Re: Wrong Perf Backtraces

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On Dienstag, 31. März 2020 15:39:18 CEST ahmadkhorrami wrote:
> But the addresses do not match. Do you confirm this as a bug in
> libdwarf,...?
> So I will ignore addresses without a matching symbol. But they do not
> seem reliable!
> Could you tell me the name of the library that generates the raw
> addresses, so that I can try to debug it?

This is a platform specific question. There are multiple ways to unwind a 
stack. If you are on x86 then by default the .eh_frame section is available 
which holds the information necessary for unwinding. It doesn't depend on 
debug symbols, that's only used for symbolization and inline-frame resolution 
as Jiri indicated.

That said, in the context of perf, there are multiple scenarios that can lead 
to broken unwinding:

a) perf record --call-graph dwarf: unwinding can overflow the stack copy 
associated with every sample, so the upper end of the stack will be broken

b) perf record --call-graph $any: when you are sampling on a precise event, 
such as cycles:P which is the default afaik, then on Intel with PEBS e.g. the 
stack copy may be "wrong". See e.g. and 
the overall thread. This is not solved yet afaik and after my initial attempt 
at workarounding this issue I stopped looking into it and instead opted for 
explicitly sampling on the non-precise events when I record call graphs... You 
could try that too: do you see the issue when you run e.g.:

`perf record --call-graph dwarf -e cycles`

This should take the non-precise version for sampling but then at least the 
call stacks are correct. I.e. you trade the accuracy of the instruction 
pointer to which a sample points with reduced call stack breakage.

c) bugs :)

Milian Wolff | milian.wolff@xxxxxxxx | Senior Software Engineer
KDAB (Deutschland) GmbH, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts

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