grttimeofday和xtime的定时精度

最新推荐文章于 2024-04-01 15:38:26 发布

最新推荐文章于 2024-04-01 15:38:26 发布 · 87 阅读

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本文探讨了gettimeofday函数在特定系统配置下（内核2.6.27.8，HZ=100）的精度问题，发现其微秒级时间戳的后四位始终为零，仅能提供10毫秒级别的精度。通过分析内核源代码，揭示了这种现象的原因在于xtime变量的更新机制及其与系统节拍的关系。

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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。