interrupts

Objective: Understand how Linux processes interrupts

When you press a button on your keyboard, what happens?

Why is the computer able to react right away?

Motivation

Why not just poll for new input?

Definition

An interrupt is a signal to the CPU to stop what
it's doing and take care of urgent business

Most important thing

1. Speed

2. Also important: Atomicity

General idea of interrupts

Something needs to be done urgently

1. Otherwise we would use polling

General idea of interrupts

First, we take care of the most urgent parts

1. This is the top-half

2. Schedule less urgent action

General idea of interrupts

Second, we take care of the less urgent parts

1. This is the bottom-half

demo

Take a look at /proc/interrupts

Two main types

1. Hardware interrupts

2. Software interrupts

Terminology

Trap can refer to

1. any interrupt

2. any software interrupt

3. any synchronous software interrupt

4. any interrupt caused by "trap" instruction

May even refer to debugger breakpoints

Exceptions

Synchronous software interrupts are generally referred to as exceptions

"Exceptional" CPU conditions

arm64 terminology

1. exception: any type of interrupt

2. interrupt: hardware interrupt

3. abort: prefetch or data abort (page fault)

   1. like an x86 exception

4. reset: reinit the processor state

5. Some instructions are exception-generating instructions

x86_64 terminology

interrupts: hardware interrupts

exceptions: software interrupts

1. fault: return to save instruction

2. trap: return to next instruction

3. aborts: don't return to program

demo

Tracing interrupts with bpftrace

Kernel execution context

Previously:

1. userspace context

2. kernelspace context

Kernel execution context

process context: kernel running attached to a process

1. current refers to running process' struct task_struct

Kernel execution context

interrupt context: kernel running NOT attached to a process

1. No sleeping

Top Half: Interrupt Service Routine (ISR)

1. Runs in interrupt context

2. No sleeping/blocking

3. Interrupts disabled

Bottom half: softirq

1. Runs in interrupt context

2. Starts immediately after the ISR

3. Interrupts enabled

Bottom half: tasklet

1. Runs in interrupt context

2. Based on softirq, but can be deferred

3. Interrupts enabled

Bottom half: workqueue

1. Runs in process context

2. Can sleep

3. For more extensive processing

demo

Look at ksoftirqd in ps

demo

A look at include/linux/interrupt.h

demo

A simple example module

demo

Use bpftrace to see the kernel stack and dive into the code

End