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

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Tue, Sep 08, 2026 at 09:20:34PM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] Unconditionally passing `-j8` or `-Zthreads=8` to `rustc` breaks single-threaded Make invocations (e.g., `make -j1` or simply `make`), forcing an 8-thread pool and creating an OOM risk.
> --
>
> commit 31e6e028120fc1bff9382e6014058f3d1bbcb274
> Author: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>
> kbuild: rust: parallelise rustc front end
>
> This commit adds parallelization flags (-j or -Zthreads) to the rustc
> invocation for Rust kernel code. It uses Kconfig checks to detect compiler
> support and applies an 8-thread default to speed up the Rust frontend build
> process.
>
> > diff --git a/Makefile b/Makefile
> > index 997fc7ebe0965..7d23d7b64be33 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))
>
> [Severity: Medium]
> Does unconditionally passing -j8 or -Zthreads=8 to rustc break
> single-threaded Make invocations (e.g. "make -j1" or just "make")?
>
> It appears this might override Make's job controls and force an 8-thread
> pool even when no jobserver is present. On resource-constrained systems
> where users intentionally use "make -j1" to limit the RAM footprint, could
> this significantly increase memory consumption per rustc invocation and
> cause an out-of-memory error during the build?

No, when a jobserver is present it doesn't override that.

When -j1 yes it will, but the memory delta is small, a few hundred megabytes
(~1.1 GiB vs. 1.4 GiB).

So it doesn't seem to be real world issue.

>
> > +endif
> > +
> >  # include additional Makefiles when needed
> >  include-y			:= scripts/Makefile.warn
>
> --
> Sashiko AI review · https://sashiko.dev/#/patchset/20260908-build-speedup-v1-0-5dc1ac01672d@kernel.org?part=20

--
Cheers, Lorenzo