Re: sched/fair: which tasks should nr_pref_llc_running be compared against?

[Chen Yu]

On Thu, Aug 27, 2026 at 01:57:33PM -0700, Tim Chen wrote:
> On Thu, 2026-08-27 at 21:50 +0800, Zhan Xusheng wrote:
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d78467ec6ee1..1673b17273c5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1544,7 +1544,14 @@ static void account_llc_dequeue(struct rq *rq, struct task_struct *p)
>  
>  	rq->nr_llc_running--;
>  	if (p->pref_llc_queued) {
> -		rq->nr_pref_llc_running--;
> +		/*
> +		 * If the task is being finally dequeued while still delayed,
> +		 * set_delayed() already removed it from nr_pref_llc_running;
> +		 * skip here to avoid underflow. Clearing pref_llc_queued also
> +		 * stops the subsequent clear_delayed() from re-adding it.
> +		 */
> +		if (!p->se.sched_delayed)
> +			rq->nr_pref_llc_running--;

Should we also do similar check in account_llc_enqueue()? It is possible during
load balance migration, a migrate_load allows the task to be migrated across
CPUs. And load balance leverages detach_tasks()/attach_tasks() to move tasks.
During this stage the p->se.sched_delayed remained unchanged:
In enqueue_task_fair(), only when if (se->on_rq && se->sched_delayed) is true,
p->se.sched_delayed will be cleared by requeue_delayed_entity() - during migration,
se->on_rq is false.

As a result, attach_tasks -> enqueue_hierarchy -> enqueue_entity -> account_llc_enqueue(rq, p)
incorrectly increase rq->nr_pref_llc_running - even that task is in delayed state, and
not runnable.

>  		/*
>  		 * Update the status in case
>  		 * other logic might query
> @@ -1572,6 +1579,24 @@ static void account_llc_dequeue(struct rq *rq, struct task_struct *p)
>  	}
>  }
>  
> +/*
> + * A task becoming delay-dequeued leaves the runnable set while staying
> + * queued. Keep nr_pref_llc_running in the runnable domain (like
> + * h_nr_runnable) so alb_break_llc() can compare the two directly.
> + */
>

Regarding alb_break_llc(), since we have compared nr_pref_llc_running vs 
nr_runnable, I wonder if we should also change the code?
	if (env->src_rq->nr_pref_llc_running &&
 	    env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable)
		unsigned long util = 0;
		struct task_struct *cur;
 
                if (env->src_rq->cfs.h_nr_runnable <= 1)
                        return true;

thanks,
Chenyu