@ -6,7 +6,7 @@ Introduction to deferred interrupts (Softirq, Tasklets and Workqueues)
It is the ninth part of the [linux-insides](https://www.gitbook.com/book/0xax/linux-insides/details) book and in the previous [Previous part](http://0xax.gitbooks.io/linux-insides/content/interrupts/interrupts-8.html) we saw implementation of the `init_IRQ` from that defined in the [arch/x86/kernel/irqinit.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/irqinit.c) source code file. So, we will continue to dive into the initialization stuff which is related to the external hardware interrupts in this part.
After the `init_IRQ` function we can see the call of the `softirq_init` function in the [init/main.c](https://github.com/torvalds/linux/blob/master/init/main.c). This function defined in the [kernel/softirq.c](https://github.com/torvalds/linux/blob/master/kernel/softirq.c) source code file and as we can understand from its name, this function makes initialization of the `softirq` or in other words initialization of the `deferred interrupts`. What is it deferreed intrrupt? We already saw a little bit about it in the ninth [part](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-9.html) of the chapter that describes initialization process of the Linux kernel. There are three types of `deffered interrupts` in the Linux kernel:
After the `init_IRQ` function we can see the call of the `softirq_init` function in the [init/main.c](https://github.com/torvalds/linux/blob/master/init/main.c). This function defined in the [kernel/softirq.c](https://github.com/torvalds/linux/blob/master/kernel/softirq.c) source code file and as we can understand from its name, this function makes initialization of the `softirq` or in other words initialization of the `deferred interrupts`. What is it deferreed intrrupt? We already saw a little bit about it in the ninth [part](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-9.html) of the chapter that describes initialization process of the Linux kernel. There are three types of `deferred interrupts` in the Linux kernel:
* `softirqs`;
* `tasklets`;
@ -27,7 +27,7 @@ As you can understand, it is almost impossible to make so that both characterist
* Top half;
* Bottom half;
Once the Linux kernel was one of the ways the organization postprocessing, and which was called: `the bottom half` of the processor, but now it is already not actual. Now this term has remained as a common noun referring to all the different ways of organizing deffered processing of an interrupt. With the advent of parallelisms in the Linux kernel, all new schemes of implementation of the bottom half handlers are built on the performance of the processor specific kernel thread that called `ksoftirqd` (will be discussed below). The `softirq` mechanism represents handling of interrupts that are `almost` as important as the handling of the hardware interrupts. The deferred processing of an interrupt suggests that some of the actions for an interrupt may be postponed to a later execution when the system will be less loaded. As you can suggests, an interrupt handler can do large amount of work that is impermissible as it executes in the context where interrupts are disabled. That's why processing of an interrupt can be splitted on two different parts. In the first part, the main handler of an interrupt does only minimal and the most important job. After this it schedules the second part and finishes its work. When the system is less busy and context of the processor allows to handle interrupts, the second part starts its work and finishes to process remaing part of a deferred interrupt. That is main explanation of the deferred interrupt handling.
Once the Linux kernel was one of the ways the organization postprocessing, and which was called: `the bottom half` of the processor, but now it is already not actual. Now this term has remained as a common noun referring to all the different ways of organizing deferred processing of an interrupt. With the advent of parallelisms in the Linux kernel, all new schemes of implementation of the bottom half handlers are built on the performance of the processor specific kernel thread that called `ksoftirqd` (will be discussed below). The `softirq` mechanism represents handling of interrupts that are `almost` as important as the handling of the hardware interrupts. The deferred processing of an interrupt suggests that some of the actions for an interrupt may be postponed to a later execution when the system will be less loaded. As you can suggests, an interrupt handler can do large amount of work that is impermissible as it executes in the context where interrupts are disabled. That's why processing of an interrupt can be splitted on two different parts. In the first part, the main handler of an interrupt does only minimal and the most important job. After this it schedules the second part and finishes its work. When the system is less busy and context of the processor allows to handle interrupts, the second part starts its work and finishes to process remaing part of a deferred interrupt. That is main explanation of the deferred interrupt handling.
As I already wrote above, handling of deferred interrupts (or `softirq` in other words) and accordingly `tasklets` is performed by a set of the special kernel threads (one thread per processor). Each processor has its own thread that is called `ksoftirqd/n` where the `n` is the number of the processor. We can see it in the output of the `systemd-cgls` util:
@ -145,7 +145,7 @@ The `raise_softirq_irqoff` function marks the softirq as deffered by setting the
__raise_softirq_irqoff(nr);
```
macro. After this, it checks the result of the `in_interrupt` that returns `irq_count` value. We already saw the `irq_count` in the first [part](http://0xax.gitbooks.io/linux-insides/content/interrupts/interrupts-1.html) of this chapter and it is used to check if a CPU is already on an interrupt stack or not. We just exit from the `raise_softirq_irqoff`, restore `IF` flang and enable interrupts on the local processor, if we are in the interrupt context, otherwise we call the `wakeup_softirqd`:
macro. After this, it checks the result of the `in_interrupt` that returns `irq_count` value. We already saw the `irq_count` in the first [part](http://0xax.gitbooks.io/linux-insides/content/interrupts/interrupts-1.html) of this chapter and it is used to check if a CPU is already on an interrupt stack or not. We just exit from the `raise_softirq_irqoff`, restore `IF` flag and enable interrupts on the local processor, if we are in the interrupt context, otherwise we call the `wakeup_softirqd`:
In the beginning of the `tasketl_action` function, we disable interrupts for the local processor with the help of the `local_irq_disable` macro (you can read about this macro in the second [part](http://0xax.gitbooks.io/linux-insides/content/interrupts/interrupts-2.html) of this chapter). In the next step, we take a head of the list that contains tasklets with normal priority and set this per-cpu list to `NULL` because all tasklets must be executed in a generaly way. After this we enable interrupts for the local processor and go through the list of taklets in the loop. In every iteration of the loop we call the `tasklet_trylock` function for the given tasklet that updates state of the given tasklet on `TASKLET_STATE_RUN`:
In the beginning of the `tasketl_action` function, we disable interrupts for the local processor with the help of the `local_irq_disable` macro (you can read about this macro in the second [part](http://0xax.gitbooks.io/linux-insides/content/interrupts/interrupts-2.html) of this chapter). In the next step, we take a head of the list that contains tasklets with normal priority and set this per-cpu list to `NULL` because all tasklets must be executed in a generally way. After this we enable interrupts for the local processor and go through the list of taklets in the loop. In every iteration of the loop we call the `tasklet_trylock` function for the given tasklet that updates state of the given tasklet on `TASKLET_STATE_RUN`:
```C
static inline int tasklet_trylock(struct tasklet_struct *t)
spacewander
8 years ago