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Subject: Re: [RFC PATCH 0/8] Move task_struct::cpu back into thread_info
To:     Ard Biesheuvel <ardb@kernel.org>, linux-kernel@vger.kernel.org
Cc:     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>,
        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
References: <20210914121036.3975026-1-ardb@kernel.org>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <c21d9ee9-d0a1-a572-dda0-6ae90b417e10@csgroup.eu>
Date:   Tue, 14 Sep 2021 15:49:08 +0200
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Le 14/09/2021 à 14:10, Ard Biesheuvel a écrit :
> 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.

I think it does explain why (init/Kconfig): "an arch will need to remove 
all thread_info fields except flags".

IIUC initially the intention with THREAD_INFO_IN_TASK was to remove 
everything from thread_info, but at the end it didn't happen it seems.

> 
> 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.

It was a pain when implementing that on powerpc, so I really like your 
idea, the series looks good to me.


> 
> 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.

Yes, if we can avoid yet another config, that's better. We already have 
so many configs that are supposed to be temporary and have lasted for 
years if not decades.

Christophe