Re: [PATCH 7/9] sched: define TIF_ALLOW_RESCHED

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Apr 02, 2023 at 10:22:31PM -0700, Ankur Arora wrote:
> Define TIF_ALLOW_RESCHED to allow threads to mark themselves as allowing
> rescheduling in the irqexit path with PREEMPTION_NONE/_VOLUNTARY.
> 
> This is meant to be used for long running tasks where it is
> not convenient to periodically call cond_resched().
> 
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
>  arch/x86/include/asm/thread_info.h |  2 ++
>  include/linux/sched.h              | 29 +++++++++++++++++++++++++++++
>  2 files changed, 31 insertions(+)
> 
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index f1cccba52eb9..8c18b9eaeec4 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -100,6 +100,7 @@ struct thread_info {
>  #define TIF_BLOCKSTEP		25	/* set when we want DEBUGCTLMSR_BTF */
>  #define TIF_LAZY_MMU_UPDATES	27	/* task is updating the mmu lazily */
>  #define TIF_ADDR32		29	/* 32-bit address space on 64 bits */
> +#define TIF_RESCHED_ALLOW	30	/* can reschedule if needed */
>  
>  #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
>  #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
> @@ -122,6 +123,7 @@ struct thread_info {
>  #define _TIF_BLOCKSTEP		(1 << TIF_BLOCKSTEP)
>  #define _TIF_LAZY_MMU_UPDATES	(1 << TIF_LAZY_MMU_UPDATES)
>  #define _TIF_ADDR32		(1 << TIF_ADDR32)
> +#define _TIF_RESCHED_ALLOW	(1 << TIF_RESCHED_ALLOW)
>  
>  /* flags to check in __switch_to() */
>  #define _TIF_WORK_CTXSW_BASE					\
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 63d242164b1a..1e7536e6d9ce 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2229,6 +2229,35 @@ static __always_inline bool need_resched(void)
>  	return unlikely(tif_need_resched());
>  }
>  
> +/*
> + * Define this in common code to avoid include hell.
> + */
> +static __always_inline bool resched_allowed(void)
> +{
> +#ifndef TIF_RESCHED_ALLOW
> +	return false;
> +#else
> +	return unlikely(test_tsk_thread_flag(current, TIF_RESCHED_ALLOW));
> +#endif
> +}
> +
> +static inline void allow_resched(void)
> +{
> +	/*
> +	 * allow_resched() allows preemption via the irqexit context.
> +	 * To ensure that we stick around on the current runqueue,
> +	 * disallow migration.
> +	 */
> +	migrate_disable();
> +	set_tsk_thread_flag(current, TIF_RESCHED_ALLOW);
> +}
> +
> +static inline void disallow_resched(void)
> +{
> +	clear_tsk_thread_flag(current, TIF_RESCHED_ALLOW);
> +	migrate_enable();
> +}

Why the migrate_disable(), that comment doesn't help much.