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

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Message-ID: <CAMj1kXF6NSy1WRrmHkg15ZD_myCa1gj7dN-CqPyz=bmRmoOdMQ@mail.gmail.com>
Date:   Thu, 30 Sep 2021 15:12:24 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
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

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.


> >       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
>
> ...