The rte_sched_queue_read_stats API has a queue_id parameter, but there is no public API to generate a queue_id from a combination of port, subport-id, pipe-id, traffic-class-id and wrr-queue-id. There is the static inline function called rte_sched_port_qindex that does what is required, perhaps it could be turned into a public API? rte_sched_queue_read_stats is the only public API to return the current length of a specific queue.