Linux时间子系统5：timekeeper、timecounter&cyclecounter

1. 前言

    前面我们介绍了用户态获取时间的接口clock_gettime，时钟的种类posix_clocks以及时钟源clocksource。那么我们思考这样一个问题，无论clock_gettime或者posix_clock定义的时间都是相对于某个起始点的时间，即相对于Linux Epoch的秒数，但是我们上一章节介绍的时钟源clocksource，它提供的read接口，我们看一下定义，它返回的是时钟源的cycle值，那么cycle和time是怎么对应的，内核是怎么通过clocksource来实现我们之前说的多种posix clock的，这就是本文要分析的内容timekeeper，同时我们再泛化到所有的硬件计时器，介绍timecounter和cyclecounter的概念。本文同样是在Linux时间子系统之（四）：timekeeping文章的基础上，增加了一点个人的笔记。

2 Timekeeping

          timekeeping模块是一个提供时间服务的基础模块。Linux内核提供各种time line，real time clock，monotonic clock、monotonic raw clock等，timekeeping模块就是负责跟踪、维护这些timeline的，并且向其他模块（timer相关模块、用户空间的时间服务等）提供服务，而timekeeping模块维护timeline的基础是基于clocksource模块和tick模块。通过tick模块的tick事件，可以周期性的更新time line，通过clocksource模块、可以获取tick之间更精准的时间信

2.1 Timekeeper数据结构

        struct  timekeeper数据结构如下：

struct timekeeper {
	struct tk_read_base	tkr_mono;
	struct tk_read_base	tkr_raw;
	u64			xtime_sec;
	unsigned long		ktime_sec;
	struct timespec64	wall_to_monotonic;
	ktime_t			offs_real;
	ktime_t			offs_boot;
	ktime_t			offs_tai;
	s32			tai_offset;
	unsigned int		clock_was_set_seq;
	u8			cs_was_changed_seq;
	ktime_t			next_leap_ktime;
	u64			raw_sec;
	struct timespec64	monotonic_to_boot;

	/* The following members are for timekeeping internal use */
	u64			cycle_interval;
	u64			xtime_interval;
	s64			xtime_remainder;
	u64			raw_interval;
	/* The ntp_tick_length() value currently being used.
	 * This cached copy ensures we consistently apply the tick
	 * length for an entire tick, as ntp_tick_length may change
	 * mid-tick, and we don't want to apply that new value to
	 * the tick in progress.
	 */
	u64			ntp_tick;
	/* Difference between accumulated time and NTP time in ntp
	 * shifted nano seconds. */
	s64			ntp_error;
	u32			ntp_error_shift;
	u32			ntp_err_mult;
	/* Flag used to avoid updating NTP twice with same second */
	u32			skip_second_overflow;
#ifdef CONFIG_DEBUG_TIMEKEEPING
	long			last_warning;
	/*
	 * These simple flag variables are managed
	 * without locks, which is racy, but they are
	 * ok since we don't really care about being
	 * sup