进程管理之schedule()

/*
 * schedule() is the main scheduler function.
 */
asmlinkage void __sched schedule(void)
{
	struct task_struct *prev, *next;
	unsigned long *switch_count;
	struct rq *rq;
	int cpu;

need_resched:
	preempt_disable(); //禁止抢占
	cpu = smp_processor_id();//获取当前CPU的ID
	rq = cpu_rq(cpu);//获取当前CPU上的rq（run queue）队列
	rcu_note_context_switch(cpu);//??????
	prev = rq->curr; //当前进程就是需要被切换出去的进程

	schedule_debug(prev);//time debugging checks and statistics

	if (sched_feat(HRTICK))
		hrtick_clear(rq);

	raw_spin_lock_irq(&rq->lock);

	switch_count = &prev->nivcsw;
	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) { //如果当前进程处于stopped状态，并且处于可抢占状态？？？
		if (unlikely(signal_pending_state(prev->state, prev))) {//如果没有待处理的信号？？？
			prev->state = TASK_RUNNING;                     //则将当前进程的状态设置为RUNNING
		} else {                                                //如果有待处理的信号
			deactivate_task(rq, prev, DEQUEUE_SLEEP);       //将当前进程从运行队列中移走
			prev->on_rq = 0;  //表明当前进程已经不在rq队列中了。

			/*
			 * If a worker went to sleep, notify and ask workqueue
			 * whether it wants to wake up a task to maintain
			 * concurrency.
			 */
			if (prev->flags & PF_WQ_WORKER) {
				struct task_struct *to_wakeup;

				to_wakeup = wq_worker_sleeping(prev, cpu);
				if (to_wakeup)
					try_to_wake_up_local(to_wakeup);
			}

			/*
			 * If we are going to sleep and we have plugged IO
			 * queued, make sure to submit it to avoid deadlocks.
			 */
			if (blk_needs_flush_plug(prev)) { //冲刷IO
				raw_spin_unlock(&rq->lock);
				blk_schedule_flush_plug(prev);
				raw_spin_lock(&rq->lock);
			}
		}
		switch_count = &prev->nvcsw;
	}

	pre_schedule(rq, prev); //如果非SMP或者非实时任务调度，则为空操作；否则，将当前rt任务放回，并挑选另一个优先级更高的rt任务放到相对空闲的cpu中运行。

	if (unlikely(!rq->nr_running))//当然，这个也是针对SMP而言的，因为在上一步过后，可能出现有的cpu空转的现象，所以要在smp间调度
		idle_balance(cpu, rq); 

	put_prev_task(rq, prev);//主要是更新待切换进程的运行时间等参数。
	next = pick_next_task(rq); //寻找下一个合适的进程以将其调度进来，这是核心函数，我们会在另外一篇博文中详述。
	clear_tsk_need_resched(prev);//TIF_NEED_RESCHED indicates that the process should be or would like to be replaced with another process by the scheduler.

	rq->skip_clock_update = 0; //？？？？？

	if (likely(prev != next)) { //如果下一个进程与当前进程不是是一个进程
		rq->nr_switches++; //？？？？
		rq->curr = next;//将运行队列的当前进程指向被选中的进程
		++*switch_count;

		context_switch(rq, prev, next); /* unlocks the rq */ /*切换进程上下文，无疑这是个核心函数，我们将在另外的博文中详细讲解*/
		/*
		 * The context switch have flipped(翻转) the stack from under us
		 * and restored the local variables which were saved when
		 * this task called schedule() in the past. prev == current
		 * is still correct, but it can be moved to another cpu/rq.
		 */
		cpu = smp_processor_id();
		rq = cpu_rq(cpu);
	} else
		raw_spin_unlock_irq(&rq->lock);

	post_schedule(rq); //和pre_schedule一样，只有在SMP中调度rt任务时才会被实际的执行，否则为空；同时与pre_schedule的作用类似，
                           
/*这个函数的作用是：
 * If the current CPU has more than one RT task, see if the non
 * running task can migrate over to a CPU that is running a task
 * of lesser priority.
 */
	preempt_enable_no_resched();
	if (need_resched()) //如果被切换进来的进程仍需要被调度，则返回至need_resched，重新调度。
		goto need_resched;
}