#define TASK_RUNNING        0
#define TASK_INTERRUPTIBLE  1
#define TASK_UNINTERRUPTIBLE    2
#define __TASK_STOPPED      4
#define __TASK_TRACED       8
/* in tsk->exit_state */
#define EXIT_ZOMBIE     16
#define EXIT_DEAD       32
/* in tsk->state again */
#define TASK_DEAD       64
#define TASK_WAKEKILL       128

#define PID_MAX_DEFAULT (CONFIG_BASE_SMALL ? 0x1000 : 0x8000)

struct sched_entity {
    /* 用于进行调度均衡的相关变量，主要跟红黑树有关 */
    struct load_weight        load; // 权重，跟优先级有关
    unsigned long            runnable_weight; // 在所有可运行进程中所占的权重
    struct rb_node            run_node; // 红黑树的节点
    struct list_head        group_node; // 所在进程组
    unsigned int            on_rq; // 标记是否处于红黑树运行队列中

    u64                exec_start; // 进程开始执行的时间
    u64                sum_exec_runtime; // 进程总运行时间
    u64                vruntime; // 虚拟运行时间，下面会给出详细解释
    u64                prev_sum_exec_runtime; // 进程在切换CPU时的sum_exec_runtime，简单说就是上个调度周期中运行的总时间

    u64                nr_migrations;

    struct sched_statistics        statistics;
    
    // 以下省略了一些在特定宏条件下才会启用的变量
}

/*
 * Update the current task‘s runtime statistics.
 */
static void update_curr(struct cfs_rq *cfs_rq)
{
    struct sched_entity *curr = cfs_rq->curr;
    u64 now = rq_clock_task(rq_of(cfs_rq));
    u64 delta_exec;

    if (unlikely(!curr))
        return;
    
    // 获得从最后一次修改负载后当前任务所占用的运行总时间
    delta_exec = now - curr->exec_start;
    if (unlikely((s64)delta_exec <= 0))
        return;
        
    // 更新执行开始时间
    curr->exec_start = now;

    schedstat_set(curr->statistics.exec_max,
              max(delta_exec, curr->statistics.exec_max));

    curr->sum_exec_runtime += delta_exec;
    schedstat_add(cfs_rq->exec_clock, delta_exec);

    // 计算虚拟时间，具体的转换算法写在clac_delta_fair函数中
    curr->vruntime += calc_delta_fair(delta_exec, curr);
    update_min_vruntime(cfs_rq);

    if (entity_is_task(curr)) {
        struct task_struct *curtask = task_of(curr);

        trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
        cgroup_account_cputime(curtask, delta_exec);
        account_group_exec_runtime(curtask, delta_exec);
    }

    account_cfs_rq_runtime(cfs_rq, delta_exec);
}

static struct sched_entity *
pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
{
    struct sched_entity *left = __pick_first_entity(cfs_rq);
    struct sched_entity *se;

    /*
     * If curr is set we have to see if its left of the leftmost entity
     * still in the tree, provided there was anything in the tree at all.
     */
    if (!left || (curr && entity_before(curr, left)))
        left = curr;

    se = left; /* ideally we run the leftmost entity */

    /*
     * 下面的过程主要针对一些特殊情况，我们在此不做讨论
     */
    if (cfs_rq->skip == se) {
        struct sched_entity *second;

        if (se == curr) {
            second = __pick_first_entity(cfs_rq);
        } else {
            second = __pick_next_entity(se);
            if (!second || (curr && entity_before(curr, second)))
                second = curr;
        }

        if (second && wakeup_preempt_entity(second, left) < 1)
            se = second;
    }

    if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
        se = cfs_rq->last;

    if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
        se = cfs_rq->next;

    clear_buddies(cfs_rq, se);

    return se;
}