Re: [PATCHv5] sched/deadline: Walk up cpuset hierarchy to decide root domain when hot-unplug

[Juri Lelli <juri.lelli@redhat.com>]

Hi,

Looks like this has two issues.

On 10/11/25 09:47, Pingfan Liu wrote:

...

> +/*
> + * This function always returns a non-empty bitmap in @cpus. This is because
> + * if a root domain has reserved bandwidth for DL tasks, the DL bandwidth
> + * check will prevent CPU hotplug from deactivating all CPUs in that domain.
> + */
> +static void dl_get_task_effective_cpus(struct task_struct *p, struct cpumask *cpus)
> +{
> +	const struct cpumask *hk_msk;
> +
> +	hk_msk = housekeeping_cpumask(HK_TYPE_DOMAIN);
> +	if (housekeeping_enabled(HK_TYPE_DOMAIN)) {
> +		if (!cpumask_intersects(p->cpus_ptr, hk_msk)) {
> +			/*
> +			 * CPUs isolated by isolcpu="domain" always belong to
> +			 * def_root_domain.
> +			 */
> +			cpumask_andnot(cpus, cpu_active_mask, hk_msk);
> +			return;
> +		}
> +	}
> +
> +	/*
> +	 * If a root domain holds a DL task, it must have active CPUs. So
> +	 * active CPUs can always be found by walking up the task's cpuset
> +	 * hierarchy up to the partition root.
> +	 */
> +	cpuset_cpus_allowed(p, cpus);

Grabs callbak_lock spin_lock (sleepable on RT) under pi_lock
raw_spin_lock.

> +}
> +
> +/* The caller should hold cpuset_mutex */
>  void dl_add_task_root_domain(struct task_struct *p)
>  {
>  	struct rq_flags rf;
>  	struct rq *rq;
>  	struct dl_bw *dl_b;
> +	unsigned int cpu;
> +	struct cpumask msk;

Potentially huge mask allocated on the stack.

>  	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
>  	if (!dl_task(p) || dl_entity_is_special(&p->dl)) {
> @@ -2891,16 +2923,22 @@ void dl_add_task_root_domain(struct task_struct *p)
>  		return;
>  	}
>  
> -	rq = __task_rq_lock(p, &rf);
> -
> +	/*
> +	 * Get an active rq, whose rq->rd traces the correct root
> +	 * domain.
> +	 * And the caller should hold cpuset_mutex, which gurantees
> +	 * the cpu remaining in the cpuset until rq->rd is fetched.
> +	 */
> +	dl_get_task_effective_cpus(p, &msk);
> +	cpu = cpumask_first_and(cpu_active_mask, &msk);
> +	BUG_ON(cpu >= nr_cpu_ids);
> +	rq = cpu_rq(cpu);
>  	dl_b = &rq->rd->dl_bw;
> -	raw_spin_lock(&dl_b->lock);
>  
> +	raw_spin_lock(&dl_b->lock);
>  	__dl_add(dl_b, p->dl.dl_bw, cpumask_weight(rq->rd->span));
> -
>  	raw_spin_unlock(&dl_b->lock);
> -
> -	task_rq_unlock(rq, p, &rf);
> +	raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);

Thanks,
Juri