Question

The following function is defined in ./kernel/sched/rt.c(v3.5.4):

static void enqueue_pushable_task(struct rq *rq, struct task_struct *p)
{
        plist_del(&p->pushable_tasks, &rq->rt.pushable_tasks);
        plist_node_init(&p->pushable_tasks, p->prio);
        plist_add(&p->pushable_tasks, &rq->rt.pushable_tasks);

        /* Update the highest prio pushable task */
        if (p->prio < rq->rt.highest_prio.next)
                rq->rt.highest_prio.next = p->prio;
}

What I am unable to understand is that why does one want to add and delete the same node form the same list over and over. First a node is deleted from the list by plist_del and same node is added to the same list by plist_add function. Any particular reasons why this is done

Answer 1

When the priority of a task changes, its position in the priority list must also change.