//Timekeeping
// Timekeeping子系统负责更新xtime, 调整误差, 及提供get/settimeofday接口.
//Times in Kernel
//kernel的time基本类型:
// 1) system time
// A monotonically increasing value that represents the amount of time the system has been running.
// 单调增长的系统运行时间, 可以通过time source, xtime及wall_to_monotonic计算出来.
// 2) wall time
// A value representing the the human time of day, as seen on a wrist-watch. Realtime时间: xtime.
// 3) time source
// A representation of a free running counter running at a known frequency, usually in hardware, e.g GPT.
// 可以通过clocksource->read()得到counter值
// 4) tick
// A periodic interrupt generated by a hardware-timer, typically with a fixed interval defined by HZ: jiffies
// 这些time之间互相关联, 互相可以转换:
// system_time = xtime + cyc2ns(clock->read() - clock->cycle_last) + wall_to_monotonic;
// real_time = xtime + cyc2ns(clock->read() - clock->cycle_last)
// 也就是说real time是从1970年开始到现在的nanosecond, 而system time是系统启动到现在的
