Fri, 17 Jan 2014 11:34:04 -0300, Arnaldo Carvalho de Melo wrote:
> > Em Fri, Jan 17, 2014 at 04:44:04PM +0900, Namhyung Kim escreveu:
> >> On Thu, 16 Jan 2014 20:23:27 +0000, Gaurav Jain wrote:
> >> > Does perf support data mappings from perf map files?

> >> IIUC there's no difference between function and data mapping.  So

> > Do the /tmp/perf mapping has any per entry indication on the type of
> > symbol it is (data, text) like ELF and kallsyms symtabs have?

> Quoting Documentation/jit-interface.txt:
 
>   Each line has the following format, fields separated with spaces:
 
>   START SIZE symbolname

> > It is possible for a function and a variable to have the same virt
> > addrin some arches (SPARC, iirc), that is why we have different MAP_
> > types (FUNCTION, VARIABLE)
 
> Hmm.. didn't know that, interesting..

> > So a 'struct map' for a data mmap should point to a different 'dso'
> > of the JIT /tmp/perf-... style if those maps don't have per entry
> > indication of text/data.
 
> Yes, but there's no way to do it currently.

Why not? Its just a matter of having a /tmp/perf-...-data file that has
the same structure as the current files for text mmaps :-)

It would then be used as the map->dso for resolving data addresses.
 
> >> only function symbols in function mappings and variables in data
> >> mapping based on the address it accesses.

> > Well, the lookup should figure out if the IP refers to TEXT or DATA and
> > use MAP__{FUNCTION, VARIABLE} accordingly when asking for symbol
> > resolution.

> Right.  But in this case we cannot determine whether a symbol in the
> /tmp/perf-... file is a function or variable.

That is why we would then need to have separate /tmp/perf-... files,
disambiguated by an extension, one for text addresses, another for data
ones.

Or change the format to match /proc/kallsyms, which probably is what we
should've done from day one...

Detecting the format change would be trivial, as we would find 3 tokens
in the new format, instead of the current 2.
 
> >> What I wasn't sure is whether JIT program also produces some dynamic data.
> >> And I think only perf mem command cares about data mappings, no?

> > Well, I think it would be great to do that kind of data resolution for
> > JITs the same way it is interesting to do for ELF ones :-)
> >
> > I need to stare harder at that patch, but with the above in mind, do we
> > really have to check if the map is MAP__FUNCTION as IIRC this patch
> > does?
> 
> Not sure.  For a JIT case, I guess the mapping is always executable and
> we don't support data mapping yet, so it seems okay for now.

Ok, agreed, applying the patch, thanks,

- Arnaldo
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