Re: [PATCH v3 08/21] sched/cache: Calculate the percpu sd task LLC preference

[Peter Zijlstra]

On Tue, Feb 10, 2026 at 02:18:48PM -0800, Tim Chen wrote:
>  static void account_llc_enqueue(struct rq *rq, struct task_struct *p)
>  {
> +	struct sched_domain *sd;
>  	int pref_llc;
>  
>  	pref_llc = p->preferred_llc;
> -	if (pref_llc < 0)
> +	if (!valid_llc_id(pref_llc))
>  		return;
>  
>  	rq->nr_llc_running++;
>  	rq->nr_pref_llc_running += (pref_llc == task_llc(p));
> +
> +	scoped_guard (rcu) {
> +		sd = rcu_dereference(rq->sd);
> +		if (valid_llc_buf(sd, pref_llc))
> +			sd->pf[pref_llc]++;
> +	}
>  }
>  
>  static void account_llc_dequeue(struct rq *rq, struct task_struct *p)
>  {
> +	struct sched_domain *sd;
>  	int pref_llc;
>  
>  	pref_llc = p->preferred_llc;
> -	if (pref_llc < 0)
> +	if (!valid_llc_id(pref_llc))
>  		return;
>  
>  	rq->nr_llc_running--;
>  	rq->nr_pref_llc_running -= (pref_llc == task_llc(p));
> +
> +	scoped_guard (rcu) {
> +		sd = rcu_dereference(rq->sd);
> +		if (valid_llc_buf(sd, pref_llc)) {
> +			/*
> +			 * There is a race condition between dequeue
> +			 * and CPU hotplug. After a task has been enqueued
> +			 * on CPUx, a CPU hotplug event occurs, and all online
> +			 * CPUs (including CPUx) rebuild their sched_domains
> +			 * and reset statistics to zero (including sd->pf).
> +			 * This can cause temporary undercount and we have to
> +			 * check for such underflow in sd->pf.
> +			 *
> +			 * This undercount is temporary and accurate accounting
> +			 * will resume once the rq has a chance to be idle.
> +			 */
> +			if (sd->pf[pref_llc])
> +				sd->pf[pref_llc]--;
> +		}
> +	}
>  }

FWIW, enqueue/dequeue must be with rq->lock held, and thus preemption
disabled and IRQs off. That RCU section is completely pointless.