How to do Linux Kernel Development?

Linux kernel development is a complex and specialized field that involves contributing to the core of the Linux operating system. The Linux kernel is the heart of the operating system, responsible for managing hardware, and system resources, and providing essential services to applications. In this article we will discuss how to do Linux kernel development so let's follow.

Linux Kernel

Linux kernel is the core component of the Linux operating system. It is an open-source monolithic Unix-like kernel designed by Linus Torvalds and first released on September 17, 1991. The kernel's primary role is to manage system resources, interact with hardware devices, and provide a platform for other software components to run on top of it.

Key features and characteristics of the Linux kernel include:

Open Source:
The Linux kernel is released under the GNU General Public License (GPL), which means its source code is freely available for anyone to view, modify, and distribute.

Monolithic Kernel:
Linux follows a monolithic kernel architecture, where most essential services and functionalities run as part of the kernel itself. This differs from microkernels, where only essential functionalities run in the kernel space, and other services are implemented as user-space processes.

Hardware Abstraction:
The kernel provides an abstraction layer between hardware devices and higher-level software. This abstraction allows developers to write device drivers that interact with the kernel, enabling seamless support for various hardware components.

Process Management:
The kernel manages processes and threads, scheduling their execution on the CPU, and allocating system resources like memory and I/O.

Memory Management:
Linux's kernel handles virtual memory management, including memory allocation, page swapping, and memory protection, ensuring that each process has its isolated address space.

File System Support:
Linux supports various file systems, such as ext4, Btrfs, XFS, and many more, allowing users to store and organize data on different storage devices.

Networking:
The kernel provides networking support for various protocols like TCP/IP, UDP, and more. It enables communication between processes and facilitates network connectivity.

Security:
Linux kernel incorporates security features such as access control lists (ACLs), capabilities, namespaces, and mandatory access control (MAC) frameworks to enhance system security.

Modularity and Loadable Kernel Modules:
Although Linux is a monolithic kernel, it allows developers to extend its functionalities by loading additional code as loadable kernel modules, enhancing the kernel's capabilities without the need to recompile the whole kernel.

Community-Driven Development:
The Linux kernel development is a collaborative effort involving thousands of developers worldwide. It is managed through mailing lists, version control systems (e.g., Git), and code review processes.

The Linux kernel is at the heart of numerous Linux-based operating systems, including popular distributions like Ubuntu, Fedora, Debian, and CentOS, among others. Its flexibility, stability, and the support of a massive developer community have made Linux one of the most widely used and influential operating systems, powering everything from personal computers to servers, mobile devices, embedded systems, and more.

Release Cycle

The release cycle refers to the process and schedule followed by software projects, including the Linux kernel, for planning, developing, and distributing new versions or updates. Each release cycle typically has a set of phases that include development, testing, bug fixing, and release. The specific details of the release cycle can vary depending on the project and its development model. Here's an overview of the Linux kernel's release cycle:

Development Phase:
The development of the Linux kernel is an ongoing process, with contributions coming from a large community of developers. New features, improvements, and bug fixes are continuously proposed and integrated into the kernel's mainline codebase. Development takes place in the "mainline" repository, hosted on sites like kernel.org.

Merge Window:
The merge window is a specific period, usually two weeks, at the beginning of each development cycle when Linus Torvalds, the creator and principal maintainer of the Linux kernel, accepts new changes and features into the mainline kernel. During this time, subsystem maintainers submit their changes, and Linus merges them into the mainline.

Stabilization Phase:
After the merge window, the kernel enters a stabilization phase, also known as the "release candidate" (RC) phase. During this phase, the focus shifts from adding new features to stabilizing the code and fixing bugs. A series of release candidates (RC1, RC2, etc.) are issued for testing and bug hunting.

Testing and Bug Fixing:
The release candidates are thoroughly tested by the community on various hardware and software configurations. Developers and testers report bugs, and the kernel maintainers work to fix them. This iterative process continues until the kernel reaches a stable state.

Release:
Once the kernel is deemed stable and all critical issues are addressed, the final version is released. The release is denoted by a version number, such as 5.0, 5.10, 5.15, etc. New releases typically happen every two to three months.

