From: Kees Cook <kees@kernel.org>
To: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>,
"Nathan Chancellor" <nathan@kernel.org>,
"Nicolas Schier" <nsc@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>,
"Mark Rutland" <mark.rutland@arm.com>,
"Ard Biesheuvel" <ardb@kernel.org>,
"Ilias Apalodimas" <ilias.apalodimas@linaro.org>,
"Josh Poimboeuf" <jpoimboe@kernel.org>,
"Peter Zijlstra" <peterz@infradead.org>,
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"Boqun Feng" <boqun@kernel.org>, "Gary Guo" <gary@garyguo.net>,
"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
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"Alice Ryhl" <aliceryhl@google.com>,
"Trevor Gross" <tmgross@umich.edu>,
"Danilo Krummrich" <dakr@kernel.org>,
"Daniel Almeida" <daniel.almeida@collabora.com>,
"Tamir Duberstein" <tamird@kernel.org>,
"Alexandre Courbot" <acourbot@nvidia.com>,
"Onur Özkan" <work@onurozkan.dev>,
"Jonathan Corbet" <corbet@lwn.net>,
"Randy Dunlap" <rdunlap@infradead.org>,
"Gustavo A. R. Silva" <gustavoars@kernel.org>,
linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
llvm@lists.linux.dev, linux-riscv@lists.infradead.org,
linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
linux-efi@vger.kernel.org, rust-for-linux@vger.kernel.org,
linux-doc@vger.kernel.org, "Jens Axboe" <axboe@kernel.dk>,
linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2 18/21] kbuild: rust: optionally parallelise rustc front end
Date: Mon, 14 Sep 2026 11:32:14 -0700 [thread overview]
Message-ID: <202609141124.F24B6F77@keescook> (raw)
In-Reply-To: <20260914-build-speedup-v2-18-39817ec5db23@kernel.org>
On Mon, Sep 14, 2026 at 10:22:17AM +0100, Lorenzo Stoakes (ARM) wrote:
> 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).
I think it's worth noting that this comes from rustc itself (it has
built-in support for the make jobserver). See commit ecab4115c44c
("kbuild: mark `rustc` (and others) invocations as recursive").
What happens when there is no jobserver, e.g. "make" without -j, or
"make -j1"?
> [...]
> rustc has a parallel front end, enabled with -Zthreads, available since
> rust 1.84 [0] and therefore on every rustc the kernel supports.
>
> It is not yet stable and its output is not reproducible, so it cannot be
> [...]
> +The number of threads rustc's parallel front end may use, passed to it as
> +``-Zthreads``. Unset by default, as the parallel front end is not yet
> +stable and its output is not reproducible. The gain levels off at 8
> +threads.
Okay, unstable, not reproducible.
> Generated output was confirmed byte-for-byte identical.
But this says otherwise? I am confused. Reproducible or not?
--
Kees Cook
next prev parent reply other threads:[~2026-09-14 18:32 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-14 9:21 [PATCH v2 00/21] kbuild: significantly speed up kernel builds Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 01/21] kbuild: do not allocate .modinfo in vmlinux Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 02/21] kallsyms: index symbols by token to speed up table compression Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 03/21] kallsyms: output binary data to speed output and kallsyms assembly Lorenzo Stoakes (ARM)
2026-09-14 20:16 ` Markus Elfring
2026-09-14 21:44 ` David Laight
2026-09-14 9:22 ` [PATCH v2 04/21] kbuild: do not sort nm output where the order is irrelevant Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 05/21] kbuild: only emit vmlinux relocations when required Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 06/21] elf-parse: add section flags, symbol binding and a read-only mapping Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 07/21] kallsyms: reimplement mksysmap in C Lorenzo Stoakes (ARM)
2026-09-14 16:33 ` Markus Elfring
2026-09-14 16:54 ` Markus Elfring
2026-09-14 17:01 ` Markus Elfring
2026-09-14 9:22 ` [PATCH v2 08/21] kbuild: cache list, composite object state per object Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 09/21] kbuild: implement and use depcheck to check dependency timestamps Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 10/21] kbuild: move the toolchain checks into init/Kconfig.toolchain Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 11/21] kbuild: avoid re-running compiler and linker probes Lorenzo Stoakes (ARM)
2026-09-14 15:02 ` John Stoffel
2026-09-14 15:24 ` Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 12/21] modpost: cache section relocation mismatch state Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 13/21] modpost: emit module descriptors as assembly Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 14/21] kbuild: batch module finalisation Lorenzo Stoakes (ARM)
2026-09-14 18:00 ` Kees Cook
2026-09-14 9:22 ` [PATCH v2 15/21] objtool: cache relocations, do less work Lorenzo Stoakes (ARM)
2026-09-14 19:44 ` Josh Poimboeuf
2026-09-14 20:06 ` Linus Torvalds
2026-09-14 22:23 ` Josh Poimboeuf
2026-09-14 22:30 ` Linus Torvalds
2026-09-14 9:22 ` [PATCH v2 16/21] objtool: size the instruction hash to the text Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 17/21] objtool: decode instructions and resolve branch targets in parallel Lorenzo Stoakes (ARM)
2026-09-14 18:20 ` Kees Cook
2026-09-14 9:22 ` [PATCH v2 18/21] kbuild: rust: optionally parallelise rustc front end Lorenzo Stoakes (ARM)
2026-09-14 18:32 ` Kees Cook [this message]
2026-09-14 9:22 ` [PATCH v2 19/21] rust: make exports.o depend on the headers generated for it Lorenzo Stoakes (ARM)
2026-09-14 9:22 ` [PATCH v2 20/21] kbuild: build rust crates in parallel with the rest of the build Lorenzo Stoakes (ARM)
2026-09-14 18:37 ` Kees Cook
2026-09-14 9:22 ` [PATCH v2 21/21] kbuild: use pigz for gzip compression if available Lorenzo Stoakes (ARM)
2026-09-14 16:39 ` Kees Cook
2026-09-14 16:49 ` H. Peter Anvin
2026-09-14 17:50 ` Kees Cook
2026-09-14 15:41 ` [PATCH v2 00/21] kbuild: significantly speed up kernel builds Kees Cook
2026-09-14 15:53 ` Linus Torvalds
2026-09-14 18:25 ` Lorenzo Stoakes (ARM)
2026-09-14 18:43 ` Kees Cook
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