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 15:22:17 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Linux Kernel Mailing List <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>,
        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 15:12, Ard Biesheuvel a écrit :
> On Thu, 30 Sept 2021 at 15:09, Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>>
>>
>>
>> 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.
>>
> 
> Because arm64 has a clear idiom here, where TSK_TI_ is used for
> thread_info fields accessed via a task_struct pointer. Also, it only
> occurs once in the code.
> 
> Power does not seem to have this idiom, and TASK_CPU is used in many
> more places, so I don't think it makes sense to change its name.

In the old days it was called TI_CPU, was changed by commit f7354ccac844 
("powerpc/32: Remove CURRENT_THREAD_INFO and rename TI_CPU") after 
commit ed1cd6deb013 ("powerpc: Activate CONFIG_THREAD_INFO_IN_TASK")

I don't have a strong opinion about it but we have:

$ git grep thread_info arch/powerpc/kernel/asm-offsets.c
arch/powerpc/kernel/asm-offsets.c:#include <asm/thread_info.h>
arch/powerpc/kernel/asm-offsets.c:      OFFSET(TI_livepatch_sp, 
thread_info, livepatch_sp);
arch/powerpc/kernel/asm-offsets.c:      OFFSET(TI_LOCAL_FLAGS, 
thread_info, local_flags);
arch/powerpc/kernel/asm-offsets.c: 
offsetof(struct task_struct, thread_info));


> 
> 
>>>        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
>>
>> ...