全局变量表示了内核上一次执行函数的时间，期间一共发生了多少次时钟中断，显然函数必须为期间所发生的每一次时钟中断补上定时器服务。-The Global variable denotes the time when the CPU running the function the latest time, the number of time interrupt between the two running. It s obvious that add timer at every time interrupt.