Re: Wrong Perf Backtraces

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And it seems that the bogus backtraces constitute only a small portion of the whole log. This seems to be good news.

On 2020-03-31 19:32, ahmadkhorrami wrote:

Hi Milian,
Thanks for the detailed answer. Well, the bug you mentioned is bad news. Because I sample using uppp. Perhaps this leads to these weird traces. Is this a purely software bug?

On 2020-03-31 19:14, Milian Wolff wrote:

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

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 :)

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