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schedule()函数分析：
/*
* ''''schedule()'''' is the scheduler function. It''''s a very simple and nice
* scheduler: it''''s not perfect, but certainly works for most things.
*
* The goto is "interesting".
*
* NOTE!! Task 0 is the ''''idle'''' task, which gets called when no other
* tasks can run. It can not be killed, and it cannot sleep. The ''''state''''
* information in task[0] is never used.
*/
asmlinkage void schedule(void)
{
struct schedule_data * sched_data;
struct task_struct *prev, *next, *p;
struct list_head *tmp;
int this_cpu, c;
spin_lock_prefetch(&runqueue_lock);
if (!current->active_mm) BUG();
need_resched_back:
prev = current;
this_cpu = prev->processor;
if (unlikely(in_interrupt())) {
printk("Scheduling in interrupt\n");
BUG();
}
release_kernel_lock(prev, this_cpu);
/*
* ''''sched_data'''' is protected by the fact that we can run
* only one process per CPU.
*/
sched_data = & aligned_data[this_cpu].schedule_data;
spin_lock_irq(&runqueue_lock);
/* move an exhausted RR process to be last.. */
if (unlikely(prev->policy == SCHED_RR))
/*
* 如果采用轮转法调度，则重新检查counter是否为0, 若是则将其挂到运行队列的最后
*/
if (!prev->counter) {
prev->counter = NICE_TO_TICKS(prev->nice);
move_last_runqueue(prev);
}
switch (prev->state) {
case TASK_INTERRUPTIBLE:
/*
* 如果是TASK_INTERRUPTIBLE,并且能够唤醒它的信号已经来临,
* 则将状态置为TASK_RUNNING
*/
if (signal_pending(prev)) {
prev->state = TASK_RUNNING;
break;
}
default:
del_from_runqueue(prev);
case TASK_RUNNING:;
}
prev->need_resched = 0;
/*
* this is the scheduler proper:
*/
repeat_schedule:
/*
* Default process to select..
*/
next = idle_task(this_cpu);
c = -1000;
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