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foreword
The interrupt mechanism of the Linux kernel is designed to handle hardware interrupts and software interrupts (also known as exceptions). An interrupt is an asynchronous event that interrupts a program being executed, causing the processor to switch execution to the interrupt handler.
The interrupt mechanism of the Linux kernel includes the following core components:
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Interrupt Controller: The interrupt controller is a hardware device responsible for receiving and distributing hardware interrupts. Common interrupt controllers are 8259A (used in old x86 systems) and APIC (Advanced Programmable Interrupt Controller, used in new x86 systems). The interrupt controller is responsible for routing hardware interrupts to the appropriate interrupt handler.
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Interrupt Descriptor Table (Interrupt Descriptor Table, IDT): IDT is a data structure used to store interrupt vectors and interrupt handler addresses. Each interrupt vector corresponds to a unique interrupt type, such as clock interrupt, keyboard interrupt, etc. When an interrupt occurs, the processor will search the IDT according to the interrupt vector and jump to the corresponding interrupt handler.
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Interrupt Handler: An interrupt handler is a specific piece of code that handles a specific type of interrupt. When an interrupt occurs, the processor will jump to the corresponding interrupt handler to perform corresponding operations. The interrupt handler is responsible for handling interrupt events, saving and restoring context, handling interrupt requests, etc.
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Interrupt Context: During the execution of the interrupt handler, the processor needs to save and restore the context information of the current program. Interrupt context includes register status, stack pointer, etc. The purpose of saving and restoring the interrupt context is to ensure that after the execution of the interrupt handler is completed, it can correctly return to the original program to continue execution.
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Interrupt control and masking: The Linux kernel provides a set of APIs for controlling and managing interrupts. These APIs allow developers to register interrupt handlers, mask and enable interrupts, set interrupt priority, and more.
When a hardware device is interrupted, the interrupt controller sends an interrupt signal to the processor. The processor will search the IDT according to the interrupt vector and jump to the corresponding interrupt handler for execution. Interrupt handlers handle interrupt events, which may include reading device state, processing data, triggering other actions, and so on. After processing is complete, the processor restores the original context and resumes execution of the interrupted program.
The interrupt mechanism is an important part of the Linux kernel, which allows the kernel to effectively manage and respond to asynchronous events of hardware devices, improving system performance and reliability.
What are the interrupt mechanisms
In Linux, the interrupt mechanism can be divided into the following types:
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Hardware Interrupts: Hardware interrupts are interrupts triggered by hardware devices, such as clock interrupts, keyboard interrupts, and network card interrupts. When a specific event occurs on a hardware device, it will send an interrupt signal to the interrupt controller, and then the interrupt controller will pass the interrupt signal to the processor, and the processor will jump to the corresponding interrupt handler for execution.
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Software Interrupts: Software interrupts, also known as exceptions, are interrupts generated by software. Software interrupts can be triggered by system calls, trap instructions, error exceptions, etc. For example, when a user process requests a service from the kernel through a system call, a software interrupt will be triggered, and the kernel will switch to the kernel mode to execute the corresponding system call handler.
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Fast Interrupts: Fast Interrupts are a special type of hardware interrupt used to handle some devices that require very high response time. Fast interrupts have lower latency and higher priority to ensure timely response. In Linux, fast interrupts are usually supported by specific hardware devices and corresponding interrupt controllers.
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Softirqs: Softirqs are a mechanism used in the kernel to handle some latency-sensitive tasks. Soft interrupts are triggered by software and do not depend on hardware interrupts. Each subsystem in the kernel can register its own soft interrupt handler, and when a specific event occurs, the soft interrupt handler will be scheduled for execution.
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Deferred Interrupts: Deferred Interrupts are a mechanism to defer interrupt processing until later execution. When an interrupt occurs, the kernel can choose to postpone the interrupt processing to an appropriate time to improve the performance and efficiency of the system. Delayed interrupts are usually used to handle some non-critical interrupt events to avoid frequent context switching.
These interrupt mechanisms together constitute the interrupt processing system of the Linux kernel, enabling the kernel to effectively manage and respond to asynchronous events of hardware devices. Different types of interrupt mechanisms are applicable to different scenarios and requirements, and an appropriate interrupt mechanism can be selected to handle interrupt events according to specific situations.
hardware interrupt
In Linux, a hardware interrupt is an interrupt event triggered by a hardware device. Here are some common hardware interrupt examples:
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Timer Interrupt: A clock interrupt is an interrupt triggered by the system clock hardware device. It fires at regular intervals and is used to maintain the system time, schedule tasks, and perform other time-related operations.
