Linux Kernel Process Management

1. Introduction to Process Management in Linux

Process management is one of the core responsibilities of the Linux kernel. It involves:

• Creating processes

• Scheduling them on the CPU

• Managing their execution

• Handling their termination

Linux follows a simple yet powerful design philosophy:

The kernel uses one unified abstraction—the task—to represent both processes and threads.

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2. The Process Abstraction in Linux

In Linux:

• A process is an instance of a program in execution

• A thread is simply a process that shares resources

Both are represented by the same kernel data structure:

struct task_struct

This unified design simplifies kernel logic and improves scalability.

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3. The task_struct: Process Descriptor

Every process in Linux is described by a process descriptor, the task_struct.

Key Information Stored in task_struct

• Process state

• Process ID (PID)

• Scheduling information

• Parent and child relationships

• Virtual memory information

• Open files

• Signal handlers

• Kernel stack pointer

📌 Each running task in the system has exactly one task_struct.

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4. Process States in Linux

Linux defines several process states (more detailed than the classic 3-state model).

State	Meaning
TASK_RUNNING	Running or ready to run
TASK_INTERRUPTIBLE	Sleeping, can be woken by signals
TASK_UNINTERRUPTIBLE	Sleeping, cannot be interrupted
TASK_STOPPED	Execution stopped (e.g., via signal)
TASK_TRACED	Being traced by debugger
EXIT_ZOMBIE	Process has exited but not reaped
EXIT_DEAD	Process is being cleaned up

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5. Process Identification: PID and TGID

PID (Process Identifier)

• Unique identifier for each task

• Assigned when the process is created

TGID (Thread Group ID)

• Used to group threads belonging to the same process

• All threads in a process share the same TGID

• The thread group leader’s PID = TGID

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6. Process Creation in Linux

fork() – Creating a Process

Linux implements fork() efficiently using Copy-On-Write (COW):

• Parent and child share memory pages

• Pages are copied only when modified

Steps:

1. Allocate new task_struct

2. Duplicate parent context

3. Assign new PID

4. Share address space initially

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vfork() – Special Case

• Child shares address space with parent

• Parent is blocked until child exits or execs

• Faster but dangerous if misused

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clone() – Generalized Creation Mechanism

clone() is the most powerful creation system call:

• Used internally by fork()

• Used to implement threads

The behavior depends on flags specifying what is shared:

• Memory

• Files

• Signals

• Filesystem context

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7. Process Termination

Normal Termination

• Process calls exit()

• Kernel releases resources

• Exit code is stored

Zombie State

• Process becomes a zombie

• task_struct remains to hold exit status

• Parent must call wait()

Orphan Processes

• If parent exits first

• Child is adopted by init (PID 1)

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8. Parent–Child Relationships

Linux maintains:

• Parent pointer

• Children list

• Sibling links

This forms a process hierarchy rooted at init.

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9. Context Switching

A context switch occurs when:

• One process stops running

• Another process starts

During a context switch:

• CPU registers are saved

• Kernel stack is switched

• Scheduler selects next task

Linux minimizes overhead by:

• Using per-task kernel stacks

• Sharing address spaces for threads

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10. Kernel Threads

What Are Kernel Threads?

• Run entirely in kernel space

• No user address space

• Used for background tasks

Examples:

• Memory management

• I/O handling

• Scheduler helpers

Kernel threads are created using:

kthread_create()

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11. Scheduling and Process Management

Although scheduling is covered in later chapters, Chapter 3 establishes:

• Linux schedules tasks, not processes

• Each task has scheduling attributes

• Supports SMP systems efficiently

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12. Signals and Process Control

Signals are used to:

• Control process execution

• Notify events

• Handle errors

Examples:

• SIGKILL

• SIGSTOP

• SIGCHLD

Signals are handled via structures referenced from task_struct.

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13. Why Linux Process Management Is Efficient

Robert Love emphasizes:

• Unified process/thread model

• Minimal abstractions

• Strong SMP support

• Clean separation of concerns

Linux avoids:

• Heavyweight process duplication

• Separate thread schedulers

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14. Summary Table

Aspect	Linux Design
Process abstraction	task_struct
Thread distinction	None (resource sharing)
Creation mechanisms	fork(), vfork(), clone()
Scheduling unit	Task
Thread model	One-to-one
Parent–child model	Hierarchical

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15. Key Takeaways 

• Linux uses a single task abstraction

• Threads are processes that share resources

• Process creation is optimized via COW

• clone() underpins both processes and threads

• Efficient process management is central to Linux’s performance