Long-Term Support (LTS) Releases:
In addition to the regular releases, certain versions of the Linux kernel are designated as Long-Term Support (LTS) releases. These versions receive extended maintenance and bug fixes for a more extended period, typically two to six years. LTS releases are favored by distributions targeting enterprise and embedded systems, providing them with stable and well-maintained kernels.

Continuous Development:
The release cycle is a continuous process, and as one development cycle completes, the next one starts with the introduction of new features and improvements.

The release cycle of the Linux kernel is tightly managed to maintain a balance between incorporating new features and ensuring stability. This iterative approach allows for a steady stream of improvements and innovations while maintaining the reliability and performance that makes Linux such a widely used and respected operating system.

Some Terminologies You Should Know

Here are some terminologies commonly used in Linux kernel development:

Kernel:
The core component of the Linux operating system that manages system resources, and hardware devices, and provides essential services to user-space applications.

Source Code:
The human-readable form of a program written in a programming language like C, which developers modify and compile to create executable binaries.

Patch:
A file containing changes to the source code that can be applied to the original codebase to modify its behavior or fix issues.

Mainline:
The main development branch of the Linux kernel, where new features and changes are initially integrated before being released.

Merge Window:
A specific period during the development cycle when new features and changes are accepted into the mainline kernel.

Subsystem:
A specific functional area of the kernel, such as networking, file systems, memory management, etc., each maintained by dedicated developers.

Maintainer:
A developer responsible for managing and reviewing the changes to a specific subsystem within the kernel.

Device Driver:
Software that allows the kernel to communicate and interact with hardware devices.

Kernel Panic:
A critical error that occurs in the kernel, resulting in the system halting to prevent potential data corruption or further damage.

Module:
A loadable piece of kernel code that can be dynamically loaded or unloaded without recompiling the entire kernel.

Sysfs:
A virtual file system in Linux that exposes kernel objects and their attributes to user space, allowing users to configure and monitor kernel settings.

Kconfig:
A configuration system used in the Linux kernel to enable or disable specific features and drivers during the build process.

Makefile:
A file containing instructions for building the kernel and its modules using the make-build automation tool.

Initramfs:
An initial RAM file system used during the boot process to load necessary kernel modules and utilities before mounting the actual root file system.

Linux Kernel Mailing List (LKML):
The primary communication channel for Linux kernel development, where patches, discussions, and announcements are sent and discussed.

GCC (GNU Compiler Collection):
The compiler used to compile the Linux kernel source code.

GNU General Public License (GPL):
The open-source license under which the Linux kernel is distributed, allowing users to view, modify, and distribute the source code.

These are just a few examples of the many terms used in Linux kernel development. The Linux kernel community has a rich and diverse set of terminologies that reflect the complexity and depth of kernel development.

Developers, Maintainers in Linux Kernel

In the Linux kernel development community, various roles and responsibilities are carried out by different individuals. Some of the key roles include:

Developers:
Developers are individuals who contribute code to the Linux kernel. They write new features, fix bugs, improve performance, and work on various subsystems of the kernel. Developers can be individuals working independently or as part of organizations contributing to the kernel.

Maintainers:
Maintainers are experienced developers responsible for specific subsystems within the Linux kernel. Each subsystem has its dedicated maintainer or a group of maintainers. They review and accept patches and changes related to their subsystem, ensuring the code adheres to the kernel's coding standards and design principles. Maintainers also work closely with developers, providing feedback and guidance.

Submaintainers:
In some cases, especially for large subsystems, some submaintainers assist the main maintainers. They have specific expertise in a subset of the subsystem and help with reviewing and integrating changes.

Linus Torvalds:
As the original creator of the Linux kernel, Linus Torvalds is the overall maintainer and final authority on the mainline kernel. During the merge window, he accepts new changes into the kernel. Linus also plays a crucial role in guiding the direction of kernel development and making critical decisions.

Contributors:
Contributors are individuals who actively participate in the development process but may not have official maintainer roles. They contribute to discussions, provide feedback, and engage in code reviews to help improve the kernel.

Reviewers:
Reviewers are experienced individuals who thoroughly review patches and changes proposed by developers. They provide feedback, suggest improvements, and ensure the quality and correctness of the code before it is merged into the kernel.

Release Managers:
Release managers are responsible for coordinating and managing the kernel release process. They oversee the stabilization phase, integrate the final set of changes, and prepare the stable release.

