perf call stacks on 32bit ARM v7

[Milian Wolff <milian.wolff@kdab.com>]

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Hello Jean, others.

Can someone please clarify the requirements for getting perf to properly 
unwind the call stack on 32bit ARM v7?

Looking at [1], it seems that I need either (a) frame pointers, or (b) DWARF 
debug information. Because (a) is often not available, and (b) is too large 
for small flash drives on embedded - how do I use perf with split debug 
information files? I.e. I have tries to record on the arm board using DWARF:

arm-v7$ perf record --call-graph dwarf ...

Then transferred the perf.data file over to my host machine. Perf archive said 
that no build-id's could be found, so I'm not using that. Instead, I try to 
ask perf to find the split debug packes using symfs:

x86-64$ perf report --symfs ... -g graph

But that does not work and I'm not seeing any backtraces. Stracing the report, 
I don't see it even trying to access files - how can I debug this and figure 
out what I'm supposed to be using?

Also according to [1], ARM .exidx unwind tables (c) are not supported by perf 
- is that still the case? If so, what is holding back support for that in 
perf, considering that libunwind supposedly supports unwinding using that 
information?

Thanks

[1]: https://archive.fosdem.org/2015/schedule/event/arm_perf/attachments/
slides/601/export/events/attachments/arm_perf/slides/601/
Fosdem_2015_perf_status_on_ARM_and_ARM64.pdf

-- 
Milian Wolff | milian.wolff@kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts

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