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

Peter Zijlstra
2023-04-05T20:09:41Z

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.