在模块的编写过程中，我们经常使用定时器来等待一段时间之后再来执行某一个操作。
为方便分析，写了下列一段测试程序：#include <linux/config.h>#include <linux/kernel.h>#include <linux/module.h>#include <linux/interrupt.h>#include <linux/delay.h>#include <linux/timer.h>MODULE_LICENSE("GPL");void test_timerfuc(unsigned long x){printk("Eric xiao test ......\n");}//声明一个定个器struct timer_list test_timer = TIMER_INITIALIZER(test_timerfuc, 0, 0);int kernel_test_init(){printk("test_init\n");//修改定时器到期时间。
为3个HZ。
一个HZ产生一个时钟中断mod_timer(&test_timer,jiffies+3*HZ);//把定时器加入时钟软中断处理链表add_timer(&test_timer);}int kernel_test_exit(){printk("test_exit\n");return 0;}module_init(kernel_test_init);module_exit(kernel_test_exit);上面的例子程序比较简单，我们从这个例子开始研究linux下的定时器实现。
TIMER_INITIALIZER（）：1):TIMER_INITIALIZER（）用来声明一个定时器，它的定义如下：#define TIMER_INITIALIZER(_function, _expires, _data) { \.function = (_function), \.expires = (_expires), \.data = (_data), \.base = NULL, \.magic = TIMER_MAGIC, \.lock = SPIN_LOCK_UNLOCKED, \}Struct timer_list定义如下：struct timer_list {//用来形成链表struct list_head entry;//定始器到达时间unsigned long expires;spinlock_t lock;unsigned long magic;//定时器时间到达后，所要运行的函数void (*function)(unsigned long);//定时器函数对应的参数unsigned long data;//挂载这个定时器的tvec_t_base_s.这个结构我们等会会看到，当该次中断顺利执行后，该值也将清空为NULLstruct tvec_t_base_s *base;};从上面的过程中我们可以看到TIMER_INITIALIZER（）只是根据传入的参数初始化了struct timer_list结构.并把magic 成员初始化成TIMER_MAGIC2): mod_timer():修改定时器的到时时间int mod_timer(struct timer_list *timer, unsigned long expires){//如果该定时器没有定义fuctionBUG_ON(!timer->function);//判断timer的magic是否为TIMER_MAGIC.如果不是,则将其修正为TIMER_MAGIC check_timer(timer);//如果要调整的时间就是定时器的定时时间而且已经被激活,则直接返回if (timer->expires == expires && timer_pending(timer))return 1;//调用_mod_timer().呆会再给出分析return __mod_timer(timer, expires);}3): add_timer()用来将定时器挂载到定时软中断队列,激活该定时器static inline void add_timer(struct timer_list * timer){__mod_timer(timer, timer->expires);}可以看到mod_timer与add_timer 最后都会调用__mod_timer().为了分析这个函数,我们先来了解一下定时系统相关的数据结构.tvec_bases: per cpu变量,它的定义如下:static DEFINE_PER_CPU(tvec_base_t, tvec_bases) = { SPIN_LOCK_UNLOCKED };由此可以看到tves_bases的数型数据为teves_base_t.数据结构的定义如下:typedef struct tvec_t_base_s tvec_base_t;struct tvec_t_base_s的定义:struct tvec_t_base_s {spinlock_t lock;//上一次运行计时器的jiffies 值这个值很关键，正是这个值保证了不会遗漏定时器中断，timer中断中每次循环查找后，该值加一unsigned long timer_jiffies;struct timer_list *running_timer;//tv1 tv2 tv3 tv4 tv5是五个链表数组tvec_root_t tv1;tvec_t tv2;tvec_t tv3;tvec_t tv4;tvec_t tv5;} ____cacheline_aligned_in_smp;Tves_root_t与tvec_t的定义如下:#define TVN_BITS 6#define TVR_BITS 8#define TVN_SIZE (1 << TVN_BITS)#define TVR_SIZE (1 << TVR_BITS)#define TVN_MASK (TVN_SIZE - 1)#define TVR_MASK (TVR_SIZE - 1)typedef struct tvec_s {struct list_head vec[TVN_SIZE];} tvec_t;typedef struct tvec_root_s {struct list_head vec[TVR_SIZE];} tvec_root_t;系统规定定时器最大超时时间间隔为0xFFFFFFFF.