void update_process_times(int user_tick)
{
struct task_struct *p = current;
int cpu = smp_processor_id();
/* Note: this timer irq context must be accounted for as well. */
account_process_tick(p, user_tick);
run_local_timers();
rcu_check_callbacks(cpu, user_tick);
printk_tick();
scheduler_tick();
run_posix_cpu_timers(p);
}
void run_local_timers(void)
{
hrtimer_run_queues();
raise_softirq(TIMER_SOFTIRQ);
softlockup_tick();
} 在分析定时器的实现之前我们先来看一看使用内核定时器的一个实例，具体使用可查看这篇文章：http://blog.csdn.net/shallnet/article/details/17734571，示例如下:#include <linux/module.h>
#include <linux/init.h>
#include <linux/version.h>
#include <linux/timer.h>
#include <linux/delay.h>
struct timer_list sln_timer;
void sln_timer_do(unsigned long l)
{
mod_timer(&sln_timer, jiffies + HZ);
printk(KERN_ALERT"param: %ld, jiffies: %ld\n", l, jiffies);
}
void sln_timer_set(void)
{
init_timer(&sln_timer);
sln_timer.expires = jiffies + HZ; //1s
sln_timer.function = sln_timer_do;
sln_timer.data = 9527;
add_timer(&sln_timer);
}
static int __init sln_init(void)
{
printk(KERN_ALERT"===%s===\n", __func__);
sln_timer_set();
return 0;
}
static void __exit sln_exit(void)
{
printk(KERN_ALERT"===%s===\n", __func__);
del_timer(&sln_timer);
}
module_init(sln_init);
module_exit(sln_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("allen"); 该示例作用是每秒钟打印出当前系统jiffies的值。struct timer_list {
struct list_head entry;
unsigned long expires;
void (*function)(unsigned long);
unsigned long data;
struct tvec_base *base;
#ifdef CONFIG_TIMER_STATS
void *start_site;
char start_comm[16];
int start_pid;
#endif
#ifdef CONFIG_LOCKDEP
struct lockdep_map lockdep_map;
#endif
}; 如示例，内核提供部分操作接口来简化管理定时器，struct timer_list sln_timer;第二步、初始化定时器数据结构的内部值。
init_timer(&sln_timer);//初始化定时器
#define init_timer(timer) init_timer_key((timer), NULL, NULL)
void init_timer_key(struct timer_list *timer,
const char *name,
struct lock_class_key *key)
{
debug_init(timer);
__init_timer(timer, name, key);
}
static void __init_timer(struct timer_list *timer,
const char *name,
struct lock_class_key *key)
{
timer->entry.next = NULL;
timer->base = __raw_get_cpu_var(tvec_bases);
#ifdef CONFIG_TIMER_STATS
timer->start_site = NULL;
timer->start_pid = -1;
memset(timer->start_comm, 0, TASK_COMM_LEN);
#endif
lockdep_init_map(&timer->lockdep_map, name, key, 0);
}sln_timer.expires = jiffies + HZ; //1s后执行 sln_timer.function = sln_timer_do; //执行函数 sln_timer.data = 9527;
add_timer(&sln_timer); //向内核注册定时器这样定时器就可以运行了。
void add_timer(struct timer_list *timer)
{
BUG_ON(timer_pending(timer));
mod_timer(timer, timer->expires);
} add_timer()调用了mod_timer()。mod_timer()用于修改定时器超时时间。del_timer(&sln_timer);该函数实现如下：
int del_timer(struct timer_list *timer)
{
struct tvec_base *base;
unsigned long flags;
int ret = 0;
timer_stats_timer_clear_start_info(timer);
if (timer_pending(timer)) {
base = lock_timer_base(timer, &flags);
if (timer_pending(timer)) {
detach_timer(timer, 1);
if (timer->expires == base->next_timer &&
!tbase_get_deferrable(timer->base))
base->next_timer = base->timer_jiffies;
ret = 1;
}
spin_unlock_irqrestore(&base->lock, flags);
}
return ret;
}
static inline void detach_timer(struct timer_list *timer,
int clear_pending)
{
struct list_head *entry = &timer->entry;
debug_deactivate(timer);
__list_del(entry->prev, entry->next);
if (clear_pending)
entry->next = NULL;
entry->prev = LIST_POISON2;
} 当使用del_timer()返回后，定时器就不会再被激活，但在多处理器机器上定时器上定时器中断可能已经在其他处理器上运行了，所以删除定时器时需要等待可能在其他处理器上运行的定时器处理I程序都退出，这时就要使用del_timer_sync()函数执行删除工作：int del_timer_sync(struct timer_list *timer)
{
#ifdef CONFIG_LOCKDEP
unsigned long flags;
local_irq_save(flags);
lock_map_acquire(&timer->lockdep_map);
lock_map_release(&timer->lockdep_map);
