Re: [RFC][PATCH 1/2] perf: Add 'perf kmem' tool

[Arnaldo Carvalho de Melo <acme@infradead.org>]

Em Mon, Nov 23, 2009 at 07:51:10AM +0100, Ingo Molnar escreveu:
> 
> * Arnaldo Carvalho de Melo <acme@infradead.org> wrote:
> 
> > Em Fri, Nov 20, 2009 at 05:41:10PM +0100, Ingo Molnar escreveu:
> > > > So we have a mechanism that is already present in several distros
> > > > (build-id), that is in the kernel build process since ~2.6.23, and that
> > > > avoids using mismatching DSOs when resolving symbols.
> > > 
> > > But what do we do if we have another box that runs say on a MIPS CPU, 
> > > uses some minimal distro - and copy that perf.data over to an x86 box.
> > 
> > There would be no problem, it would be just a matter of installing the
> > right -debuginfo packages, for MIPS.
> 
> I havent tried this - is this really possible to do on an x86 box, with 
> a typical distro? Can i install say Fedora PowerPC debuginfo packages on 
> an x86 box, while also having the x86 debuginfo packages there?

I should have added "in theory", as I haven't tested this as well using
the current tools, but it should :)
 
> > Or the original, unstripped FS image sent to the machine with the MIPS 
> > cpu, if there aren't -debuginfo packages.
> > 
> > Either one, the right DSOs would be found by the buildids.
> > 
> > There are other scenarios, like a binary that gets updated while a long
> > running perf record session runs, the way to differentiate between the
> > two DSOs wouldn't be the name, but the buildid.
> > 
> > > The idea is there to be some new mode of perf.data where all the 
> > > relevant DSO contents (symtabs but also sections with instructions for 
> > > perf annotate to work) are copied into perf.data, during or after data 
> > > capture - on the box that does the recording.
> > > 
> > > Once we have everything embedded in the perf.data, analysis passes only 
> > > have to work based on that particular perf.data - no external data.
> > 
> > Well, we can that, additionally, but think about stripped binaries, we 
> > would lose potentially a lot because the symtabs on that small machine 
> > would have poorer symtabs than the ones in an unstriped binary (or in 
> > a -debuginfo package).
> 
> We should definitely use the widest and best quality information we can 
> - if it's available.
> 
> So even if we 'inline' any information from the box, if there's better 
> info available at the time of analysis, we should use that too.
> 
> Basically what matters is the principle of 'what is possible'.
> 
> If a user records on a box and analyses on a different box, and we end 
> up not doing something (and printing an error or displaying an empty 
> profile) that could reasonably have been done, then the user will be 
> unhappy and we might lose that user.
> 
> The user wont be unhappy about us using a big set of data sources that 
> we can recover information from transparently. The user will be unhappy 
> if we insist on (and force) a certain form of information source - such 
> as debuginfo.

Sure thing, I'm thinking about how to encode the perf.data file inside
an ELF section while merging all symtabs to reduce size by sharing the
strings table, etc.

The dso__load routine already does that fallback from what is best
(debuginfo packages) to what is available (the symtab, dynsym tables in
the DSO itself), its just a matter of efficiently encoding the symtabs
into the perf.data file and that will be another source of symbols if
the preferred one (debuginfo) is not available.

- Arnaldo