This is yet another post that opens new chapter in the [linux-insides](http://0xax.gitbooks.io/linux-insides/content/) book. The previous [part](https://0xax.gitbooks.io/linux-insides/content/SysCall/syscall-4.html) was a list part of the chapter that describes [system call](https://en.wikipedia.org/wiki/System_call) concept and now time is to start new chapter. As you can understand from the post's title, this chapter will be devoted to the `timers` and `time management` in the Linux kernel. The choice of topic for the current chapter is not accidental. Timers and generally tim management are very important and widely used in the Linux kernel. The Linux kernel uses timers for various tasks, different timeouts for example in [TCP](https://en.wikipedia.org/wiki/Transmission_Control_Protocol) implementation, the kernel must know current time, scheduling asynchronous functions, next event interrupt scheduling and many many more.
This is yet another post that opens new chapter in the [linux-insides](http://0xax.gitbooks.io/linux-insides/content/) book. The previous [part](https://0xax.gitbooks.io/linux-insides/content/SysCall/syscall-4.html) was a list part of the chapter that describes [system call](https://en.wikipedia.org/wiki/System_call) concept and now time is to start new chapter. As you can understand from the post's title, this chapter will be devoted to the `timers` and `time management` in the Linux kernel. The choice of topic for the current chapter is not accidental. Timers and generally time management are very important and widely used in the Linux kernel. The Linux kernel uses timers for various tasks, different timeouts for example in [TCP](https://en.wikipedia.org/wiki/Transmission_Control_Protocol) implementation, the kernel must know current time, scheduling asynchronous functions, next event interrupt scheduling and many many more.
So, we will start to learn implementation of the different time management related stuff in this part. We will see different types of timers and how how do different Linux kernel subsystems use them. As always we will start from the earliest part of the Linux kernel and will go through initialization process of the Linux kernel. We already did it in the special [chapter](https://0xax.gitbooks.io/linux-insides/content/Initialization/index.html) which describes initialization process of the Linux kernel, but as you may remember we missied some things there. And one of these is initialization of timers.
So, we will start to learn implementation of the different time management related stuff in this part. We will see different types of timers and how do different Linux kernel subsystems use them. As always we will start from the earliest part of the Linux kernel and will go through initialization process of the Linux kernel. We already did it in the special [chapter](https://0xax.gitbooks.io/linux-insides/content/Initialization/index.html) which describes initialization process of the Linux kernel, but as you may remember we missied some things there. And one of these is initialization of timers.
Where the `x86_init_noop` is just function that does not nothing:
Where the `x86_init_noop` is just a function that does nothing:
```C
void __cpuinit x86_init_noop(void) { }
@ -128,7 +128,7 @@ This definition is very similar to the `jiffy` in the Linux kernel. There is glo
extern unsigned long volatile __jiffy_data jiffies;
```
during initialization process. This global variable will be incremented each time each time during timer interrupt. Besides this, near the `jiffies` variable we can see definition of the similar variable
during initialization process. This global variable will be incremented each time during timer interrupt. Besides this, near the `jiffies` variable we can see definition of the similar variable
```C
extern u64 jiffies_64;
@ -251,13 +251,13 @@ by default, and the `shift` is
#endif
```
The `jiffies` clock source uses the `NSEC_PER_JIFFY` multiplier conversion to specify the nanosecond over cycle ratio. Note that values of the `JIFFIES_SHIFT` and `NSEC_PER_JIFFY` depend on `HZ` value. The `HZ` represents the frequency of the system timer. This macro defined in the [include/asm-generic/param.h](https://github.com/torvalds/linux/blob/master/include/asm-generic/param.h) and depends on the `CONFIG_HZ` kernel configuration option. The value of `HZ` differs for each supported architecture, but for `x86` it defined like:
The `jiffies` clock source uses the `NSEC_PER_JIFFY` multiplier conversion to specify the nanosecond over cycle ratio. Note that values of the `JIFFIES_SHIFT` and `NSEC_PER_JIFFY` depend on `HZ` value. The `HZ` represents the frequency of the system timer. This macro defined in the [include/asm-generic/param.h](https://github.com/torvalds/linux/blob/master/include/asm-generic/param.h) and depends on the `CONFIG_HZ` kernel configuration option. The value of `HZ` differs for each supported architecture, but for `x86` it's defined like:
```C
#define HZ CONFIG_HZ
```
Where `CONFIG_HZ` can be on of following values:
Where `CONFIG_HZ` can be one of the following values:
function from the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup.c#L842) source code file.
As I already wrote, the main purpose of the `register_refined_jiffies` function is to register `refined_jiffies` clocksource. We already saw the `clocksource_jiffies` structure represents standard `jiffies` clock source. Now, if you will look in the [kernel/time/jiffies.c](https://github.com/torvalds/linux/blob/master/kernel/time/jiffies.c) source code file, you will find yet another clock source definition:
As I already wrote, the main purpose of the `register_refined_jiffies` function is to register `refined_jiffies` clocksource. We already saw the `clocksource_jiffies` structure represents standard `jiffies` clock source. Now, if you look in the [kernel/time/jiffies.c](https://github.com/torvalds/linux/blob/master/kernel/time/jiffies.c) source code file, you will find yet another clock source definition:
Anton Tiurin
9 years ago