From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3987E381B1A; Mon, 14 Sep 2026 18:32:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789410736; cv=none; b=hCgxWav00xixAnW1OHdILFemqIltfiMMvLpIBp6UQOKdU8ZVE+/6z4u8dhngz89IKeialhPh/VXXTLsiLnTPw6D/MnmMQ303uJ7YJPkrZFv8q2ymZoh6De1smbKl9mxByyEpDRrH+UsLxEJOXoH+o0jSCu73gQabrgaUcTiY0ho= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789410736; c=relaxed/simple; bh=D2+hleGXIMDN227b5SNY36r7EzpYxCREKmva1l2nkHY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Nv8YZUORR9KkQpXQ/WxoTj3VMl+zvfVc2n39giYCHlYb0Y6+D3kxMYM2GCbMVC4KO3pp3+2BpscVskTrOBXfn6wq98hR9khr9LkrLhYntrHeByGvSPP754tz3+vq/JimqnucM5Zpg1JJtP++O3vjMIlNvziOFEn6pm35sRfBwlI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LT1cFNgk; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="LT1cFNgk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CFDD31F000FF; Mon, 14 Sep 2026 18:32:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789410734; bh=qh1ZerciXS6mhhgUbgOWtjBtb3wIfa62KF3pRdi9HRg=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=LT1cFNgkvbD3eoVT/UaBEH3+r1ZmImzoNjg1sB0fKgYfiWphX+2k3leDEJjTKPvkI UCeZKsCtpOsU1Qh6ZRgd9PGKscvkwgEZ/CXkgLZsggzcTd3LcxREE5AMbc7rSZ176a 7gNUxLvYBfZ5rTEMblheppVavtWDrYGJ9aihGEv2SwKX80uE2nVnkBoFYyryneR1eY l99LJZaV59sYUO8uU85RDxazHEX6uhwHzX4FGdH4s0ESmZYKyjLD5malFkq7HqRZYp MO93SpFQpfLWTJCfXhlr1iU+iBNmOcV7d+vp6+IkNPD3Hj/IjfJ9MaHA5ebXh1WWUJ +Cz7fAfk/ogdA== Date: Mon, 14 Sep 2026 11:32:14 -0700 From: Kees Cook To: "Lorenzo Stoakes (ARM)" Cc: Linus Torvalds , Nathan Chancellor , Nicolas Schier , Nick Desaulniers , Bill Wendling , Justin Stitt , Masahiro Yamada , Alexey Gladkov , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Arnd Bergmann , Catalin Marinas , Will Deacon , Mark Rutland , Ard Biesheuvel , Ilias Apalodimas , Josh Poimboeuf , Peter Zijlstra , Miguel Ojeda , Boqun Feng , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Daniel Almeida , Tamir Duberstein , Alexandre Courbot , Onur =?iso-8859-1?Q?=D6zkan?= , Jonathan Corbet , Randy Dunlap , "Gustavo A. R. Silva" , 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 , linux-hardening@vger.kernel.org Subject: Re: [PATCH v2 18/21] kbuild: rust: optionally parallelise rustc front end Message-ID: <202609141124.F24B6F77@keescook> References: <20260914-build-speedup-v2-0-39817ec5db23@kernel.org> <20260914-build-speedup-v2-18-39817ec5db23@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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