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

[Peter Zijlstra]

On Fri, Feb 20, 2026 at 12:02:22PM +0100, Peter Zijlstra wrote:
> 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.

That is, use rcu_dereference_all() and observe the warning go away.