gettimeofday和xtime的定时精度

客户反映，gettimeofday获取的时间us部分总是零。 自己修改测试代码有如下结果： ./lpc3250test 1 0 50000 divsor is 1 sec is 0 usec is 50000 *** current time: sec = 1289836976 usec = 390000 *** current time: sec = 1289836976 usec = 390000 *** current time: sec = 1289836976 usec = 390000 *** current time: sec = 1289836976 usec = 450000 cnt = 1 *** current time: sec = 1289836976 usec = 450000 cnt = 1 *** current time: sec = 1289836976 usec = 500000 cnt = 2 *** current time: sec = 1289836976 usec = 500000 cnt = 2 *** current time: sec = 1289836976 usec = 550000 cnt = 3 *** current time: sec = 1289836976 usec = 550000 cnt = 3 *** current time: sec = 1289836976 usec = 600000 cnt = 4 *** current time: sec = 1289836976 usec = 600000 cnt = 4 *** current time: sec = 1289836976 usec = 650000 cnt = 5 *** current time: sec = 1289836976 usec = 650000 cnt = 5 *** current time: sec = 1289836976 usec = 700000 cnt = 6 *** current time: sec = 1289836976 usec = 700000 cnt = 6 *** current time: sec = 1289836976 usec = 750000 cnt = 7 *** current time: sec = 1289836976 usec = 750000 cnt = 7 *** current time: sec = 1289836976 usec = 800000 cnt = 8 *** current time: sec = 1289836976 usec = 800000 cnt = 8 *** current time: sec = 1289836976 usec = 850000 cnt = 9 *** 可以发现gettimeofday的us后四位总为零，只能精确到10ms. 调用关系：gettimeofday->sys_gettimeofday->do_gettimeofday. do_gettimeofday的代码在内核代码的kernel/time/timekeeping.c 函数定义在114行，do_gettimeofday又调用了同一文件中的getnstimeofday。 void getnstimeofday(struct timespec *ts) { unsigned long seq; s64 nsecs;

do { seq = read_seqbegin(&xtime_lock);

*ts = xtime; nsecs = __get_nsec_offset();

} while (read_seqretry(&xtime_lock, seq));

timespec_add_ns(ts, nsecs); } 可见getnstimeofday直接使用了xtime。 现在问题转化为xtime的精度到底如何。 在驱动模块中打印current_kernel_time(void)读取的xtime如下： isoinit, sec:1289833417, ns:950000000 isoinit, sec:1289833417, ns:950000000 isoinit, sec:1289833417, ns:960000000 isoinit, sec:1289833417, ns:960000000 isoinit, sec:1289833417, ns:960000000 isoinit, sec:1289833417, ns:970000000 isoinit, sec:1289833417, ns:970000000

isoinit, sec:1289833425, ns:780000000 isoinit, sec:1289833425, ns:780000000 isoinit, sec:1289833425, ns:790000000 isoinit, sec:1289833425, ns:790000000 isoinit, sec:1289833425, ns:790000000 isoinit, sec:1289833425, ns:800000000 isoinit, sec:1289833425, ns:800000000 多次试验的结果都是最后七位总为零。所以xtime的精度不会超过10ms。 现在可以看到gettimeofday和xtime的精度同为10ms。与系统时钟的精度相同（HZ=100）. xtime是一个全局变量。 kernel/time/timekeeping.c中更新xtime的函数为update_wall_time(void) /** * update_wall_time - Uses the current clocksource to increment the wall time * * Called from the timer interrupt, must hold a write on xtime_lock. */ void update_wall_time(void) { 。。。。。。。。。。。 /* store full nanoseconds into xtime */ xtime.tv_nsec = (s64)clock->xtime_nsec >> clock->shift; clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift; 。。。。。。。。。。。 } 由于此函数是由系统定时中断调用的，所以xtime精度和系统时钟精度相同也就不足为怪了。

总结：getrimeofday和xtime的精度和系统节拍有关，我的系统HZ=100,因此只能精确到10ms。（内核2.6.27.8）