即为一个32位数.即使在64位系统上.如果超过此值也会将其强制设这oxFFFFFFFF(这在后面的代码分析中可以看到).内核最关心的就是间隔在0~255个HZ之间的定时器.次重要的是间隔在255~1<<(8+6)之间的定时器.第三重要的是间隔在1<<(8+6) ~ 1<<(8+6+6)之间的定器.依次往下推.也就是把32位的定时间隔为份了五个部份.1个8位.4个6位.所以内核定义了五个链表数组.第一个链表数组大小为8位大小,也即上面定义的 #define TVR_SIZE (1 << TVR_BITS).其它的四个数组大小为6位大小.即上面定义的#define TVN_SIZE (1 << TVN_BITS)在加入定时器的时候,按照时间间隔把定时器加入到相应的数组即可.了解这点之后,就可以来看__mod_timer()的代码了://修改timer或者新增一个timer都会调用此接口int __mod_timer(struct timer_list *timer, unsigned long expires){tvec_base_t *old_base, *new_base;unsigned long flags;int ret = 0;//入口参数检测BUG_ON(!timer->function);check_timer(timer);spin_lock_irqsave(&timer->lock, flags);//取得当前CPU对应的tvec_basesnew_base = &__get_cpu_var(tvec_bases);repeat://该定时器所在的tvec_bases.对于新增的timer.它的base字段为NULLold_base = timer->base;/** Prevent deadlocks via ordering by old_base < new_base.*///在把timer从当前tvec_bases摘下来之前,要充分考虑好竞争的情况if (old_base && (new_base != old_base)) {//按次序获得锁if (old_base < new_base) {spin_lock(&new_base->lock);spin_lock(&old_base->lock);} else {spin_lock(&old_base->lock);spin_lock(&new_base->lock);}/** The timer base might have been cancelled while we were* trying to take the lock(s):*///如果timer->base != old_base.那就是说在Lock的时候.其它CPU更改它的值 //那就解锁.重新判断if (timer->base != old_base) {spin_unlock(&new_base->lock);spin_unlock(&old_base->lock);goto repeat;}} else {//old_base == NULl 或者是 new_base==old_base的情况//获得锁spin_lock(&new_base->lock);//同理,在Lock的时候timer会生了改变if (timer->base != old_base) {spin_unlock(&new_base->lock);goto repeat;}}/** Delete the previous timeout (if there was any), and install* the new one:*///将其从其它的tvec_bases上删除.注意运行到这里的话,说话已经被Lock了 if (old_base) {list_del(&timer->entry);ret = 1;}//修改它的定时器到达时间timer->expires = expires;//将其添加到new_base中internal_add_timer(new_base, timer);//修改base字段timer->base = new_base;//操作完了,解锁if (old_base && (new_base != old_base))spin_unlock(&old_base->lock);spin_unlock(&new_base->lock);spin_unlock_irqrestore(&timer->lock, flags);return ret;}internal_add_timer()的代码如下:static void internal_add_timer(tvec_base_t *base, struct timer_list *timer){//定时器到达的时间unsigned long expires = timer->expires;//计算时间间间隔unsigned long idx = expires - base->timer_jiffies;struct list_head *vec;//根据时间间隔,将timer放入相应数组的相应位置if (idx < TVR_SIZE) {int i = expires & TVR_MASK;vec = base->tv1.vec + i;} else if (idx < 1 << (TVR_BITS + TVN_BITS)) {int i = (expires >> TVR_BITS) & TVN_MASK;vec = base->tv2.vec + i;} else if (idx < 1 << (TVR_BITS + 2 * TVN_BITS)) {int i = (expires >> (TVR_BITS + TVN_BITS)) & TVN_MASK;vec = base->tv3.vec + i;} else if (idx < 1 << (TVR_BITS + 3 * TVN_BITS)) {int i = (expires >> (TVR_BITS + 2 * TVN_BITS)) & TVN_MASK;vec = base->tv4.vec + i;} else if ((signed long) idx < 0) {/** Can happen if you add a timer with