Re: perf not showing me call graph for memcpy no matter what

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

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On Donnerstag, 16. Dezember 2021 14:37:03 CET Timothy Miller wrote:
> Hi,
> 
> I am doing some software profiling on an aarch64 system, and I’m using the
> Linux perf tool. The problem I’m running into is that “__GI___memcpy_simd”
> keeps showing up as the function with the most CPU usage.
> 
> Unfortunately, no matter what I do, this function keeps showing up as
> orphaned. That is, I cannot get a stack trace for it so I can find out who
> is calling it.
> 
> I have tried using dwarf mode, but it always gets overloaded.
> 
> I have tried using lbr mode, but I get the following error:
>    Error:
>    PMU Hardware doesn't support sampling/overflow-interrupts.
> 
> I’ve rebuilt my application and all relevant libraries with
> -no-omit-frame-pointer so that I could use the default frame pointer mode.
> Unfortunately, I still can’t get a call graph for this function.
> 
> I emailed the glibc mailing list about this, trying to find out how to work
> around this problem, perhaps adding frame pointer to the assembly
> implementation of memcpy. They suggested I try attaching a debugger, and
> I’ve found that I can get stack traces just fine. They suggest that I seem
> to be running into some kind of bug in perf.
> 
> Any help/advice would be appreciated.

Hey Timothy,

I haven't been following upstream perf development closely in the past, but a 
few years ago perf was missing support for arm <-> x86 cross machine 
unwinding. Maybe that's the issue you are running into?

Can you try the AppImage of hotspot available at [1] and see if that one works 
when you pass the right sysroot and potentially other flags, see [2] and 
`hotspot --help` for more information.

Hotspot uses a different unwinding mechanism and at least in the past used to 
be better at unwinding `perf.data` files recorded on arm on a x86 machine.

One way or another though, it would be great if you can share an MWE - i.e. 
the `perf.data` file together with the `perf archive` for a minimal example 
that just calls `memcpy` in a loop.

Cheers

[1]: https://github.com/KDAB/hotspot/releases/tag/continuous
[2]: https://github.com/KDAB/hotspot#embedded-systems

-- 
Milian Wolff | milian.wolff@kdab.com | 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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