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

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Message-ID: <20260107131822.GD3707837@noisy.programming.kicks-ass.net>
Date: Wed, 7 Jan 2026 14:18:22 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Alexandre Courbot <acourbot@...dia.com>,
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Uladzislau Rezki <urezki@...il.com>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	linux-kbuild@...r.kernel.org, linux-mm@...ck.org,
	nouveau@...ts.freedesktop.org, Matthew Maurer <mmaurer@...gle.com>
Subject: Re: [PATCH 0/4] Inline helpers into Rust without full LTO

On Wed, Jan 07, 2026 at 02:12:10PM +0100, Andreas Hindborg wrote:
> "Peter Zijlstra" <peterz@...radead.org> writes:
> 
> > On Wed, Jan 07, 2026 at 01:23:38PM +0100, Andreas Hindborg wrote:
> >> Hi Alice,
> >>
> >> Alice Ryhl <aliceryhl@...gle.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@...gle.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!
> >
> > 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.