1. Processes David Ferry, Chris Gill
CSE 422S - Operating Systems Organization
Washington University in St. Louis
St. Louis, MO 63143

2. CSE 422S –Operating Systems Organization
Processes in Linux
Fundamental process abstraction:
Original: Each process behaves as though it has total control over the system
Modern: Threads still execute as though they monopolize the system, but the process abstraction also facilitates multi-threading, signals, and inter-process communication
More features:
Multi-threading
Scheduling
Shared memory
Virtual Memory
Sockets
Process accounting
etc.
Signals
Process groups
Synchronization
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3. What is a Process, Really?
A process is an execution stack together with data that describes the process state. (also describes threads)
Two stacks:
Kernel mode
User mode
Data:
thread_info: small, efficient
task_struct: large, statically allocated by slab allocator, points to other data structs
User side:
Kernel side:
User Stack
thread_info
Kernel stack
task_struct
.bss
.data
.text
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4. Creating a New Process: fork() and clone()
Duplicate existing process
Initialize per-PID data
PID, real_parent, statistics
If new process:
Copies file handles, signal handlers, process address space, etc.
Resources shared via page-level lazy copy-on-write*
If new thread:
Shares pointers to above resources
thread_info
Kernel stack
task_struct
ptr* 0x1
ptr* 0x2
ptr* 0x3
thread_info
Kernel stack
task_struct
ptr* 0x1
ptr* 0x2
ptr* 0x3
*E.g. Every process in the system shares the same “copy” of the C library
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Process States
Alive states:
TASK_RUNNING: running or runnable
TASK_INTERRUPTIBLE: blocked on wait queue
TASK_UNINTERRUPTIBLE: short duration blocking, can’t receive signals
__TASK_TRACED: being traced with syscall ptrace
__TASK_STOPPED: cannot execute (SIGSTOP, debugger)
Exit states:
EXIT_ZOMBIE: exited but has not been waited for (kernel structures still exit)
EXIT_DEAD: exited and has been waited for
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6. Handling Processes in Linux
Linux uses many containers to ensure efficient access to processes:
Linked list of all processes
Lists of child processes
PID hash table
Processor run queues
Blocking I/O wait queues
Helper functions:
for_each_process()
list_for_each(list, task->children)
task_struct
ptr* 0x1
ptr* 0x2
ptr* 0x3
task_struct
ptr* 0x1
ptr* 0x2
ptr* 0x3
task_struct
ptr* 0x1
ptr* 0x2
ptr* 0x3
task_struct
ptr* 0x1
ptr* 0x2
ptr* 0x3
task_struct
ptr* 0x1
ptr* 0x2
ptr* 0x3
task_struct
ptr* 0x1
ptr* 0x2
ptr* 0x3
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7. CSE 422S – Operating Systems Organization
One Size Fits All
Processes, threads, and kernel threads are all implemented with the same data structures (thread_info and task_struct) and functions.
C has limited classes / polymorphism
Logic and code for handling all runnable objects is the same
Threads are like user processes but they share their parent’s address space.
Kernel threads don’t have a user-side address space (i.e. mm_struct pointer is NULL) and not all data values are used.
CSE 422S – Operating Systems Organization