Kernel Maintainers' Tree:
The Linux kernel source code is maintained in various trees (Git repositories) managed by maintainers. Developers send their patches to the relevant maintainer's tree for review and inclusion in the mainline kernel.

It's essential to understand that Linux kernel development is a collaborative effort involving thousands of contributors from all over the world. The kernel development process is highly organized and follows specific guidelines to ensure that the code is of high quality and meets the stringent requirements of a critical system component like the Linux kernel.

Development Process

The development process of the Linux kernel involves multiple branches and trees to manage the flow of changes and ensure stability before releasing new versions. Let's break down each component mentioned:

Main 4.x Kernel Tree

This refers to the official mainline Linux kernel tree, maintained by Linus Torvalds. It contains the latest development version of the Linux kernel. New features and changes are continuously integrated into this tree during the development cycle.

4.x.y -stable Kernel Tree

The stable kernel tree holds the stable releases of the Linux kernel. After the mainline development cycle, specific versions are marked as stable and receive additional bug fixes and security updates. The format "4.x.y" represents the version number, where "x" is the major version, and "y" is the minor version of the stable release.

4.x -git Kernel Patches

Between major stable releases, developers submit patches containing bug fixes and smaller updates directly to the mainline kernel's Git repository. These patches are reviewed and integrated into the mainline tree by the maintainers.

Subsystem-Specific Kernel Trees and Patches

Various subsystems in the Linux kernel have their dedicated maintainers and Git repositories. Developers working on specific subsystems submit their changes to these subsystem-specific trees for review and integration. Once reviewed and tested, the changes are then sent to the mainline tree.

4.x -next Kernel Tree for Integration Tests

The "4.x-next" tree is a staging area for proposed changes that are not yet ready for the mainline kernel. It acts as an integration and testing tree for new features and larger changes. The "next" tree allows developers to collaborate, test interactions between different subsystems, and identify any potential conflicts or issues.

The overall development process can be summarized as follows:

• Developers propose changes and new features to specific subsystem maintainers through patches.
• Subsystem maintainers review the patches and, if accepted, merge them into their subsystem-specific trees.
• The changes from various subsystem trees are then sent to Linus Torvalds for integration into the mainline tree during the merge window.
• After the merge window, the kernel goes through a stabilization phase with several release candidates (RCs) to address bugs and issues.
• Once the kernel is stable, a new major release is tagged and published.
• After the major release, stable maintenance releases are periodically made from the stable tree (4.x.y) containing bug fixes and security updates.

This development process allows for a controlled and collaborative approach to kernel development, ensuring that the Linux kernel remains reliable, secure, and actively maintained with new features and improvements.

Subsystem Maintainers

In the Linux kernel development process, subsystem maintainers play a critical role in managing and maintaining specific functional areas or subsystems within the kernel. They are responsible for reviewing, accepting, and integrating changes related to their respective subsystems. Subsystem maintainers are typically experienced developers who have deep knowledge and expertise in their assigned areas. Here's a more detailed overview of the role of subsystem maintainers:

Responsibilities:
Subsystem maintainers are responsible for a particular portion of the Linux kernel codebase, which can include areas such as networking, file systems, memory management, device drivers, security, etc. They act as gatekeepers for their subsystems, ensuring that only high-quality and relevant changes are merged into their tree.

Code Review:
Subsystem maintainers thoroughly review code changes submitted by developers. They assess the correctness, efficiency, and adherence to the coding standards of the proposed changes. Reviewers also consider the impact of the changes on other parts of the kernel and their interactions with the broader system.

Integration:
Once the changes pass the review process, subsystem maintainers integrate them into their subsystem-specific Git repositories. These repositories are separate from the mainline kernel tree and serve as staging areas for the respective subsystems.

Collaboration:
Subsystem maintainers work closely with developers who contribute to their subsystems. They provide feedback, offer guidance, and collaborate to ensure that the overall quality and cohesiveness of the codebase are maintained.

Stability and Maintenance:
Subsystem maintainers are responsible for the long-term stability and maintenance of their respective areas. They monitor the health of their subsystem, identify and fix bugs, and apply critical updates to ensure the proper functioning of the kernel.

Merge Window:
During the merge window, which is a specific period at the beginning of each development cycle, subsystem maintainers submit their changes to the mainline kernel tree maintained by Linus Torvalds. The merge window is when new features and significant changes are accepted into the mainline kernel.

