Re: [PATCH 20/23] kbuild: rust: parallelise rustc front end

From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
To: Nicolas Schier <n.schier@fritz.com>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Bill Wendling" <morbo@google.com>,
	"Justin Stitt" <justinstitt@google.com>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Alexey Gladkov" <legion@kernel.org>,
	"Thomas Gleixner" <tglx@kernel.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Paul Walmsley" <pjw@kernel.org>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Will Deacon" <will@kernel.org>,
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	linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
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	linux-efi@vger.kernel.org, rust-for-linux@vger.kernel.org,
	linux-doc@vger.kernel.org, "Jens Axboe" <axboe@kernel.dk>
Subject: Re: [PATCH 20/23] kbuild: rust: parallelise rustc front end
Date: Sun, 13 Sep 2026 22:30:41 +0100	[thread overview]
Message-ID: <aqcTSgN7Qui-NYKS@gremlin> (raw)
In-Reply-To: <20260911-impressive-heavy-bobcat-6ceeb1@l-nschier-aarch64>

On Fri, Sep 11, 2026 at 09:00:40PM +0200, Nicolas Schier wrote:
> On Tue, Sep 08, 2026 at 09:55:20PM +0100, Lorenzo Stoakes (ARM) wrote:
> > Rust crates are compiled in a serial chain on the critical path - core,
> > bindings, kernel crates and then the drivers - each a rustc invocation.
> >
> > With CONFIG_RUST set and every rust driver enabled, the entire chain takes
> > around 25 seconds on a 128-thread machine, with the associated C code
> > taking 24 seconds.
> >
> > Each time a change is made to any of the bindings, it triggers a rebuild.
> >
> > Use the -Zthreads option to have rustc perform builds in parallel. This has
> > been available since rust 1.84 (rust-lang/rust#132282), and the kernel
> > requires rust 1.85 or above.
> >
> > A future -j/--jobs option is planned for
> > rustc:(rust-lang/compiler-team#1005), so check to see if this available and
> > if so use it.
> >
> > If the user's rustc supports neither, then it falls back gracefully and
> > neither are used.
> >
> > The threads are taken from make's jobserver, so a parallel build is not
> > oversubscribed. It was found that benefits level off at 8 threads (16 was
> > found to be around the same, and 32 slower).
> [...]
>
> > diff --git a/Makefile b/Makefile
> > index 997fc7ebe096..7d23d7b64be3 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -1204,6 +1204,10 @@ KBUILD_RUSTFLAGS += --remap-path-prefix=$(srcroot)/= --remap-path-scope=macro
> >  endif
> >  endif
> >
> > +ifdef CONFIG_RUST
> > +KBUILD_RUSTFLAGS += $(if $(CONFIG_RUSTC_HAS_JOBS),-j8,$(if $(CONFIG_RUSTC_HAS_ZTHREADS),-Zthreads=8))
> > +endif
>
> Is there a reason for the 'ifdef CONFIG_RUST'?
>
> Hard-coding '8' feels a bit odd.  If some kind of KBUILD_RUST_JOBS would
> be used, it could allow users to disable or override the parallelism for
> rust.  Does it make sense to have rust parallelism configurable?

Have decided to go with a build var for v2 - KBUILD_RUST_THREADS=n - which
solves the default thread count, the instability issues and the confiigurability
all at once! :)

>
> --
> Nicolas

--
Cheers, Lorenzo