Linux下的hrtimer高精度定时器【转】

转自：http://blog.csdn.net/waverider2012/article/details/38305785

hrtimer高精度定时器的interval由ktime_set(const long secs, const unsigned long nsecs)决定，可做到ns级。此处的例子为5ms interval：

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/hrtimer.h>
#include <linux/ktime.h>
MODULE_LICENSE("GPL");
static struct hrtimer hr_timer;
static struct work_struct wq_hrtimer;
static ktime_t ktime;
static unsigned int interval=5000; /* unit: us */
struct timespec uptimeLast;
static unsigned int count=0;
#define COUNT_INTERVAL 4000
unsigned long long diff_tv(struct timespec start, struct timespec end) {
return (end.tv_sec-start.tv_sec)*1000000000+(end.tv_nsec-start.tv_nsec);
}
enum hrtimer_restart my_hrtimer_callback( struct hrtimer *timer )
{
schedule_work(&wq_hrtimer);
return HRTIMER_NORESTART;
}
static void wq_func_hrtimer(struct work_struct *work)
{
struct timespec uptime;
hr_timer.function = my_hrtimer_callback;
ktime = ktime_set( interval/1000000, (interval%1000000)*1000 );
hrtimer_start(&hr_timer, ktime, HRTIMER_MODE_REL );
/* print time every COUNT_INTERVAL*interval second*/
if(count%COUNT_INTERVAL==0)
{
do_posix_clock_monotonic_gettime(&uptime);
printk(KERN_INFO"hrtimer:%9lu sec, %9lu ns, interval_delay=%lu ns\n",
(unsigned long) uptime.tv_sec, uptime.tv_nsec,
(unsigned long)(diff_tv(uptimeLast, uptime)-interval*1000*COUNT_INTERVAL) \
/COUNT_INTERVAL);
uptimeLast=uptime;
}
count++;
}
static int __init module_hrtimer_init( void )
{
struct timespec uptime;
printk(KERN_INFO"HR Timer module installing\n");
hrtimer_init( &hr_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL );
ktime = ktime_set( interval/1000000, (interval%1000000)*1000 );
hr_timer.function = my_hrtimer_callback;
hrtimer_start( &hr_timer, ktime, HRTIMER_MODE_REL );
do_posix_clock_monotonic_gettime(&uptime);
uptimeLast = uptime;
printk(KERN_INFO"hrtimer:%9lu sec, %9lu ns\n", (unsigned long) uptime.tv_sec,
uptime.tv_nsec );
INIT_WORK(&wq_hrtimer, wq_func_hrtimer);
return 0;
}
static void __exit module_hrtimer_exit( void )
{
int ret;
ret = hrtimer_cancel( &hr_timer );
if (ret)
printk("The timer was still in use...\n");
printk("HR Timer module uninstalling\n");
return;
}
module_init(module_hrtimer_init);
module_exit(module_hrtimer_exit);

如果在my_hrtimer_callback()里面直接返回HRTIMER_RESTART会导致立即重新进入my_hrtimer_callback()。这时shell对输入没有任何响应。

所以为了解决这个问题，创建了一个work queue，由my_hrtimer_callback() enqueue这个工作队列。在work queue的处理函数里面重启hrtimer。

但是这样带来的负面影响是进入hrtimer_callback和wq_func被调用之间有Linux系统调度引入的延迟，导致interval出现误差。经过实测，在ZC706缺省配置下，这个延迟大约是17.5us (hrtimer interval为5ms，每20秒计算一次interval误差)。

root@zynq:~/nfs/hrtimer# insmod hrtimer.ko
HR Timer module installing
hrtimer: 2900 sec, 993366078 ns
hrtimer: 2900 sec, 998395278 ns, interval_delay=369966 ns
hrtimer: 2921 sec, 69525447 ns, interval_delay=17782 ns
hrtimer: 2941 sec, 139764655 ns, interval_delay=17559 ns
hrtimer: 2961 sec, 210029519 ns, interval_delay=17566 ns
hrtimer: 2981 sec, 280465631 ns, interval_delay=17609 ns
hrtimer: 3001 sec, 350677038 ns, interval_delay=17552 ns
hrtimer: 3021 sec, 420625114 ns, interval_delay=17487 ns
hrtimer: 3041 sec, 490744847 ns, interval_delay=17529 ns

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Linux下的hrtimer高精度定时器【转】

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