Re: [PATCH V7 02/11] sched: Indicate if thread got rescheduled

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, Jul 24 2025 at 16:16, Prakash Sangappa wrote:

Indicate this to whom? Can you please write descriptive subject lines
which summarize the change in a way that is comprehensible?

> +void rseq_delay_resched_clear(struct task_struct *tsk)
> +{
> +	u32 flags;
> +
> +	if (tsk->rseq_delay_resched == RSEQ_RESCHED_DELAY_REQUESTED) {
> +		tsk->rseq_delay_resched = RSEQ_RESCHED_DELAY_PROBE;
> +		if (copy_from_user_nofault(&flags, &tsk->rseq->flags, sizeof(flags)))
> +                        return;
> +                flags |= RSEQ_CS_FLAG_RESCHEDULED;
> +                copy_to_user_nofault(&tsk->rseq->flags, &flags, sizeof(flags));
> +	}
> +}
>  #endif /* CONFIG_RSEQ_RESCHED_DELAY */
>  
>  #ifdef CONFIG_DEBUG_RSEQ
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e75ecbb2c1f7..ba1e4f6981cd 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6752,9 +6752,8 @@ static void __sched notrace __schedule(int sched_mode)
>  picked:
>  	clear_tsk_need_resched(prev);
>  	clear_preempt_need_resched();
> -	if (IS_ENABLED(CONFIG_RSEQ_RESCHED_DELAY) &&
> -	    prev->rseq_delay_resched == RSEQ_RESCHED_DELAY_REQUESTED)
> -		prev->rseq_delay_resched = RSEQ_RESCHED_DELAY_PROBE;
> +	if(IS_ENABLED(CONFIG_RSEQ_RESCHED_DELAY))
> +		rseq_delay_resched_clear(prev);

Yet another unconditional function call for the sake of something which
is only used by special applications. This is the scheduler hotpath and
not a dump ground for random functionality, which is even completely
redundant. Why redundant?

The kernel already handles in rseq, that a task was scheduled out:

    schedule()
       prepare_task_switch()
         rseq_preempt()

rseq_preempt() sets RSEQ_EVENT_PREEMPT_BIT and TIF_NOTIFY_RESUME, which
causes exit to userspace to invoke __rseq_handle_notify_resume(). That's
the obvious place to handle this instead of inflicting it into the
scheduler hotpath.

No?

Thanks,

        tglx