（十二）linux内核定时器

内核定时器并不是用来简单的定时操作，而是在定时时间后，触发事件的操作，类似定时器中断，是内核用来控制在未来某个时间点（基于jiffies）调度执行某个函数的一种机制，内核中采用的定时器以jiffies为单位。单位秒=jiffies/HZ

几个重要跟时间有关的名词或变数

HZ：Linux核心每隔固定周期会发出timer interrupt (IRQ 0)，HZ是用来定义每一秒有几次timer interrupts。举例来说，HZ为1000，代表每秒有1000次timer interrupts。
Tick：Tick是HZ的倒数，意即timer interrupt每发生一次中断的时间。如HZ为250时，tick为4毫秒(millisecond)。
Jiffies：Jiffies为Linux核心变数(unsigned long)，它被用来记录系统自开机以来，已经过了多少tick。每发生一次timer interrupt，Jiffies变数会被加一。值得注意的是，Jiffies于系统开机时，并非初始化成零，而是被设为-300*HZ ，类似Linux系统中time（日历时间）

（二）内核定时器相关接口

和定时器先关的数据结构：

struct timer_list {unsigned long expires;	//未来时间点，即超时时间void (*function)(unsigned long);//超时回调函数unsigned long data;	//传递给回调函数的数据，也就是定时器数据		
};

初始化定时器相关的数据结构

1. 静态定以定时器数据结构：
#define DEFINE_TIMER(_name, _function, _expires, _data)		\struct timer_list _name =				\TIMER_INITIALIZER(_function, _expires, _data)2. 动态初始化：-----手动对变量进行初始化
#define init_timer(timer)						\do {								\static struct lock_class_key __key;			\init_timer_key((timer), #timer, &__key);		\} while (0)

可延时定时：

#define init_timer_deferrable(timer)					\do {								\static struct lock_class_key __key;			\init_timer_deferrable_key((timer), #timer, &__key);	\} while (0)

设置定时时间

#define setup_timer(timer, fn, data)					\
do {								\static struct lock_class_key __key;			\setup_timer_key((timer), #timer, &__key, (fn), (data));\
} while (0)

向内核添加定时器:

/*** add_timer - start a timer* @timer: the timer to be added** The kernel will do a ->function(->data) callback from the* timer interrupt at the ->expires point in the future. The* current time is 'jiffies'.** The timer's ->expires, ->function (and if the handler uses it, ->data)* fields must be set prior calling this function.** Timers with an ->expires field in the past will be executed in the next* timer tick.*/
void add_timer(struct timer_list *timer)
{BUG_ON(timer_pending(timer));mod_timer(timer, timer->expires);
}

从内核删除定时器：

/*** del_timer - deactive a timer.* @timer: the timer to be deactivated** del_timer() deactivates a timer - this works on both active and inactive* timers.** The function returns whether it has deactivated a pending timer or not.* (ie. del_timer() of an inactive timer returns 0, del_timer() of an* active timer returns 1.)*/
int del_timer(struct timer_list *timer)

修改定时时间：

/*** mod_timer - modify a timer's timeout* @timer: the timer to be modified* @expires: new timeout in jiffies** mod_timer() is a more efficient way to update the expire field of an* active timer (if the timer is inactive it will be activated)** mod_timer(timer, expires) is equivalent to:**     del_timer(timer); timer->expires = expires; add_timer(timer);** Note that if there are multiple unserialized concurrent users of the* same timer, then mod_timer() is the only safe way to modify the timeout,* since add_timer() cannot modify an already running timer.** The function returns whether it has modified a pending timer or not.* (ie. mod_timer() of an inactive timer returns 0, mod_timer() of an* active timer returns 1.)*/
int mod_timer(struct timer_list *timer, unsigned long expires)

jiffies和时间的转换：

extern unsigned int jiffies_to_msecs(const unsigned long j);
extern unsigned int jiffies_to_usecs(const unsigned long j);
extern unsigned long msecs_to_jiffies(const unsigned int m);
extern unsigned long usecs_to_jiffies(const unsigned int u);

（三）使用步骤

1、向内核添加定时器

   setup_timer();设置定时器add_timer();.

2、解绑定时器

   int del_timer(struct timer_list *timer)

（四）实例代码

#include <linux/timer.h>
#include <linux/kernel.h>
#include <linux/module.h>struct timer_list timer;
void function(unsigned long data)
{static int count=0;printk("this is timer test:%d\n",count++);mod_timer(&timer,jiffies+1*HZ);
}
static int __init timer_module_init(void)
{timer.expires = jiffies+5*HZ;setup_timer(&timer,function,0);add_timer(&timer);return 0;
}
static void __exit timer_module_cleanup(void)
{del_timer(&timer);
}module_init(timer_module_init);
module_exit(timer_module_cleanup);
MODULE_LICENSE("GPL");