Summary

Processes have to share the processor with other processes and define individual contexts of execution that hold all the information necessary to run the process. In the course of their execution processes, they go through various states that can be abstracted into blocked states, running states, and ready-to-be-run states.

The kernel stores information regarding tasks in a task_struct descriptor. The task_struct fields can be split up according to different functions that involve the process, including process attributes, process relationships, process memory access, process-related file management, credentials, resource limits, and scheduling. All these fields are necessary to keep track of the process context. A process can be composed of one or more threads that share the memory address space. Each thread has its own structure.

Process creation comes about with a call to one of fork(), vfork(), or clone() system calls. All three system calls end up calling the kernel routine do_fork(), which performs the bulk of the new process creation. During execution, a process goes from one state to another. A process goes from a ready state to a running state by way of scheduler selection, from a running state to a ready state if its timeslice ends or if it yields to another process, from a blocked state to a ready state if an awaited signal comes in, and from running state to a blocked state when awaiting a resource or when sleeping. Process death comes about with a call to the exit() system call.

We then delved into the basics of scheduler construction and the structures it uses, including the run queues and wait queues, and how it manages these structures to keep track of how processes are to be scheduled.

This chapter closed with a discussion of the asynchronous flows of process execution, which include exceptions and interrupts, by looking at how the x86 and the PPC hardware handle interrupts. We explored how the Linux kernel manages an interrupt after the hardware delivers it by using the system timer interrupt as an example.