高精度 schedule_hrtimeout

精确纳秒级调度，但是原理上会更多的消耗CPU

/**
 * schedule_hrtimeout - sleep until timeout
 * @expires: timeout value (ktime_t)
 * @mode: timer mode, HRTIMER_MODE_ABS or HRTIMER_MODE_REL
 *
 * Make the current task sleep until the given expiry time has
 * elapsed. The routine will return immediately unless
 * the current task state has been set (see set_current_state()).
 *
 * You can set the task state as follows -
 *
 * %TASK_UNINTERRUPTIBLE - at least @timeout time is guaranteed to
 * pass before the routine returns.
 *
 * %TASK_INTERRUPTIBLE - the routine may return early if a signal is
 * delivered to the current task.
 *
 * The current task state is guaranteed to be TASK_RUNNING when this
 * routine returns.
 *
 * Returns 0 when the timer has expired otherwise -EINTR
 */
int __sched schedule_hrtimeout(ktime_t *expires,
      const enum hrtimer_mode mode)
{
return schedule_hrtimeout_range(expires, 0, mode);
}