Re: [RFC PATCH v5 22/29] sched/rt: Add rt-cgroup migration functions

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 30, 2026 at 11:38:26PM +0200, Yuri Andriaccio wrote:

> +static int group_find_lowest_rt_rq(struct task_struct *task, struct rt_rq *task_rt_rq)
> +{
> +	struct sched_domain *sd;
> +	struct cpumask lowest_mask;
> +	struct sched_dl_entity *dl_se;
> +	struct rt_rq *rt_rq;
> +	int prio, lowest_prio;
> +	int cpu, this_cpu = smp_processor_id();
> +
> +	if (task->nr_cpus_allowed == 1)
> +		return -1; /* No other targets possible */
> +
> +	lowest_prio = task->prio - 1;
> +	cpumask_clear(&lowest_mask);
> +	for_each_cpu_and(cpu, cpu_online_mask, task->cpus_ptr) {
> +		dl_se = task_rt_rq->tg->dl_se[cpu];
> +		rt_rq = &dl_se->my_q->rt;
> +		prio = rt_rq->highest_prio.curr;
> +
> +		/*
> +		 * If we're on asym system ensure we consider the different capacities
> +		 * of the CPUs when searching for the lowest_mask.
> +		 */
> +		if (dl_se->dl_throttled || !rt_task_fits_capacity(task, cpu))
> +			continue;
> +
> +		if (prio >= lowest_prio) {
> +			if (prio > lowest_prio) {
> +				cpumask_clear(&lowest_mask);
> +				lowest_prio = prio;
> +			}
> +
> +			cpumask_set_cpu(cpu, &lowest_mask);
> +		}
> +	}
> +
> +	if (cpumask_empty(&lowest_mask))
> +		return -1;
> +
> +	/*
> +	 * At this point we have built a mask of CPUs representing the
> +	 * lowest priority tasks in the system.  Now we want to elect
> +	 * the best one based on our affinity and topology.
> +	 *
> +	 * We prioritize the last CPU that the task executed on since
> +	 * it is most likely cache-hot in that location.
> +	 */
> +	cpu = task_cpu(task);
> +	if (cpumask_test_cpu(cpu, &lowest_mask))
> +		return cpu;
> +
> +	/*
> +	 * Otherwise, we consult the sched_domains span maps to figure
> +	 * out which CPU is logically closest to our hot cache data.
> +	 */
> +	if (!cpumask_test_cpu(this_cpu, &lowest_mask))
> +		this_cpu = -1; /* Skip this_cpu opt if not among lowest */
> +
> +	scoped_guard(rcu) {
> +	for_each_domain(cpu, sd) {
> +		if (sd->flags & SD_WAKE_AFFINE) {
> +			int best_cpu;
> +
> +			/*
> +			 * "this_cpu" is cheaper to preempt than a
> +			 * remote processor.
> +			 */
> +			if (this_cpu != -1 &&
> +			    cpumask_test_cpu(this_cpu, sched_domain_span(sd)))
> +				return this_cpu;
> +
> +			best_cpu = cpumask_any_and_distribute(&lowest_mask,
> +							      sched_domain_span(sd));
> +			if (best_cpu < nr_cpu_ids)
> +				return best_cpu;
> +		}
> +	}
> +	}

I appreciate you trying to save on indent, but this does violate
coding-style, please indent as normal.

> +
> +	/*
> +	 * And finally, if there were no matches within the domains
> +	 * just give the caller *something* to work with from the compatible
> +	 * locations.
> +	 */
> +	if (this_cpu != -1)
> +		return this_cpu;
> +
> +	cpu = cpumask_any_distribute(&lowest_mask);
> +	if (cpu < nr_cpu_ids)
> +		return cpu;
> +
> +	return -1;
> +}