Re: [PATCH v5 3/5] sched/fair: Skip core update if task pending

[Chen Yu]

On 2022-09-09 at 13:53:02 +0800, Abel Wu wrote:
> The function __update_idle_core() considers this cpu is idle so
> only checks its siblings to decide whether the resident core is
> idle or not and update has_idle_cores hint if necessary. But the
> problem is that this cpu might not be idle at that moment any
> more, resulting in the hint being misleading.
> 
> It's not proper to make this check everywhere in the idle path,
> but checking just before core updating can make the has_idle_core
> hint more reliable with negligible cost.
> 
> Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7abe188a1533..fad289530e07 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6294,6 +6294,9 @@ void __update_idle_core(struct rq *rq)
>  	int core = cpu_of(rq);
>  	int cpu;
>  
> +	if (rq->ttwu_pending)
> +		return;
> +
Is it to deal with the race condition? I'm thinking of the
following scenario: task p1 on rq1 is about to switch to idle.
However when p1 reaches __update_idle_core(), someone on other
CPU tries to wake up p2, and leverages rq1 to queue p2
thus set the ttwu_pending flag on rq1. It is likely that
rq1 becomes idle but soon finds that TF_NEED_RESCHED is set, thus
quits the idle loop. As a result rq will not be idle and we will
get false positive here.

thanks,
Chenyu
>  	rcu_read_lock();
>  	if (test_idle_cores(core, true))
>  		goto unlock;
> -- 
> 2.37.3
>