expires == jiffies,* or you set a timer to go off in the past*///如果间隔小于0vec = base->tv1.vec + (base->timer_jiffies & TVR_MASK);} else {int i;/* If the timeout is larger than 0xffffffff on 64-bit* architectures then we use the maximum timeout:*///时间间隔超长,将其设为oxFFFFFFFFif (idx > 0xffffffffUL) {idx = 0xffffffffUL;expires = idx + base->timer_jiffies;}i = (expires >> (TVR_BITS + 3 * TVN_BITS)) & TVN_MASK;vec = base->tv5.vec + i;}/** Timers are FIFO:*///加入到链表末尾list_add_tail(&timer->entry, vec);}计算时间间隔即可知道要加入到哪一个数组.哪又怎么计算加入到该数组的那一项呢?对于间隔时间在0~255的定时器: 它的计算方式是将定时器到达时间的低八位与低八位为1的数相与而成对于第1个六位,它是先将到达时间右移8位.然后与低六位全为1的数相与而成对于第2个六位, 它是先将到达时间右移8+6位.然后与低六位全为1的数相与而成依次为下推…在后面结合超时时间到达的情况再来分析相关部份4):定时器更新每过一个HZ,就会检查当前是否有定时器的定时器时间到达.如果有,运行它所注册的函数,再将其删除.为了分析这一过程,我们先从定时器系统的初始化看起.asmlinkage void __init start_kernel(void){……init_timers();……}Init_timers()的定义如下:void __init init_timers(void){timer_cpu_notify(&timers_nb, (unsigned long)CPU_UP_PREPARE,(void *)(long)smp_processor_id());register_cpu_notifier(&timers_nb);//注册TIMER_SOFTIRQ软中断open_softirq(TIMER_SOFTIRQ, run_timer_softirq, NULL);}timer_cpu_notify()àinit_timers_cpu():代码如下:static void /* __devinit */ init_timers_cpu(int cpu){int j;tvec_base_t *base;//初始化各个数组中的链表base = &per_cpu(tvec_bases, cpu);spin_lock_init(&base->lock);for (j = 0; j < TVN_SIZE; j++) {INIT_LIST_HEAD(base->tv5.vec + j);INIT_LIST_HEAD(base->tv4.vec + j);INIT_LIST_HEAD(base->tv3.vec + j);INIT_LIST_HEAD(base->tv2.vec + j);}for (j = 0; j < TVR_SIZE; j++)INIT_LIST_HEAD(base->tv1.vec + j);//将最近到达时间设为当前jiffiesbase->timer_jiffies = jiffies;}我们在前面分析过,每当时钟当断函数到来的时候,就会打开定时器的软中断.运行其软中断函数.run_timer_softirq()代码如下:static void run_timer_softirq(struct softirq_action *h){//取得当于CPU的tvec_base_t结构tvec_base_t *base = &__get_cpu_var(tvec_bases);//如果jiffies > base->timer_jiffiesif (time_after_eq(jiffies, base->timer_jiffies))__run_timers(base);}__run_timers()代码如下:static inline void __run_timers(tvec_base_t *base){struct timer_list *timer;unsigned long flags;spin_lock_irqsave(&base->lock, flags);//因为CPU可能关闭中断,引起时钟中断信号丢失.可能jiffies要大base->timer_jiffies 好几个//HZwhile (time_after_eq(jiffies, base->timer_jiffies)) {//定义并初始化一个链表struct list_head work_list = LIST_HEAD_INIT(work_list);struct list_head *head = &work_list;int index = base->timer_jiffies & TVR_MASK;/** Cascade timers:*///当index == 0时,说明已经循环了一个周期//则将tv2填充tv1.如果tv2为空,则用tv3填充tv2.依次类推......if (!index &&(!cascade(base, &base->tv2, INDEX(0))) &&(!cascade(base, &base->tv3, INDEX(1))) &&!cascade(base, &base->tv4, INDEX(2)))cascade(base, &base->tv5, INDEX(3));//更新base->timer_jiffies++base->timer_jiffies;//将base->tv1.vec项移至work_list.并将base->tv1.vec置空list_splice_init(base->tv1.vec + index, &work_list);repeat://work_List中的定时器是已经到时的定时器if (!list_empty(head)) {void (*fn)(unsigned long);unsigned long data;//遍历链表中的每一项.