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Date:   Tue, 14 Sep 2021 14:55:27 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-kernel@vger.kernel.org, Keith Packard <keithpac@amazon.com>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Paul Mackerras <paulus@samba.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        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>
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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.rutland@arm.com>

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 <keithpac@amazon.com>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Paul Walmsley <paul.walmsley@sifive.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Cc: Albert Ou <aou@eecs.berkeley.edu>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> 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
>