Date:   Tue, 14 Sep 2021 14:55:27 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Keith Packard <keithpac@...zon.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Paul Mackerras <paulus@...ba.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org
Subject: Re: [RFC PATCH 0/8] Move task_struct::cpu back into thread_info

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@....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@...zon.com>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Paul Walmsley <paul.walmsley@...ive.com>
> Cc: Palmer Dabbelt <palmer@...belt.com>
> Cc: Albert Ou <aou@...s.berkeley.edu>
> Cc: Heiko Carstens <hca@...ux.ibm.com>
> Cc: Vasily Gorbik <gor@...ux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@...ibm.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linuxppc-dev@...ts.ozlabs.org
> Cc: linux-riscv@...ts.infradead.org
> Cc: linux-s390@...r.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
> 

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