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

["Chen, Yu C" <yu.c.chen@intel.com>]

On 2/20/2026 10:02 PM, Peter Zijlstra wrote:
> 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.

OK we will remove rcu_read_lock() and use rcu_dereference_all() directly.

thanks,
Chenyu