linux-kernel - Re: [PATCH v2 5/7] sched: move CPU field back into thread_info if THREAD_INFO_IN

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Date:   Thu, 30 Sep 2021 15:08:56 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org
Cc:     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>,
        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>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2 5/7] sched: move CPU field back into thread_info if
 THREAD_INFO_IN_TASK=y



Le 30/09/2021 à 14:58, Ard Biesheuvel a écrit :
> THREAD_INFO_IN_TASK moved the CPU field out of thread_info, but this
> causes some issues on architectures that define raw_smp_processor_id()
> in terms of this field, due to the fact that #include'ing linux/sched.h
> to get at struct task_struct is problematic in terms of circular
> dependencies.
> 
> Given that thread_info and task_struct are the same data structure
> anyway when THREAD_INFO_IN_TASK=y, let's move it back so that having
> access to the type definition of struct thread_info is sufficient to
> reference the CPU number of the current task.
> 
> Note that this requires THREAD_INFO_IN_TASK's definition of the
> task_thread_info() helper to be updated, as task_cpu() takes a
> pointer-to-const, whereas task_thread_info() (which is used to generate
> lvalues as well), needs a non-const pointer. So make it a macro instead.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> Acked-by: Catalin Marinas <catalin.marinas@....com>
> Acked-by: Mark Rutland <mark.rutland@....com>
> Acked-by: Michael Ellerman <mpe@...erman.id.au>
> ---
>   arch/arm64/kernel/asm-offsets.c   |  1 -
>   arch/arm64/kernel/head.S          |  2 +-
>   arch/powerpc/kernel/asm-offsets.c |  2 +-
>   arch/powerpc/kernel/smp.c         |  2 +-
>   include/linux/sched.h             | 13 +------------
>   kernel/sched/sched.h              |  4 ----
>   6 files changed, 4 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index cee9f3e9f906..0bfc048221af 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -27,7 +27,6 @@
>   int main(void)
>   {
>     DEFINE(TSK_ACTIVE_MM,		offsetof(struct task_struct, active_mm));
> -  DEFINE(TSK_CPU,		offsetof(struct task_struct, cpu));
>     BLANK();
>     DEFINE(TSK_TI_CPU,		offsetof(struct task_struct, thread_info.cpu));
>     DEFINE(TSK_TI_FLAGS,		offsetof(struct task_struct, thread_info.flags));
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 17962452e31d..6a98f1a38c29 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -412,7 +412,7 @@ SYM_FUNC_END(__create_page_tables)
>   	scs_load \tsk
>   
>   	adr_l	\tmp1, __per_cpu_offset
> -	ldr	w\tmp2, [\tsk, #TSK_CPU]
> +	ldr	w\tmp2, [\tsk, #TSK_TI_CPU]

Why do you need to change the name ?

For powerpc64, you leave TASK_CPU.

>   	ldr	\tmp1, [\tmp1, \tmp2, lsl #3]
>   	set_this_cpu_offset \tmp1
>   	.endm
> diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
> index e563d3222d69..e37e4546034e 100644
> --- a/arch/powerpc/kernel/asm-offsets.c
> +++ b/arch/powerpc/kernel/asm-offsets.c
> @@ -93,7 +93,7 @@ int main(void)
>   #endif /* CONFIG_PPC64 */
>   	OFFSET(TASK_STACK, task_struct, stack);
>   #ifdef CONFIG_SMP
> -	OFFSET(TASK_CPU, task_struct, cpu);
> +	OFFSET(TASK_CPU, task_struct, thread_info.cpu);
>   #endif
>   
>   #ifdef CONFIG_LIVEPATCH

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