Long-Term Support (LTS):
Some subsystem maintainers are also involved in maintaining LTS releases. LTS maintainers backport important bug fixes and security updates from the mainline kernel to LTS branches, providing stable and well-maintained kernels for long-term support users.

The role of subsystem maintainers is essential for ensuring the overall health, stability, and progress of the Linux kernel. Their expertise and dedication contribute to the success of the collaborative and distributed development model that has made the Linux kernel one of the most widely used and respected open-source projects in the world.

Working with the Community

Working with the community is a fundamental aspect of contributing to open-source projects like the Linux kernel. The Linux kernel development community is vibrant and diverse, comprising thousands of developers and enthusiasts worldwide. Here are some essential principles and practices for effectively collaborating with the Linux kernel community:

Understand the Community Culture:
Familiarize yourself with the community's values, communication norms, and development processes. Respectful and constructive communication is highly encouraged, and being open to feedback and learning from others is crucial.

Engage in Mailing Lists and Forums:
Participate in mailing lists like LKML (Linux Kernel Mailing List) and relevant forums to join discussions, ask questions, and contribute to ongoing conversations. Mailing lists are the primary communication channels for kernel development.

Contribute Incrementally:
Start with small and manageable contributions to build your reputation within the community. Gradually take on more substantial tasks as you gain experience and establish trust among other developers.

Submit Patches and Contributions:
Follow the established processes for submitting patches. Ensure that your code adheres to the kernel's coding standards and is well-documented. Be patient and responsive during the review process.

Participate in Code Reviews:
Reviewing other developers' patches is an excellent way to learn from their code and provide valuable feedback. It also helps you gain visibility within the community.

Acknowledge and Respect Maintainers:
Respect the decisions of subsystem maintainers and be understanding if your contribution is not immediately accepted. Engage in meaningful discussions and iterate on your work based on feedback.

Collaborate and Coordinate:
If you're working on a feature or change that affects multiple subsystems, collaborate and coordinate with relevant maintainers and developers to ensure smooth integration.

Contribute to Documentation and Testing:
Help improve documentation to make it easier for others to understand your work and contribute effectively. Also, participate in testing efforts to identify and report bugs.

Follow Licensing and Copyright Guidelines:
Ensure that your contributions comply with the licensing and copyright requirements of the Linux kernel and its components.

Working with the Linux kernel community is an opportunity to learn from top-notch developers, gain valuable experience, and make a meaningful impact on a critical piece of technology. By embracing the principles of open-source collaboration and contributing positively, you can become an integral part of the Linux kernel development ecosystem.

Linux Kernel Release Cycle

The Linux kernel follows a predictable and well-defined release cycle, which includes the following key phases:

Development Phase:
The development phase is an ongoing process where new features, enhancements, and bug fixes are continuously integrated into the mainline kernel. This phase occurs between major stable releases and is driven by the contributions of developers worldwide.

Merge Window:
At the beginning of each development cycle, there is a two-week period known as the merge window. During this time, Linus Torvalds, the creator and principal maintainer of the Linux kernel, accepts new features and changes from various subsystem maintainers into the mainline kernel.

Stabilization Phase (Release Candidates):
After the merge window, the kernel enters a stabilization phase. During this phase, which typically lasts for several weeks, the focus shifts from adding new features to stabilizing the code and fixing bugs. A series of release candidates (RCs) are released for testing and bug fixing.

Final Release:
Once the kernel is deemed stable, a final version is released. The version number is determined based on the year of release and the order of releases within that year. For example, the first stable release of the year 2023 would be version 5.16, and subsequent stable releases would be 5.17, 5.18, and so on.

Long-Term Support (LTS) Releases:
In addition to regular releases, certain versions of the Linux kernel are designated as Long-Term Support (LTS) releases. LTS kernels receive extended maintenance and bug fixes for a more extended period, typically two to six years, depending on the specific LTS designation.

Continuous Development:
The release cycle is continuous, meaning that as one development cycle completes, the next one begins. The development process keeps evolving, and new features and improvements are continuously added in subsequent cycles.

The Linux kernel development process strictly adheres to a time-based release cycle. New major releases typically occur every two to three months, while the stable release candidates are released every week during the stabilization phase. This regular and predictable release cycle allows users to stay up-to-date with the latest kernel features and bug fixes and also provides stability and reliability through the availability of LTS releases for those who require long-term support.

