Re: [RFC PATCH 1/2] timer: Use is_idle_task() check instead of idle_cpu() on irq exit

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, May 30, 2024 at 08:24:00AM -0400, Ze Gao a écrit :
> idle_cpu() was initially introduced in irq_enter()/exit() to check
> whether an irq interrupts an idle cpu or not since commit
> 79bf2bb335b8 ("[PATCH] tick-management: dyntick / highres functionality")
> and at that time, it's implemented via a simple check if the curr
> of task of that rq is idle or not. And then commit 6378ddb59215 ("time:
> track accurate idle time with tick_sched.idle_sleeptime") uses the same
> check to do accurate idle time accounting.
> 
> But since commit 908a3283728d ("sched: Fix idle_cpu()"), idle_cpu()
> takes scheduler stats into consideration and becomes more constrained,
> and therefore it tells more than if we have interrupted an idle
> process but also whether a cpu is going to be idle or not since it
> takes queued tasks and queued to be woken tasks into account.
> 
> However for tick user, it is too much as now we only rely on this check
> to do nohz idle time accounting in tick_nohz_start_idle() just in case
> that tick_nohz_stop_idle() is called upon irq_enter() if we actually
> rupture an idle cpu(process). The use of idle_cpu() simply complicates
> things here, and the introduction of sched_core_idle_cpu() in
> commit 548796e2e70b ("sched/core: introduce sched_core_idle_cpu()")
> proves this.
> 
> The use of is_idle_task() just like in commit 0a8a2e78b7ee ("timer: Fix
> bad idle check on irq entry") helps to save one unnecessary fix for idle
> time accounting for the newly force idle state. Note this also preps for
> the remove of sched_core_idle_cpu() in the following patch.
> 
> Signed-off-by: Ze Gao <zegao@tencent.com>
> ---
>  kernel/softirq.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 02582017759a..24c7bf3c3f6c 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -617,7 +617,7 @@ static inline void tick_irq_exit(void)
>  	int cpu = smp_processor_id();
>  
>  	/* Make sure that timer wheel updates are propagated */
> -	if ((sched_core_idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
> +	if ((is_idle_task(current) && !need_resched()) || tick_nohz_full_cpu(cpu)) {

The reason why there is a check here for idle_cpu() (or sched_core_idle_cpu())
is to avoid calling again tick_nohz_start_idle() and then again
tick_nohz_stop_idle() later from tick_nohz_idle_exit(). This can save two costly
calls to ktime_get() when a real task is waiting for the CPU. So any quick clue to
know if a task is going to be scheduled is good to get. And idle_cpu() gives
them all:

int idle_cpu(int cpu)
{
	struct rq *rq = cpu_rq(cpu);

	if (rq->curr != rq->idle)
		return 0;

// This is the necessary is_idle_task() check

	if (rq->nr_running)
		return 0;

// This tells if there is a real task pending. Ok that check
// is perhaps a bit redundant with need_resched()...

#ifdef CONFIG_SMP
	if (rq->ttwu_pending)
		return 0;
#endif

// This one tells if there is a remote wakeup pending for this CPU.
// And need_resched() doesn't tell about that yet...

	return 1;
}

So it looks to me that idle_cpu() is still a good fit at this place.
And sched_core_idle_cpu() doesn't bring more overhead since the static
key in sched_core_enabled() is rarely active (I guess...). And if it is,
then the check is even more simple.

Thanks.

>  		if (!in_hardirq())
>  			tick_nohz_irq_exit();
>  	}
> -- 
> 2.41.0
>