"Peter Zijlstra" <[email protected]> writes:
> On Wed, Jan 07, 2026 at 02:12:10PM +0100, Andreas Hindborg wrote:
>> "Peter Zijlstra" <[email protected]> writes:
>>
>> > On Wed, Jan 07, 2026 at 01:23:38PM +0100, Andreas Hindborg wrote:
>> >> Hi Alice,
>> >>
>> >> Alice Ryhl <[email protected]> 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/[email protected]/
>> >> >
>> >> > Signed-off-by: Alice Ryhl <[email protected]>
>> >>
>> >> 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!
>> >
>> > Looks like it used the wrong static_call_mod() version, was MODULE
>> > defined?
>>
>> CONFIG_MODULES=y, yes
>>
>> I built without the patch first, then applied the series, enabled the
>> option via menuconfig and ran the build. I thought maybe some dependency
>> check is messed up so I retried the build from a clean state. Same
>> result.
>
> No, I mean -DMODULE. Note how the quiet_cmd_bindgen target has -DMODULE,
> but the new quiet_cmd_rust_helper target does not.
Ah, that did the trick, thanks!
diff --git a/rust/Makefile b/rust/Makefile
index 5365d53b6cf96..08d3dc1038cf5 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -483,7 +483,7 @@ $(obj)/bindings/bindings_helpers_generated.rs:
$(src)/helpers/helpers.c FORCE
quiet_cmd_rust_helper = HELPER $@
cmd_rust_helper = \
- $(CC) $(filter-out $(CFLAGS_REMOVE_helpers/helpers.o), $(c_flags)) -c
-g0 $< -emit-llvm -o $@
+ $(CC) $(filter-out $(CFLAGS_REMOVE_helpers/helpers.o), $(c_flags))
-DMODULE -c -g0 $< -emit-llvm -o $@
$(obj)/helpers/helpers.bc: $(obj)/helpers/helpers.c FORCE
+$(call if_changed_dep,rust_helper)
Is -DMOUDLE always appropriate to pass to the helpers? The helpers are
also inlined into non module code.
Best regards,
Andreas Hindborg
