Re: [PATCH v2 00/17] Support dynamic opening of capstone/llvm remove BUILD_NONDISTRO

Ian Rogers
2025-01-22T16:11:50Z

On Wed, Jan 22, 2025 at 7:21 AM Andi Kleen <ak@linux.intel.com> wrote:
>
> On Tue, Jan 21, 2025 at 10:23:15PM -0800, Ian Rogers wrote:
> > Linking against libcapstone and libLLVM can be a significant increase
> > in dependencies and size of memory footprint. For something like `perf
> > record` the disassembler and addr2line functionality won't be
> > used. Support dynamically loading these libraries using dlopen and
> > then calling the appropriate functions found using dlsym.
>
> It's unclear to me what this actually fixes. If the code is not used
> it should not be faulted in and the dynamic linker is lazy too, so
> if it's not used, it won't even be linked.
>
> I don't see any numbers, but it won't surprise me if it improved
> actual run time or memory usage significantly.

In certain scenarios, like data centers, it can be useful to
statically link all your dependencies to avoid dll hell. The X86
disassembler alone in libllvm is of a size comparable to the perf tool
- I think this speaks to us doing a reasonably good job of size
optimization of the events/metrics in the perf tool. We want these
dependencies for the performance over forking objdump and addr2line,
but we don't want it baked in - unless the person doing the build
wants this and this is still the default if the libraries are detected
by Makefile.config. Using dlopen also means distributions can have a
perf tool that doesn't drag in libLLVM.so and a universe of
dependencies, but when it is installed get the performance advantages.
In data centres having fast disassembly/addr2line is less of a
priority over the binary size cost replicated over 10,000s of machines
because those machines don't tend to be running the annotate/report
commands.

Fwiw, Namhyung's uftrace is doing something similar for python:
https://github.com/namhyung/uftrace/blob/master/utils/script-python.c#L139
and I wish the perf tool were also doing this. I think it is much
nicer to have the tool fail at runtime because of a missing dependency
which you can then install should you want it, rather than doing an
equivalent within the code base with #ifdefs and needing users to
recompile. This patch series significantly reduces the #ifdefs in
places like the core disasm code.

Thanks,
Ian