From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id C2715C88E75 for ; Tue, 15 Sep 2026 11:17:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=1D/jPavzhyAyEznqwRA8qV5XXPF70dg+DgLFTvtgeuE=; b=JpnxTBtoSBP7N/ fzpUEI7uqmQHpokTt4tUtEWe0jWgLKdueYARkL3IgrdiF+V5khiL7tzf11c1DwSN2ZfKCq4WXrgbn iE8ZNjv7VvR3nNk1K8KQxJwqXvRGtim4YVQz+xRS81zQ3gKV699bpk8Kk8cfwDqrl52VkhMr0ACP8 t2ien2Y9eUwM1pOsiZRQsMu0YjeP4XbOMM+VEFI0YJes1x6j3tqyTztKrgbgZs03PLG6NdBBWEcVV C4lvWfRJV5bCBBW6MEUaLcwgjRkSj+W9LmZoqrgflpiuhGq6esum708Fp/HYVjKJGTbyoEOlv2RIR gFgMeWsghRm/hrJ8oBdw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x6R9o-000000065A7-30w1; Tue, 15 Sep 2026 11:17:00 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1x6R9U-0000000651E-15Fd; Tue, 15 Sep 2026 11:16:40 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 04F7440D88; Tue, 15 Sep 2026 11:16:40 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5C04A1F000FF; Tue, 15 Sep 2026 11:16:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789470999; bh=8jGKX9lxad0nC5LwC+m3/xZlEuIw43z0LUda/hlK958=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=Q++L+p3WYA6AP7ADzOymmxif7GVxlUsd1TMY/VWqmWYAk0XvwziMicEQbz+6DNao7 XcPR++La0+G90EC9iwDjmpnkVeOifjMFofsHBu77Xi5mnhjyguFYPGvKBIUrquwQCI iiCNgS0iUL+FOLtDfRnYLFB+s6SvqNFqz3K69R94xTah6cUkSNdn52CWUZue0M+5dj +CrqVU1SNkWozON17mn8scd2A3/hV0UD6WFULz9ATw+O6zePrJc7pOK13WWXtgy/yq DD0zN4DWDoecYccw8W4FesguEdT1rAdhHxZy1ZHBQenSINumCyPKtJXWVqnfdmsQoZ Uqd2TxsmCMfLg== Date: Tue, 15 Sep 2026 12:16:25 +0100 From: "Lorenzo Stoakes (ARM)" To: Kees Cook 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 , =?utf-8?B?QmrDtnJu?= Roy Baron , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Daniel Almeida , Tamir Duberstein , Alexandre Courbot , Onur =?utf-8?B?w5Z6a2Fu?= , 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: References: <20260914-build-speedup-v2-0-39817ec5db23@kernel.org> <20260914-build-speedup-v2-18-39817ec5db23@kernel.org> <202609141124.F24B6F77@keescook> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On Tue, Sep 15, 2026 at 12:09:04PM +0100, Lorenzo Stoakes (ARM) wrote: > On Mon, Sep 14, 2026 at 11:32:14AM -0700, Kees Cook wrote: > > 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? On this though - the lack of reproducibility was the reports in the rust world of the -Zthreads flag sometimes resulting in that. I never observed that locally at all, and so locally it was all byte-for-byte identical. It could have been worded better :) > > Well, we can set all of this aside as, based on Miguel's feedback, I think > dropping this commit is best :) > > > > > -- > > Kees Cook > > -- > Cheers, Lorenzo -- Cheers, Lorenzo _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv