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Re: How to associate Elf with Dwfl_Module returned by dwfl_report_module

On Donnerstag, 22. März 2018 10:11:29 CET Ulf Hermann wrote:
> Hi Milian,
> > I am regularly seeing broken backtraces for samples where I have
> > the gut feeling that missing reported ELFs are to blame. But we report
> > everything, except for scenarios where the mmap events seemingly overlap.
> Actually, at least for perfparser that's not quite true. When perfparser
> encounters an overlap error, it will throw out the entire set of mappings
> and restart reporting, with the addresses from the current sample (see
> PerfSymbolTable::reportElf() and PerfSymbolTable::clearCache()).

Yes, I know :) I was more talking about the libdwfl integraiton in perf there.

> If that
> still gives you overlapping ranges, it means perf has not sent all the mmap
> events and therefore we're reporting the wrong ELF for some address in your
> sample. That wrong ELF may be larger than the one we actually want and
> therefore it can overlap some other ELF an address in your sample points
> to.
> I've seen that happen. Make sure to keep your sample rate low enough to
> prevent perf from dropping anything.
> I realize we could optimize the reporting a bit, with the dwfl_report_end
> callback Mark mentioned, but if you have addresses into two overlapping
> ELFs in one sample, that's fundamentally impossible to unwind.

In the concrete data file I have at hand, no chunks got lost, so I don't think 
that mmap events could have been lost?


Milian Wolff

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