Re: [PATCH 1/3] sched/core,x86: make struct thread_info arch specific again

[Mark Rutland <mark.rutland@arm.com>]

Hi,

On Thu, Oct 13, 2016 at 01:57:10PM +0200, Heiko Carstens wrote:
> commit c65eacbe290b ("sched/core: Allow putting thread_info into
> task_struct") made struct thread_info a generic struct with only a
> single flags member if THREAD_INFO_IN_TASK_STRUCT is selected.
> 
> This change however seems to be quite x86 centric, since at least the
> generic preemption code (asm-generic/preempt.h) assumes that struct
> thread_info also has a preempt_count member, which apparently was not
> true for x86.
> 
> We could add a bit more ifdefs to solve this problem too, but it seems
> to be much simpler to make struct thread_info arch specific
> again. This also makes the conversion to THREAD_INFO_IN_TASK_STRUCT a
> bit easier for architectures that have a couple of arch specific stuff
> in their thread_info definition.
> 
> The arch specific stuff _could_ be moved to thread_struct. However
> keeping them in thread_info makes it easier: accessing thread_info
> members is simple, since it is at the beginning of the task_struct,
> while the thread_struct is at the end. At least on s390 the offsets
> needed to access members of the thread_struct (with task_struct as
> base) are too large for various asm instructions.  This is not a
> problem when keeping these members within thread_info.

The exact same applies for arm64 on all counts. This is also simpler than both
attempts I had at this, so FWIW:

Acked-by: Mark Rutland <mark.rutland@arm.com>

To make merging less painful, I guess we'll need a stable branch with (just)
this and whatever patch we end up with for fixing current_thread_info(), so we
can independently merge the arch-specific parts.

I guess it'd make sense for the tip tree to host that?

Thanks,
Mark.

> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> ---
>  arch/x86/include/asm/thread_info.h |  9 +++++++++
>  include/linux/thread_info.h        | 11 -----------
>  2 files changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index 2aaca53c0974..ad6f5eb07a95 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -52,6 +52,15 @@ struct task_struct;
>  #include <asm/cpufeature.h>
>  #include <linux/atomic.h>
>  
> +struct thread_info {
> +	unsigned long		flags;		/* low level flags */
> +};
> +
> +#define INIT_THREAD_INFO(tsk)			\
> +{						\
> +	.flags		= 0,			\
> +}
> +
>  #define init_stack		(init_thread_union.stack)
>  
>  #else /* !__ASSEMBLY__ */
> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
> index 45f004e9cc59..2873baf5372a 100644
> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -14,17 +14,6 @@ struct timespec;
>  struct compat_timespec;
>  
>  #ifdef CONFIG_THREAD_INFO_IN_TASK
> -struct thread_info {
> -	unsigned long		flags;		/* low level flags */
> -};
> -
> -#define INIT_THREAD_INFO(tsk)			\
> -{						\
> -	.flags		= 0,			\
> -}
> -#endif
> -
> -#ifdef CONFIG_THREAD_INFO_IN_TASK
>  #define current_thread_info() ((struct thread_info *)current)
>  #endif
>  
> -- 
> 2.8.4
>