1. CSC 660: Advanced Operating Systems
CSC 660: Advanced OS
Processes
CSC 660: Advanced Operating Systems

2. CSC 660: Advanced Operating Systems
Topics
What is a Process?
Task Descriptor
Process Lifecycle
Process Address Space
Context Switch
Linked lists
Creation
fork() and clone()
Process Relationships
Termination
Zombies
CSC 660: Advanced Operating Systems

3. CSC 660: Advanced Operating Systems
What is a process?
A process is a program in execution.
Program code + dynamic execution context.
Virtualization
Processes provide virtual CPU + virtual memory.
Kernel refers to processes as tasks.
CSC 660: Advanced Operating Systems

4. CSC 660: Advanced Operating Systems
What is in a process?
A process consists of.
Program code.
Address space.
Data.
Resources: open files, signals.
At least one thread of execution.
Threads contain:
Program counter.
Stack.
Register set.
CSC 660: Advanced Operating Systems

5. Process Control Block: task_struct
Diagram from Understanding the Linux Kernel, 3rd edition, Chapter 3
task_struct declared in include/linux/sched.h
CSC 660: Advanced Operating Systems

6. CSC 660: Advanced Operating Systems
Process Identity
Kernel: Address of task_struct
current macro
User space: PID
current->pid
32768 possible PIDs
pidhash
CSC 660: Advanced Operating Systems

7. CSC 660: Advanced Operating Systems
Process Lifecycle
CSC 660: Advanced Operating Systems

8. CSC 660: Advanced Operating Systems
Process State
TASK_RUNNING
Process is ready or running.
TASK_INTERRUPTIBLE
Waiting, able to receive interrupts.
TASK_UNINTERRUPTIBLE
Waiting, unable to receive interrupts.
TASK_STOPPED
Execution stopped, can resume with SIGCONT.
TASK_TRACED
Execution stopped by the ptrace() debugging mechanism.
EXIT_ZOMBIE
Execution terminated, but parent hasn’t wait()ed.
EXIT_DEAD
Being removed from system as parent has wait()ed.
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9. CSC 660: Advanced Operating Systems
Process Address Space
Process appears to have flat memory space.
Discontinguous areas of physical memory mapped via page table to produce virtual memory.
Described by mm_struct
Virtual memory space
3GB virtual memory on 32-bit architectures.
High 1GB reserved for mapping kernel memory.
CSC 660: Advanced Operating Systems

10. 32-bit Process Address Space
0xBFFFFFFF
stack
ESP
heap
bss
data
text
EIP 0x
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11. CSC 660: Advanced Operating Systems
Context Switch
When does scheduler switch out process?
Blocked on I/O.
Time slice expires.
Where is switching done?
In schedule() function.
How does it switch?
Switch to new page table.
Save hardware context to stack.
Switch stack pointers.
Load hardware context of next process from stack.
CSC 660: Advanced Operating Systems

12. Context Switch Example
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13. CSC 660: Advanced Operating Systems
Your own task_struct
How to find your task_struct?
1. stack pointer points to top of kernel stack
2. thread_info stored at bottom of kernel stack
3. mask 13 lsb’s of stack pointer to get address
4. thread_info has pointer to task_struct
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14. CSC 660: Advanced Operating Systems
Your own task_struct
CSC 660: Advanced Operating Systems

15. CSC 660: Advanced Operating Systems
Kernel linked lists
Circular doubly-linked list.
No special first or last node.
Defined in <linux/list.h>
struct list_head {
struct list_head *next, *prev; }
Use by embedding in structures:
struct my_struct {
struct list_head list;
void *data;
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Kernel linked lists
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Kernel Linked Lists
struct my_struct *p;
struct list_head *new, *x, *my;
INIT_LIST_HEAD(&p->list)
Create a new linked list of my_struct’s.
list_add(new, p)
Add new list_head to list after p.
list_del(struct list_head *x)
Remove structure containing list_head pointer x.
list_for_each(p, &my->list)
Iterate over list starting with my, with p as pointer to current element.
list_entry(p, struct my_struct, list)
Return pointer to my_struct whose list_head is p.
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18. CSC 660: Advanced Operating Systems
Process Lists
#define next_task(p) list_entry((p)->tasks.next, struct task_struct, tasks)
#define prev_task(p) list_entry((p)->tasks.prev, struct task_struct, tasks)
#define for_each_process(p) \
for (p = &init_task ; (p = next_task(p)) != &init_task ; )
include/linux/sched.h
CSC 660: Advanced Operating Systems

19. Where do processes come from?
Kernel creates some processes.
Kernel threads.
init
Processes create all other processes.
Process copies itself with fork().
Original is parent, new process is child.
Child can load its own program with exec().
CSC 660: Advanced Operating Systems

20. Process Creation and Termination
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21. CSC 660: Advanced Operating Systems
fork()
System call creates a new process
Creates a new task_struct.
Initializes with copy of parent resources.
Creates a new address space.
Page table is copy-on-write pointer to parent.
Places child process on ready queue.
Returns in both parent and child.
Parent return value is PID of child process.
Error return value of -1 indicates no child.
Child return value is 0.
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22. CSC 660: Advanced Operating Systems
clone()
Threads are just a type of process.
Share address space, files, signal handlers.
All processes/threads created by clone()
Process: clone(SIGCHLD, 0)
Thread: clone(CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND, 0);
CSC 660: Advanced Operating Systems

23. Process Relationships
real_parent
Process that created p or process 1 (init).
parent
Process to be signaled on termination.
If ptrace(), is debugging process, else real_parent.
children
List of all children created by p.
sibling
Pointers to next and previous elements of sibling list.
CSC 660: Advanced Operating Systems

24. Process Relationships
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25. Process Tree (Solaris)
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Process Termination
Voluntary
exit() system call.
C compiler automatically adds to binaries.
Involuntary
signal
exception
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do_exit()
Sets PF_EXITING flag.
Releases resources:
Address space.
File handles.
Signal handlers and timers.
Sets exit_code member of task_struct.
Notifies parent of termination (SIGCHLD)
Reparents any children of process.
Set process state to EXIT_ZOMBIE.
Calls schedule() to switch to new process.
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28. CSC 660: Advanced Operating Systems
The Undead
Why do we need zombies?
Must keep task_struct so parents can get exit_code member.
Parent issues wait() system call to get exit.
If parent predeceases child, init will be parent.
Calls release_task() to free resources.
Removes process from lists, hashes, caches.
Frees kernel stack and thread_info struct.
Frees task_struct.
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29. CSC 660: Advanced Operating Systems
Key Points
Process = program + execution context
Execution context stored in PCB (task_struct.)
Tasks organized in a linked list.
Referenced by PID from user space.
Process provides virtual CPU + memory.
Scheduler manages context switches.
Processes and threads created by clone().
Parent/child relationship.
Kernel reclaims resources on exit().
CSC 660: Advanced Operating Systems

30. CSC 660: Advanced Operating Systems
References
Daniel P. Bovet and Marco Cesati, Understanding the Linux Kernel, 3rd edition, O’Reilly, 2005.
Robert Love, Linux Kernel Development, 2nd edition, Prentice-Hall, 2005.
Claudia Rodriguez et al, The Linux Kernel Primer, Prentice-Hall, 2005.
Peter Salzman et. al., Linux Kernel Module Programming Guide, version 2.6.1, 2005.
Avi Silberchatz et. al., Operating System Concepts, 7th edition, 2004.
Andrew S. Tanenbaum, Modern Operating Systems, 3nd edition, Prentice-Hall, 2005.
CSC 660: Advanced Operating Systems