local_irq_restore(flags);
#endif
for (;;) { //一直循环，直到删除timer成功再退出
int ret = try_to_del_timer_sync(timer);
if (ret >= 0)
return ret;
cpu_relax();
}
}
int try_to_del_timer_sync(struct timer_list *timer)
{
struct tvec_base *base;
unsigned long flags;
int ret = -1;
base = lock_timer_base(timer, &flags);
if (base->running_timer == timer)
goto out;
ret = 0;
if (timer_pending(timer)) {
detach_timer(timer, 1);
if (timer->expires == base->next_timer &&
!tbase_get_deferrable(timer->base))
base->next_timer = base->timer_jiffies;
ret = 1;
}
out:
spin_unlock_irqrestore(&base->lock, flags);
return ret;
} 一般情况下应该使用del_timer_sync()函数代替del_timer()函数，因为无法确定在删除定时器时，他是否在其他处理器上运行。为了防止这种情况的发生，应该调用del_timer_sync()函数而不是del_timer()函数。否则，对定时器执行删除操作后，代码会继续执行，但它有可能会去操作在其它处理器上运行的定时器正在使用的资源，因而造成并发访问，所有优先使用删除定时器的同步方法。unsigned long delay = jiffies+10; //延迟10个节拍
while(time_before(jiffies,delay))
； 这种方法当代码等待时，处理器只能在原地旋转等待，它不会去处理其他任何任务。最好在任务等待时，允许内核重新调度其它任务执行。将上面代码修改如下：unsigned long delay = jiffies+10; //10个节拍
while(time_before(jiffies,delay))
cond_resched(); 看一下cond_resched()函数具体实现代码：#define cond_resched() ({ __might_sleep(__FILE__, __LINE__, 0); _cond_resched(); })
int __sched _cond_resched(void)
{
if (should_resched()) {
__cond_resched();
return 1;
}
return 0;
}
static void __cond_resched(void)
{
add_preempt_count(PREEMPT_ACTIVE);
schedule(); //最终还是调用schedule()函数来重新调度其它程序运行
sub_preempt_count(PREEMPT_ACTIVE);
} 函数cond_resched()将重新调度一个新程序投入运行，但它只有在设置完need_resched标志后才能生效。换句话说，就是系统中存在更重要的任务需要运行。再由于该方法需要调用调度程序，所以它不能在中断上下文中使用----只能在进程上下文中使用。事实上，所有延迟方法在进程上下文中使用，因为中断处理程序都应该尽可能快的执行。另外，延迟执行不管在哪种情况下都不应该在持有锁时或者禁止中断时发生。void udelay(unsigned long usecs); //arch/x86/include/asm/delay.h void ndelay(unsigned long nsecs); //arch/x86/include/asm/delay.h void mdelay(unsigned long msecs);udelay()使用忙循环将任务延迟指定的ms后执行,其依靠执行数次循环达到延迟效果，mdelay()函数是通过udelay()函数实现，如下：
#define mdelay(n) (\
(__builtin_constant_p(n) && (n)<=MAX_UDELAY_MS) ? udelay((n)*1000) : ({unsigned long __ms=(n); while (__ms--) udelay(1000);}))
#endif udelay()函数仅能在要求的延迟时间很短的情况下执行，而在高速机器中时间很长的延迟会造成溢出。对于较长的延迟，mdelay()工作良好。 set_current_state(TASK_INTERRUPTIBLE); //将任务设置为可中断睡眠状态
schedule_timeout(s*HZ); //小睡一会儿，“s”秒后唤醒signed long __sched schedule_timeout(signed long timeout)
{
struct timer_list timer;
unsigned long expire;
switch (timeout)
{
case MAX_SCHEDULE_TIMEOUT:
/*
* These two special cases are useful to be comfortable
* in the caller. Nothing more. We could take
* MAX_SCHEDULE_TIMEOUT from one of the negative value
* but I' d like to return a valid offset (>=0) to allow
* the caller to do everything it want with the retval.
*/
schedule();
goto out;
default:
/*
* Another bit of PARANOID. Note that the retval will be
* 0 since no piece of kernel is supposed to do a check
* for a negative retval of schedule_timeout() (since it
* should never happens anyway). You just have the printk()
* that will tell you if something is gone wrong and where.
*/
if (timeout < 0) {
printk(KERN_ERR "schedule_timeout: wrong timeout "
"value %lx\n", timeout);
dump_stack();
current->state = TASK_RUNNING;
goto out;
}
}
expire = timeout + jiffies;
//下一行代码设置了超时执行函数process_timeout()。
setup_timer_on_stack(&timer, process_timeout, (unsigned long)current);
__mod_timer(&timer, expire, false, TIMER_NOT_PINNED); //激活定时器
schedule(); //调度其他新任务
del_singleshot_timer_sync(&timer);
/* Remove the timer from the object tracker */
destroy_timer_on_stack(&timer);
timeout = expire - jiffies;
out:
return timeout < 0 ? 0 : timeout;
} 当定时器超时时，process_timeout()函数被调用：static void process_timeout(unsigned long __data)
{
wake_up_process((struct task_struct *)__data);
}版权声明：本文为博主原创文章，未经博主允许不得转载。
原文地址：http://blog.csdn.net/shallnet/article/details/47133031
