Re: [PATCH v2 5/7] sched: move CPU field back into thread_info if THREAD_INFO_IN_TASK=y

[Christophe Leroy]

Le 30/09/2021 à 15:22, Christophe Leroy a écrit :
> Le 30/09/2021 à 15:12, Ard Biesheuvel a écrit :
> > On Thu, 30 Sept 2021 at 15:09, Christophe Leroy
> > <christophe.leroy@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> > > > Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> > > > Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > > > ---
> > > >    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));

Forget that. I didn't realise that your change keeps it based on 
'task_struct' so it makes sense to keep it named 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
> > >
> > > ...