Types of Releases

In the context of the Linux kernel development, there are several types of releases, each serving a specific purpose. These releases help manage the development process and provide various levels of support and stability to different users. The main types of releases are as follows:

Mainline Releases:
Mainline releases refer to the regular stable releases of the Linux kernel that occur approximately every two to three months. These releases include all the new features, enhancements, and bug fixes that have been developed and stabilized during the development cycle. Mainline releases are intended for users who want the latest features and improvements but are willing to accept the possibility of encountering some minor issues.

Long-Term Support (LTS) Releases:
LTS releases are specific versions of the Linux kernel that receive extended maintenance and support for an extended period. LTS releases are typically designated every two to three years. They are meant for users who prioritize stability and require a kernel version with long-term maintenance and bug fixes. LTS releases are widely used in enterprise and embedded systems, providing a reliable and stable platform.

Release Candidates (RCs):
Release candidates are pre-release versions of the Linux kernel that are made available during the stabilization phase before a mainline release. They are labeled as RC1, RC2, and so on. RCs are intended for testing purposes, allowing users and developers to identify and report bugs before the final stable release.

Next-Tree:
The "next-tree" is a staging area where proposed changes and features that are not yet ready for the mainline kernel are collected. The "next-tree" acts as an integration and testing area to identify potential conflicts and issues between different subsystems.

-rc Regression Fixes:
After a mainline release, if critical regressions (unexpected issues introduced in the new version) are discovered, regression fixes in the form of -rc patches are released. These patches address the regressions and provide a more stable experience for users until the next stable release.

Each type of release caters to different user needs and provides a balance between the latest features and long-term stability. Users can choose the appropriate type of release based on their specific requirements and risk tolerance. The Linux kernel development process, with its various types of releases, ensures that users have access to a reliable, up-to-date, and continuously improving operating system.

Differences between the Kernel Community and Corporate Structures

The Linux kernel development community and corporate structures are distinct entities with different organizational and decision-making approaches. Here are some key differences between them:

Organizational Structure

Linux Kernel Development:
The Linux kernel development community is an open-source, collaborative, and decentralized organization. It operates as a meritocracy, where contributors earn authority and recognition based on the quality and impact of their contributions. There is no formal hierarchy, and decision-making is distributed among maintainers, reviewers, and developers who earn the trust of their peers.

Corporate Structures:
Corporations typically have hierarchical organizational structures with clear chains of command. Decision-making often follows a top-down approach, where decisions are made by executives and management teams, and instructions are passed down to lower-level employees.

Motivations

Linux Kernel Development:
Contributors to the Linux kernel are often motivated by the desire to improve the open-source ecosystem, solve technical challenges, and contribute to a widely-used and respected project. Their contributions are driven by passion and a commitment to the principles of open-source software.

Corporate Structures:
Corporations are driven by various motives, including profit, market dominance, and meeting business objectives. While some companies may actively contribute to the Linux kernel development for strategic reasons, others may focus on specific areas of the kernel that align with their commercial interests.

Decision-Making Process

Linux Kernel Development:
The decision-making process in the Linux kernel development community is open and transparent. Major decisions are made through discussions on mailing lists, public forums, and in-person conferences. Consensus-building is valued, and the input of multiple stakeholders is considered before finalizing decisions.

Corporate Structures:
Decision-making in corporations often involves closed-door meetings among top executives and management. While some decisions may involve consultation with relevant teams, the ultimate authority lies with the company's leadership.

Ownership and Intellectual Property

Linux Kernel Development:
The Linux kernel is released under the GNU General Public License (GPL), which ensures that the source code is freely available, and users have the right to view, modify, and distribute it. Contributions to the kernel become part of the open-source codebase, and developers retain copyright ownership of their contributions.

Corporate Structures:
Corporations typically retain ownership of intellectual property and code developed by their employees. Contributions to open-source projects may be subject to specific licensing agreements based on the project's licensing terms.

Despite these differences, the Linux kernel development community and corporate involvement are not mutually exclusive. Many corporations actively participate in Linux kernel development, recognizing its strategic importance and benefits to the broader technology ecosystem. Corporate contributions often coexist and complement the efforts of individual developers within the vibrant and collaborative Linux kernel development community.