3.7.5.4. CFS队列操作

完全公平调度器CFS中两个函数可用来增删队列的成员：enqueue_task_fair和dequeue_task_fair分别来向CFS就绪队列中添加或者删除进程

3.7.5.4.1. enqueue_task_fair入队操作

3.7.5.4.1.1. enqueue_task_fair函数

向就绪队列中放置新进程的工作由函数enqueue_task_fair函数完成，该函数定义在 kernel/sched/fair.c中

该函数将task_struct *p所指向的进程插入到rq所在的就绪队列中,除了指向所述的就绪队列rq和task_struct的指针外，该函数还有另外一个参数wakeup.这使得可以指定 入队的进程是否最近才被唤醒并转换为运行状态(此时需指定wakeup=1),还是此前就是可运行的(那么wakeup=0)

/*
 * The enqueue_task method is called before nr_running is
 * increased. Here we update the fair scheduling stats and
 * then put the task into the rbtree:
 */
static void
enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
{
    struct cfs_rq *cfs_rq;
    struct sched_entity *se = &p->se;
    int idle_h_nr_running = task_has_idle_policy(p);

    /*
     * The code below (indirectly) updates schedutil which looks at
     * the cfs_rq utilization to select a frequency.
     * Let's add the task's estimated utilization to the cfs_rq's
     * estimated utilization, before we update schedutil.
     */
    util_est_enqueue(&rq->cfs, p);

    /*
     * If in_iowait is set, the code below may not trigger any cpufreq
     * utilization updates, so do it here explicitly with the IOWAIT flag
     * passed.
     */
    if (p->in_iowait)
        cpufreq_update_util(rq, SCHED_CPUFREQ_IOWAIT);

    for_each_sched_entity(se) {
        if (se->on_rq)
            break;
        cfs_rq = cfs_rq_of(se);
        enqueue_entity(cfs_rq, se, flags);

        /*
         * end evaluation on encountering a throttled cfs_rq
         *
         * note: in the case of encountering a throttled cfs_rq we will
         * post the final h_nr_running increment below.
         */
        if (cfs_rq_throttled(cfs_rq))
            break;
        cfs_rq->h_nr_running++;
        cfs_rq->idle_h_nr_running += idle_h_nr_running;

        flags = ENQUEUE_WAKEUP;
    }

    for_each_sched_entity(se) {
        cfs_rq = cfs_rq_of(se);
        cfs_rq->h_nr_running++;
        cfs_rq->idle_h_nr_running += idle_h_nr_running;

        if (cfs_rq_throttled(cfs_rq))
            break;

        update_load_avg(cfs_rq, se, UPDATE_TG);
        update_cfs_group(se);
    }

    if (!se) {
        add_nr_running(rq, 1);
        if (flags & ENQUEUE_WAKEUP)
            update_overutilized_status(rq);

    }

    if (cfs_bandwidth_used()) {
        for_each_sched_entity(se) {
            cfs_rq = cfs_rq_of(se);

            if (list_add_leaf_cfs_rq(cfs_rq))
                break;
        }
    }

    assert_list_leaf_cfs_rq(rq);

    hrtick_update(rq);
}