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Keyboard Interrupt: A keyboard interrupt is an interrupt triggered by a keyboard hardware device. When the user presses a key on the keyboard, the keyboard controller will send an interrupt signal to the processor, and then the processor will execute the corresponding interrupt handler and pass the key information to the operating system.
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Network Interrupt: A network interrupt is an interrupt triggered by a network interface card (NIC) hardware device. When a network data packet arrives at the network card, the network card will trigger an interrupt and pass the data packet to the processor for processing, such as processing the network protocol stack or passing the data packet to the application program.
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Disk Interrupt: A disk interrupt is an interrupt triggered by the hard disk controller. When the disk operation is completed or an error occurs, the hard disk controller will send an interrupt signal to the processor, and the processor will execute the corresponding interrupt handler to process the result of the disk operation or perform error handling.
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USB Interrupt: A USB interrupt is an interrupt triggered by a USB device. When the USB device needs to communicate with the host or transmit data, it will trigger an interrupt, pass the related interrupt request to the processor, and the processor will execute the corresponding interrupt handler.
These are examples of common hardware interrupts, and different types of hardware devices may trigger different types of interrupts. The Linux kernel manages and handles these interrupt events through the interrupt controller to ensure timely response and proper processing.
tasklet
In computer science, a Tasklet (task subroutine) is a lightweight software mechanism for implementing asynchronous event processing. It is commonly used in real-time operating systems and embedded systems to handle events that require fast response.
Tasklet can be regarded as a software interrupt handling mechanism. It is similar to an interrupt handler, but is more lightweight than an interrupt handler. Tasklets are not directly associated with hardware interrupts like interrupt handlers are, but are explicitly scheduled and executed by software.
Tasklets can be executed in kernel context, they do not cause a process switch. This makes Tasklets useful for handling events that require a quick response, since their execution time is relatively short and does not block the execution of other processes or threads.
Tasklets are usually used to handle hardware interrupts, network packet reception, timer events, etc. They are designed as small, fast-executing functions that handle certain types of events. When an event occurs, a Tasklet can be dispatched and execute the corresponding processing code.
In summary, Tasklet is a lightweight software mechanism for asynchronous event processing. It is widely used in real-time operating systems and embedded systems to handle events that require fast response without causing process switching.
The following is a simplified Linux kernel code example showing how to use Tasklets in the kernel.
#include <linux/module.h>
#include <linux/init.h>
#include <linux/interrupt.h>
static void my_tasklet_handler(unsigned long data);
DECLARE_TASKLET(my_tasklet, my_tasklet_handler, 0);
// Tasklet 处理函数
static void my_tasklet_handler(unsigned long data)
{
printk(KERN_INFO "Tasklet executed!\n");
}
static int __init tasklet_init(void)
{
// 调度 Tasklet
tasklet_schedule(&my_tasklet);
printk(KERN_INFO "Tasklet module loaded.\n");
return 0;
}
static void __exit tasklet_exit(void)
{
// 停止 Tasklet
tasklet_kill(&my_tasklet);
printk(KERN_INFO "Tasklet module unloaded.\n");
}
module_init(tasklet_init);
module_exit(tasklet_exit);
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Tasklet Module");
In the above code, a my_taskletTasklet variable named is first declared to represent the Tasklet.
my_tasklet_handlerThen, a Tasklet handler named is defined . When the Tasklet is scheduled for execution, this function is called and a message is printed.
In tasklet_initthe function, schedule the execution of the Tasklet by calling tasklet_schedulethe function.
In tasklet_exitthe function, call tasklet_killthe function to stop the Tasklet.
Finally, define the module's initialization and exit functions using module_initthe and module_exitmacros , and define the module's license and description information using MODULE_LICENSEthe and MODULE_DESCRIPTIONmacros .
Please note that the execution of Tasklet is completed in the interrupt context, so you need to pay attention to follow the requirements of the interrupt context in the processing function, and avoid using operations that may cause sleep. Additionally, Tasklets are a lightweight mechanism in the kernel for asynchronous event handling without causing process switching. In the actual development, it is also necessary to consider the lock mechanism and synchronization issues to ensure the correctness and reliability of Tasklet.
workqueue
Workqueue (work queue) is a mechanism in the operating system for asynchronous execution of tasks that have a long delay or need to be processed for a long time. It is a queue for scheduling and executing background work.
