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

[Lorenzo Stoakes (ARM)]

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