内核的定时和延迟

1. 定时： 一般是通过在当前 jiffies 上加上一个 delay 得到目标 jiffies ，然后比较目标 jiffies 实现定时。

2. 延时： 短延时（一般通过忙等待实现），长延时（一般通过使进程让出处理器的方式实现） 。

3. 系统中记录时间的计数器：（1） jiffies ,(2) 处理器特定的寄存器

4. 内核定时器: 内核定时器常常作为"软件中断"而异步运行。

实例：参考ldd3

//jittimer.c

//#include <linux/config.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/init.h>

#include <linux/time.h>
#include <linux/timer.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include <linux/types.h>
#include <linux/spinlock.h>
#include <linux/interrupt.h>

#include <asm/hardirq.h>
/*
 * This module is a silly one: it only embeds short code fragments
 * that show how time delays can be handled in the kernel.
 */

int delay = HZ; /* the default delay, expressed in jiffies */

MODULE_AUTHOR("Alessandro Rubini");
MODULE_LICENSE("Dual BSD/GPL");


/*
 * The timer example follows
 */

int tdelay = 10;
module_param(tdelay, int, 0);

/* This data structure used as "data" for the timer and tasklet functions */
struct jit_data {
    struct timer_list timer;
    wait_queue_head_t wait;
    unsigned long prevjiffies;
    unsigned char *buf;
    int loops;
};
#define JIT_ASYNC_LOOPS 5

void jit_timer_fn(unsigned long arg)
{
    struct jit_data *data = (struct jit_data *)arg;
    unsigned long j = jiffies;
    data->buf += sprintf(data->buf, "%9li  %3li     %i    %6i   %i   %s
",
                 j, j - data->prevjiffies, in_interrupt() ? 1 : 0,
                 current->pid, smp_processor_id(), current->comm);

    if (--data->loops) {
        data->timer.expires += tdelay;
        data->prevjiffies = j;
        add_timer(&data->timer);
    } else {
        wake_up_interruptible(&data->wait);
    }
}

/* the /proc function: allocate everything to allow concurrency */
int jit_timer(char *buf, char **start, off_t offset,
          int len, int *eof, void *unused_data)
{
    struct jit_data *data;
    char *buf2 = buf;
    unsigned long j = jiffies;

    data = kmalloc(sizeof(*data), GFP_KERNEL);
    if (!data)
        return -ENOMEM;

    init_timer(&data->timer);
    init_waitqueue_head (&data->wait);

    /* write the first lines in the buffer */
    buf2 += sprintf(buf2, "   time   delta  inirq    pid   cpu command
");
    buf2 += sprintf(buf2, "%9li  %3li     %i    %6i   %i   %s
",
            j, 0L, in_interrupt() ? 1 : 0,
            current->pid, smp_processor_id(), current->comm);

    /* fill the data for our timer function */
    data->prevjiffies = j;
    data->buf = buf2;
    data->loops = JIT_ASYNC_LOOPS;
    
    /* register the timer */
    
    data->timer.data = (unsigned long)data;
    data->timer.function = jit_timer_fn;
    
    
    data->timer.expires = j + tdelay; /* parameter */
    add_timer(&data->timer);

    /* wait for the buffer to fill */
    wait_event_interruptible(data->wait, !data->loops);
    
    if (signal_pending(current))
        return -ERESTARTSYS;
    buf2 = data->buf;
    kfree(data);
    *eof = 1;
    return buf2 - buf;
}





int __init jit_init(void)
{
    
    create_proc_read_entry("jitimer", 0, NULL, jit_timer, NULL);
    

    return 0; /* success */
}

void __exit jit_cleanup(void)
{

    remove_proc_entry("jitimer", NULL);
    
}

module_init(jit_init);
module_exit(jit_cleanup);

 程序主要通过定时器组织起来，注册定时器后主程序休眠，等待唤醒。当读取次数到达时，通过唤醒等待队列使程序退出。