更新時間片 -- scheduler_tick()

時鐘中斷處理程序中會調用函數scheduler_tick():
    函數scheduler_tick()更新當前進程的time_slice;並根據time_slice的使用情況(剩餘還是耗盡)，來做進一步處理.scheduler_tick( ) Keeps the time_slice counter of current up-to-date


void scheduler_tick(void)
{
    int cpu = smp_processor_id();
    runqueue_t *rq = this_rq();
    task_t *p = current;
    unsigned long long now = sched_clock();
    update_cpu_clock(p, rq, now);

更新運行隊列的最新時鐘中斷時間戳爲當前時間
|----------------------------------|
|   rq->timestamp_last_tick = now; |
|----------------------------------|

如果當前進程爲本地CPU的swapper進程
|-------------------------------------------|
|   if (p == rq->idle) {                    |
|       if (wake_priority_sleeper(rq))      |
|           goto out;                       |
|       rebalance_tick(cpu, rq, SCHED_IDLE);|
|       return;                             |
|   }                                       |
|-------------------------------------------|

如果當前進程不是活躍(active)進程
|---------------------------------|
|   if (p->array != rq->active) { |
|       set_tsk_need_resched(p);  |
|       goto out;                 |
|   }                             |
|---------------------------------|

    spin_lock(&rq->lock);

如果當前進程是實時進程；如果調度策略爲SCHED_RR，並且其時間片已經耗盡；爲該進程重新計算時間片；清除first_time_slice域；設置task_struct.thread_info.flags域;將該任務插入其所在活躍隊列的尾部.
|----------------------------------------------------------|
|   if (rt_task(p)) {                                      |
|       if ((p->policy == SCHED_RR) && !--p->time_slice) { |
|           p->time_slice = task_timeslice(p);             |
|           p->first_time_slice = 0;                       |
|           set_tsk_need_resched(p);                       |
|           requeue_task(p, rq->active);                   |
|       }                                                  |
|       goto out_unlock;                                   |
|   }                                                      |
|----------------------------------------------------------|


當前進程爲普通進程，如果其時間片已經耗盡；將該進程從活躍隊列中刪除；設置task_struct.thread_info.flags域；重新計算動態優先級prio；重新計算時間片；清除first_time_slice域；如果過期隊列爲空，將更新 runqueue.expired_timestamp；Inserts the current process either in the active set or in the expired set
|------------------------------------------------------------|
|   if (!--p->time_slice) {                                  |
|       dequeue_task(p, rq->active);                         |
|       set_tsk_need_resched(p);                             |
|       p->prio = effective_prio(p);                         |
|       p->time_slice = task_timeslice(p);                   |
|       p->first_time_slice = 0;                             |
|                                                            |
|       if (!rq->expired_timestamp)                          |
|           rq->expired_timestamp = jiffies;                 |
|       if (!TASK_INTERACTIVE(p) || EXPIRED_STARVING(rq)) {  |
|           enqueue_task(p, rq->expired);                    |
|           if (p->static_prio < rq->best_expired_prio)      |
|               rq->best_expired_prio = p->static_prio;      |
|       } else                                               |
|           enqueue_task(p, rq->active);                     |
|------------------------------------------------------------|

如果時間片沒有耗盡，並且剩餘時間片過長;則將該進程插入其所在隊列的尾部；並設置task_struct.thread_info.flags域
if the time quantum is not exhausted (current->time_slice is not zero), checks whether the remaining time slice of the current process is too long.
|-----------------------------------------------------------------------------|
|   } else {                                                                  |
|       if ( TASK_INTERACTIVE(p) &&                                           |
|         !((task_timeslice(p) - p->time_slice) % TIMESLICE_GRANULARITY(p)) &&|
|         (p->time_slice >= TIMESLICE_GRANULARITY(p)) &&                      |
|         (p->array == rq->active)) {                                         |
|           requeue_task(p, rq->active);                                      |
|                                                                             |
|           set_tsk_need_resched(p);                                          |
|       }                                                                     |
|   }                                                                         |
|-----------------------------------------------------------------------------|
out_unlock:
    spin_unlock(&rq->lock);
out:
    rebalance_tick(cpu, rq, NOT_IDLE);
}