1. The kernel’s task list
Introduction to process descriptors and their related data-structures for Linux kernel version

2. Multi-tasking
Modern operating systems allow multiple users to share a computer’s resources
Users are allowed to run multiple tasks
The OS kernel must protect each task from interference by other tasks, while allowing every task to take its turn using some of the processor’s available time

3. Stacks and task-descriptors
To manage multitasking, the OS needs to use a data-structure which can keep track of every task’s progress and usage of the computer’s available resourcres (physical memory, open files, pending signals, etc.)
Such a data-structure is called a ‘process descriptor’ – every active task needs one
Every task needs its own ‘private’ stack

4. What’s on a program’s stack?
Upon entering ‘main()’:
A program’s exit-address is on user stack
Command-line arguments on user stack
Environment variables are on user stack
During execution of ‘main()’:
Function parameters and return-addresses
Storage locations for ‘automatic’ variables

5. Entering the kernel… A user process enters ‘kernel-mode’:
when it decides to execute a system-call
when it is ‘interrupted’ (e.g. by the timer)
when ‘exceptions’ occur (e.g. divide by 0)

6. Switching to a different stack
Entering kernel-mode involves not only a ‘privilege-level transition’ (from level 3 to level 0), but also a stack-area ‘switch’
This is necessary for robustness:
e.g., user-mode stack might be exhausted
This is desirable for security:
e.g, privileged data might be accessible

7. What’s on the kernel stack?
Upon entering kernel-mode:
task’s registers are saved on kernel stack
(e.g., address of task’s user-mode stack)
During execution of kernel functions:
Function parameters and return-addresses
Storage locations for ‘automatic’ variables

8. Supporting structures
So every task, in addition to having its own code and data, will also have a stack-area that is located in user-space, plus another stack-area that is located in kernel-space
Each task also has a process-descriptor which is accessible only in kernel-space

9. A task’s virtual-memory layout
Process descriptor
and
kernel-mode stack
Kernel space
User space
Privilege-level 0
User-mode stack-area
Privilege-level 3
Task’s code and data

10. Something new in 2.6 Linux uses part of a task’s kernel-stack
page-frame to store ‘thread information’
The thread-info includes a pointer to the task’s process-descriptor data-structure
Task’s kernel-stack
struct task_struct
Task’s
process-descriptor
Task’s thread-info
kernel page-frame

11. Tasks have ’states’ From kernel-header: <linux/sched.h>
#define TASK_RUNNING 0
#define TASK_INTERRUPTIBLE 1
#define TASK_UNINTERRUPTIBLE 2
#define TASK_ZOMBIE 4
#define TASK_STOPPED 8

12. Fields in a process-descriptor
struct task_struct {
volatile long state;
struct thread_into *thread_info;
unsigned long flags;
struct mm_struct *mm;
pid_t pid;
char comm[16];
/* plus many other fields */ };

13. Finding a task’s ‘thread-info’
During a task’s execution in kernel-mode, it’s very quick to find that task’s thread-info object
Just use two assembly-language instructions:
movl $0xFFFFF000, %eax
andl %esp, %eax
Ok, now %eax = the thread-info’s base-address
There’s a macro that implements this computation

14. Finding the task-descriptor
Use a macro ‘current_thread_info()’ to get a pointer to the ‘thread_info’ structure:
struct thread_info *info = current_thread_info();
Then one more step gets you the address of the task’s process-descriptor:
struct task_struct *task = info->task;
You can also use ‘current’ to perform this two-step assignment: task = current;

15. Parenthood New tasks get created by calling ‘fork()’
Old tasks get terminated by calling ‘exit()’
When ‘fork()’ is called, two tasks return
One task is known as the ‘parent’ process
And the other is called the ‘child’ process
The kernel keeps track of this relationship

16. A parent can have many children
If a user task calls ‘fork()’ twice, that will create two distinct ‘child’ processes
These children are called ‘siblings’
Kernel track of all this with lists of pointers

17. Parenthood relationships
See “Linux Kernel Programming”
(Chapter 3) for additional details

18. The kernel’s ‘task-list’
Kernel keeps a list of process descriptors
A ‘doubly-linked’ circular list is used
The ‘init_task’ serves as a fixed header
Other tasks inserted/deleted dynamically
Tasks have forward & backward pointers, implemented as fields in the ‘tasks’ field
To go forward: task = next_task( task );
To go backward: task = prev_task( task );

19. Doubly-linked circular list
next_task
init_task
(pid=0)
newest
task …
prev_task

20. Demo
We can write a module that lets us create a pseudo-file (named ‘/proc/tasklist’) for viewing the list of all currently active tasks
Our ‘tasklist.c’ module shows the name and process-ID of each task, along with the task’s current state
Use the command: $ cat /proc/tasklist

21. In-class exercise #1
Different versions of the 2.6 Linux kernel use slightly different definitions for these task-related kernel data-structures (e.g, our kernel uses a smaller-sized ‘thread-info’ structure than did)
So can you write an installable kernel module that will tell you:
the size of a ‘task_struct’ object (in bytes)?
the size of a ‘thread_info’ object (in bytes)?

22. ‘Kernel threads’
Some tasks don’t have a page-directory of their own – because they don’t need one
They can just ‘borrow’ the page-dirtectory that belongs to another task
These ‘kernel thread’ tasks will have an NULL value (i.e., zero) stored in the ‘mm’ field of their ‘task_struct’ descriptor

23. In-class exercise #2
Can you modify our ‘tasklist.c’ module so it will display a list of only those tasks which are kernel threads?