Workqueue is usually used to handle some non-real-time tasks, such as background data processing, timing tasks, asynchronous I/O, etc. Compared with tasks with higher real-time requirements, Workqueue can more flexibly schedule and execute these tasks without blocking the execution of the main thread or other critical tasks.
The basic principle of Workqueue is to add tasks to the queue, and then the worker thread (worker thread) will take the tasks out of the queue and execute them. These worker threads are usually threads in the thread pool pre-created by the system, and they will continuously obtain tasks from the queue and execute them until the queue is empty.
Workqueue provides a mechanism for executing tasks asynchronously, so that the execution of tasks can be performed in parallel with the main thread or other tasks. It can improve the response performance and throughput of the system, especially suitable for scenarios that need to process a large number of time-consuming tasks.
In the Linux kernel, there are two types of Workqueue: System Workqueue and High Priority Workqueue. System Workqueue is used to handle general background tasks, while High Priority Workqueue is used to handle some higher priority tasks.
To sum up, Workqueue is a mechanism in the operating system for asynchronously executing tasks that have a long delay or require a long time to process. It implements the background processing of tasks by adding tasks to the queue, and taking them out of the queue and executing them by the worker thread, which improves the response performance and throughput of the system.
timer interrupt
Timer interrupt refers to the use of timers in computer systems to generate periodic interrupt signals. It is a hardware mechanism used to periodically trigger the execution of an interrupt handler.
Timers in computer systems are usually provided by the system clock or dedicated hardware timers. The principle of the timer interrupt is that within the set time interval, the timer will count or decrement. When the counter reaches the preset threshold, an interrupt signal will be triggered to notify the processor to execute the corresponding interrupt handler.
Timer interrupts are widely used in operating systems, such as:
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Time slice round-robin scheduling: The operating system uses timer interrupts to divide time slices. When the time slice of a process runs out, the timer interrupt will be triggered, and the operating system will perform context switching and switch to the next ready process for execution.
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Timing task scheduling: The operating system can use timer interrupts to trigger periodic task execution, such as refreshing the screen at regular intervals, sending heartbeat packets at regular intervals, and so on.
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Real-time task processing: In real-time operating systems, timer interrupts can be used to precisely control the execution time of tasks to meet real-time requirements.
The timer interrupt can provide the system time base, so that the operating system and application programs can perform various operations and scheduling according to the time. By reasonably setting the time interval of the timer interrupt, the response performance and resource utilization of the system can be balanced.
The following is a simplified Linux kernel code example showing how timer interrupts are used in the kernel.
#include <linux/module.h>
#include <linux/init.h>
#include <linux/timer.h>
static struct timer_list my_timer;
// 定时器中断处理函数
static void timer_handler(struct timer_list *t)
{
printk(KERN_INFO "Timer expired!\n");
// 重新设置定时器
mod_timer(&my_timer, jiffies + msecs_to_jiffies(1000));
}
static int __init timer_init(void)
{
// 初始化定时器
timer_setup(&my_timer, timer_handler, 0);
// 设置定时器的超时时间
my_timer.expires = jiffies + msecs_to_jiffies(1000);
// 启动定时器
add_timer(&my_timer);
printk(KERN_INFO "Timer module loaded.\n");
return 0;
}
static void __exit timer_exit(void)
{
// 停止定时器
del_timer(&my_timer);
printk(KERN_INFO "Timer module unloaded.\n");
}
module_init(timer_init);
module_exit(timer_exit);
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Timer Module");
In the above code, a global variable named of type is first defined my_timerto struct timer_listrepresent the timer.
Then, a function named is defined timer_handleras the timer interrupt processing function. When a timer interrupt occurs, this function will be called and a message will be printed. In the processing function, the timeout time of the timer is also reset to realize regular periodic triggering.
In timer_initthe function, the timer is initialized by calling timer_setupthe function, and timer_handlerthe function is passed in as the processing function. Then, set the timeout time of the timer to 1 second, and call add_timerthe function to start the timer.
In timer_exitthe function, call del_timerthe function to stop the timer.
Finally, define the module's initialization and exit functions using module_initthe and module_exitmacros , and define the module's license and description information using MODULE_LICENSEthe and MODULE_DESCRIPTIONmacros .
Note that this is just a simplified example, real kernel timer interrupts may involve more complex processing and configuration. In actual development, issues such as lock mechanism and context switching also need to be considered to ensure the correctness and reliability of timer interrupts.