（一）內核定時器介紹

內核定時器并不是用來簡單的定時操作，而是在定時時間后，觸發事件的操作，類似定時器中斷，是內核用來控制在未來某個時間點（基于jiffies）調度執行某個函數的一種機制，內核中采用的定時器以jiffies為單位。 單位秒=jiffies/HZ

幾個重要跟時間有關的名詞或變數

HZ：Linux核心每隔固定周期會發出timer interrupt (IRQ 0)，HZ是用來定義每一秒有幾次timer interrupts。舉例來說，HZ為1000，代表每秒有1000次timer interrupts。
Tick：Tick是HZ的倒數，意即timer interrupt每發生一次中斷的時間。如HZ為250時，tick為4毫秒(millisecond)。
Jiffies：Jiffies為Linux核心變數(unsigned long)，它被用來記錄系統自開機以來，已經過了多少tick。每發生一次timer interrupt，Jiffies變數會被加一。值得注意的是，Jiffies于系統開機時，并非初始化成零，而是被設為-300*HZ ，類似Linux系統中time（日歷時間）

（二）內核定時器相關接口

和定時器先關的數據結構：

struct timer_list {unsigned long expires;	//未來時間點，即超時時間void (*function)(unsigned long);//超時回調函數unsigned long data;	//傳遞給回調函數的數據，也就是定時器數據		
};

初始化定時器相關的數據結構

1. 靜態定以定時器數據結構：
#define DEFINE_TIMER(_name, _function, _expires, _data)		\struct timer_list _name =				\TIMER_INITIALIZER(_function, _expires, _data)2. 動態初始化：-----手動對變量進行初始化
#define init_timer(timer)						\do {								\static struct lock_class_key __key;			\init_timer_key((timer), #timer, &__key);		\} while (0)

可延時定時：

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

設置定時時間

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

向內核添加定時器:

/*** 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);
}

從內核刪除定時器：

/*** 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)

修改定時時間：

/*** 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和時間的轉換：

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）使用步驟

1、向內核添加定時器

   setup_timer();設置定時器add_timer();.

2、解綁定時器

   int del_timer(struct timer_list *timer)

（四）實例代碼

#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");

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