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 X-Spam-Level: X-Spam-Status: No, score=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B7F49C433F5 for ; Tue, 14 Sep 2021 13:55:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9EC3B6113B for ; Tue, 14 Sep 2021 13:55:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233537AbhINN5B (ORCPT ); Tue, 14 Sep 2021 09:57:01 -0400 Received: from foss.arm.com ([217.140.110.172]:45208 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233631AbhINN4w (ORCPT ); Tue, 14 Sep 2021 09:56:52 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9D62A1FB; Tue, 14 Sep 2021 06:55:33 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.21.233]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8D39B3F5A1; Tue, 14 Sep 2021 06:55:29 -0700 (PDT) Date: Tue, 14 Sep 2021 14:55:27 +0100 From: Mark Rutland To: Ard Biesheuvel Cc: linux-kernel@vger.kernel.org, Keith Packard , Russell King , Catalin Marinas , Will Deacon , Michael Ellerman , Benjamin Herrenschmidt , Christophe Leroy , Paul Mackerras , Paul Walmsley , Palmer Dabbelt , Albert Ou , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Peter Zijlstra , Kees Cook , Andy Lutomirski , Linus Torvalds , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org Subject: Re: [RFC PATCH 0/8] Move task_struct::cpu back into thread_info Message-ID: <20210914135527.GC30247@C02TD0UTHF1T.local> References: <20210914121036.3975026-1-ardb@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210914121036.3975026-1-ardb@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On Tue, Sep 14, 2021 at 02:10:28PM +0200, Ard Biesheuvel wrote: > Commit c65eacbe290b ("sched/core: Allow putting thread_info into > task_struct") mentions that, along with moving thread_info into > task_struct, the cpu field is moved out of the former into the latter, > but does not explain why. >From what I recall of talking to Andy around that time, when converting arm64 over, the theory was that over time we'd move more and more out of thread_info and into task_struct or thread_struct, until task_struct supplanted thread_info entirely, and that all became generic. I think the key gain there was making things more *generic*, and there are other ways we could do that in future without moving more into task_struct (e.g. with a geenric thread_info and arch_thread_info inside that). With that in mind, and given the diffstat, I think this is worthwhile. FWIW, for the series: Acked-by: Mark Rutland Mark. > While collaborating with Keith on adding THREAD_INFO_IN_TASK support to > ARM, we noticed that keeping CPU in task_struct is problematic for > architectures that define raw_smp_processor_id() in terms of this field, > as it requires linux/sched.h to be included, which causes a lot of pain > in terms of circular dependencies (or 'header soup', as the original > commit refers to it). > > For examples of how existing architectures work around this, please > refer to patches #6 or #7. In the former case, it uses an awful > asm-offsets hack to index thread_info/current without using its type > definition. The latter approach simply keeps a copy of the task_struct > CPU field in thread_info, and keeps it in sync at context switch time. > > Patch #8 reverts this latter approach for ARM, but this code is still > under review so it does not currently apply to mainline. > > We also discussed introducing yet another Kconfig symbol to indicate > that the arch has THREAD_INFO_IN_TASK enabled but still prefers to keep > its CPU field in thread_info, but simply keeping it in thread_info in > all cases seems to be the cleanest approach here. > > Cc: Keith Packard > Cc: Russell King > Cc: Catalin Marinas > Cc: Will Deacon > Cc: Michael Ellerman > Cc: Benjamin Herrenschmidt > Cc: Christophe Leroy > Cc: Paul Mackerras > Cc: Paul Walmsley > Cc: Palmer Dabbelt > Cc: Albert Ou > Cc: Heiko Carstens > Cc: Vasily Gorbik > Cc: Christian Borntraeger > Cc: Thomas Gleixner > Cc: Ingo Molnar > Cc: Borislav Petkov > Cc: Peter Zijlstra > Cc: Kees Cook > Cc: Andy Lutomirski > Cc: Linus Torvalds > Cc: Arnd Bergmann > Cc: linux-arm-kernel@lists.infradead.org > Cc: linuxppc-dev@lists.ozlabs.org > Cc: linux-riscv@lists.infradead.org > Cc: linux-s390@vger.kernel.org > > Ard Biesheuvel (8): > arm64: add CPU field to struct thread_info > x86: add CPU field to struct thread_info > s390: add CPU field to struct thread_info > powerpc: add CPU field to struct thread_info > sched: move CPU field back into thread_info if THREAD_INFO_IN_TASK=y > powerpc: smp: remove hack to obtain offset of task_struct::cpu > riscv: rely on core code to keep thread_info::cpu updated > ARM: rely on core code to keep thread_info::cpu updated > > arch/arm/include/asm/switch_to.h | 14 -------------- > arch/arm/kernel/smp.c | 3 --- > arch/arm64/include/asm/thread_info.h | 1 + > arch/arm64/kernel/asm-offsets.c | 2 +- > arch/arm64/kernel/head.S | 2 +- > arch/powerpc/Makefile | 11 ----------- > arch/powerpc/include/asm/smp.h | 17 +---------------- > arch/powerpc/include/asm/thread_info.h | 3 +++ > arch/powerpc/kernel/asm-offsets.c | 4 +--- > arch/powerpc/kernel/smp.c | 2 +- > arch/riscv/kernel/asm-offsets.c | 1 - > arch/riscv/kernel/entry.S | 5 ----- > arch/riscv/kernel/head.S | 1 - > arch/s390/include/asm/thread_info.h | 1 + > arch/x86/include/asm/thread_info.h | 3 +++ > include/linux/sched.h | 6 +----- > kernel/sched/sched.h | 4 ---- > 17 files changed, 14 insertions(+), 66 deletions(-) > > -- > 2.30.2 >