Re: [PATCH 0/4] Inline helpers into Rust without full LTO

[Andreas Hindborg <a.hindborg@kernel.org>]

Hi Alice,

Alice Ryhl <aliceryhl@google.com> writes:

> Currently the only way for Rust code to call a static inline function is
> to go through a helper in rust/helpers/. This introduces performance
> costs due to additional function calls and also clutters backtraces and
> flame graphs with helper symbols.
>
> To get rid of these helper symbols, provide functionality to inline
> helpers into Rust using llvm-link. This option complements full LTO, by
> being much cheaper and avoiding incompatibility with BTF.
>
> I ran a microbenchmark showing the benefit of this. All the benchmark
> does is call refcount_inc() in a loop. This was chosen since refcounting
> is quite hot in Binder. The results are that Rust spends 6.35 ns per
> call vs 5.73 ns per call in C. When enabling this option, the two
> languages become equally fast, and disassembly confirms the exact same
> machine code is used (in particular there is no call to
> rust_helper_refcount_inc). Benchmarking Binder also results in an
> improvement from this change.
>
> This patch is complementary to:
> https://lore.kernel.org/all/20251202-define-rust-helper-v1-0-a2e13cbc17a6@google.com/
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

I get the following modpost errors when building with this applied on top
of v6.19-rc4:

ERROR: modpost: "__SCK__WARN_trap" [drivers/gpu/drm/nova/nova.ko] undefined!
ERROR: modpost: "__SCK__WARN_trap" [drivers/gpu/nova-core/nova_core.ko] undefined!
ERROR: modpost: "__SCK__WARN_trap" [drivers/block/rnull/rnull_mod.ko] undefined!
ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_minimal.ko] undefined!
ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_misc_device.ko] undefined!
ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_print.ko] undefined!
ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_dma.ko] undefined!
ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_driver_pci.ko] undefined!
ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_driver_platform.ko] undefined!
ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_driver_faux.ko] undefined!

I also applied the series adding the inline macro [1].


Best regards,
Andreas Hindborg


[1] https://lore.kernel.org/rust-for-linux/20260105-define-rust-helper-v2-0-51da5f454a67@google.com/