bfq_entity_of
child_entity = bfq_entity_of(rb_first(child_active));
while ((entity = bfq_entity_of(rb_first(active))))
struct bfq_entity *bfq_entity_of(struct rb_node *node);
st->first_idle = bfq_entity_of(next);
st->last_idle = bfq_entity_of(next);