Linux下的hrtimer高精度定時器

hrtimer高精度定時器的interval由ktime_set(const long secs, const unsigned long nsecs)決定，可做到ns級。此處的例子爲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()裏面直接返回HRTIMER_RESTART會導致立即重新進入my_hrtimer_callback()。這時shell對輸入沒有任何響應。

所以爲了解決這個問題，創建了一個work queue，由my_hrtimer_callback() enqueue這個工作隊列。在work queue的處理函數裏面重啓hrtimer。

但是這樣帶來的負面影響是進入hrtimer_callback和wq_func被調用之間有Linux系統調度引入的延遲，導致interval出現誤差。經過實測，在ZC706缺省配置下，這個延遲大約是17.5us (hrtimer interval爲5ms，每20秒計算一次interval誤差)。

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