> For the complete documentation index, see [llms.txt](https://trance.gitbook.io/blog/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://trance.gitbook.io/blog/time.md).

# Time Keeping

## 什么是 jiffies

一个全局变量，系统初始化时钟之后，每一个时钟中断到来就会增加`1`，记载着启动的时间，也可以作为简单的时钟计数

## 序

以 x86 和 ARM 两个角度出发，说说时钟（SoC，Arch timer， DTS的交互），就是说，理一理这个框架，config->driver/clocksource/->of->kernel\_init->probe->match\_ids

## Draft

timer\_init 

of\_probe

of\_xx\_init -> DECLARE-> init\_func

interfacing the kernel framework\~

clock\_source: 提供日期，是个长期的时钟

clock\_event:  reverse of ..source, 这就是个 timer，周期性的调用

## tick device

```c
/*
 * Tick devices
 */
DEFINE_PER_CPU(struct tick_device, tick_cpu_device);
/*
 * Tick next event: keeps track of the tick time
 */
ktime_t tick_next_period;
ktime_t tick_period;

/*
 * tick_do_timer_cpu is a timer core internal variable which holds the CPU NR
 * which is responsible for calling do_timer(), i.e. the timekeeping stuff. This
 * variable has two functions:
 *
 * 1) Prevent a thundering herd issue of a gazillion of CPUs trying to grab the
 *    timekeeping lock all at once. Only the CPU which is assigned to do the
 *    update is handling it.
 *
 * 2) Hand off the duty in the NOHZ idle case by setting the value to
 *    TICK_DO_TIMER_NONE, i.e. a non existing CPU. So the next cpu which looks
 *    at it will take over and keep the time keeping alive.  The handover
 *    procedure also covers cpu hotplug.
 */
int tick_do_timer_cpu __read_mostly = TICK_DO_TIMER_BOOT;
```

clock\_event 是否会被接受，看的是 tick\_device set\_up  之类的，对比的是，是否支持 oneshot, 也就是手动启动下一次，以及 rating 的大小，越大越好。

这样就是说，处理时钟中断的过程，没有唤醒时钟，不在计数范围内，这对调度来说是公平的，但是我目前还不清楚，为什么 tick 的时候，需要判断是否本CPU的中断，难以理解。

很简单，因为只有一个`cpu`负责增加 `jiffies`，除了引导的 CPU 其它都是被唤醒的，并不能作为 jiifies，但是每个进程的计数却是分开的，所以才有了那段经典的判断。

logical\_id 就是 sched\_init 递增的而已，而其它 CPU 都是由引导 CPU 唤醒的，唤醒的位置也不同，看看 secondary\_startup  

## tick 的矫正

![](/files/-Lo6AGaGoNUP6yQTzNh8)

![](/files/-Lo6ASD6n6YjS54eiDW8)

ktime\_get 在 update\_wall\_time 更新

![](/files/-Lo6Ac2bPLHhw-iBBcEY)

所以说有俩个 clock

![](/files/-Lo6AwBhCYCuOlL-SsIW)

一个是 tick\_device 一个是 clock\_source，后者让前者更加的精确

![](/files/-Lo6BBq2z4kHwE-tmOq8)

## mul & shift

$$
(B\~*~~\lfloor \frac{A~~*2^{shift} + \lfloor B/2\rfloor}{B}\rfloor \gg\~shift) \~\approx A
$$

![](/files/-Lo9OVo8WjROJ_73DgpP)

这里前面计算 sftacc 就是为了限制 mul 的大小，如果 mul 太大，我们转换的公式就会出问题

$$
to = max ~~\*mul~~\gg shift
$$

`tmp`  如果超过了 32位，那么超过的位数就得从 `mul` 中拿去，因为 `max = maxsec * from` 假设，此时的 `mul` 的位数 + max 的位数 超过`64`，那么出现了数据丢失

所以前面进行这一步计算是为了这个考虑，当然，从公式中我们也可以看出，`mul` 小了一些，那么 `shift` 也小一些就好了，所以这是双向的限制，保证了两点

`from * mul` 以及 `to << shift` 不会越界，当然是是 `64 bit` 的界

## END


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://trance.gitbook.io/blog/time.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.