运行它所对应的函数,并将定时器从链表上脱落timer = list_entry(head->next,struct timer_list,entry);fn = timer->function;data = timer->data;list_del(&timer->entry);set_running_timer(base, timer);smp_wmb();timer->base = NULL;spin_unlock_irqrestore(&base->lock, flags);fn(data);spin_lock_irq(&base->lock);goto repeat;}}set_running_timer(base, NULL);spin_unlock_irqrestore(&base->lock, flags);}如果base->timer_jiffies低八位为零.说明它向第九位有进位.所以把第九位到十五位对应的定时器搬到前八位对应的数组.如果第九位到十五位为空的话.就到它的上个六位去搬数据.上面的代码也说明.要经过1<<8个HZ才会更新全部数组中的定时器.这样做的效率是很高的. 分析下里面的两个重要的子函数:static int cascade(tvec_base_t *base, tvec_t *tv, int index){/* cascade all the timers from tv up one level */struct list_head *head, *curr;//取数组中序号对应的链表head = tv->vec + index;curr = head->next;/** We are removing _all_ timers from the list, so we don't have to* detach them individually, just clear the list afterwards.*///遍历这个链表,将定时器重新插入到base中while (curr != head) {struct timer_list *tmp;tmp = list_entry(curr, struct timer_list, entry);BUG_ON(tmp->base != base);curr = curr->next;internal_add_timer(base, tmp);}//将链表设为初始化状态INIT_LIST_HEAD(head);return index;}//将list中的数据放入head中,并将list置为空static inline void list_splice_init(struct list_head *list, struct list_head *head) {if (!list_empty(list)) {__list_splice(list, head);INIT_LIST_HEAD(list);}}//将list中的数据放入headstatic inline void __list_splice(struct list_head *list, struct list_head *head){//list的第一个元素struct list_head *first = list->next;//list的最后一个元素struct list_head *last = list->prev;//head的第一个元素struct list_head *at = head->next;将first对应的链表链接至headfirst->prev = head;head->next = first;//将head 原有的数据加入到链表末尾last->next = at;at->prev = last;}5):del_timer()删除定时器//删除一个timerint del_timer(struct timer_list *timer){unsigned long flags;tvec_base_t *base;check_timer(timer);repeat:base = timer->base;//该定时器没有被激活if (!base)return 0;//加锁spin_lock_irqsave(&base->lock, flags);//如果在加锁的过程中,有其它操作改变了timerif (base != timer->base) {spin_unlock_irqrestore(&base->lock, flags);goto repeat;}//将timer从链表中删除list_del(&timer->entry);timer->base = NULL;spin_unlock_irqrestore(&base->lock, flags);return 1;}6): del_timer_sync()有竞争情况下的定时器删除在SMP系统中,可能要删除的定时器正在某一个CPU上运行.为了防止这种在情况.在删除定时器的时候,应该优先使用del_timer_synsc().它会一直等待所有CPU上的定时器执行完成. int del_timer_sync(struct timer_list *timer){tvec_base_t *base;int i, ret = 0;check_timer(timer);del_again://删除些定时器ret += del_timer(timer);//遍历CPUfor_each_online_cpu(i) {base = &per_cpu(tvec_bases, i);//如果此CPU正在运行这个timerif (base->running_timer == timer) {//一直等待,直到这个CPU执行完while (base->running_timer == timer) {cpu_relax();preempt_check_resched();}break;}}smp_rmb();//如果这个timer又被调用.再删除if (timer_pending(timer))goto del_again;return ret;}定时器部份到这里就介绍完了.为了管理定时器.内核用了一个很巧妙的数据结构.值得好好的体会.。
