KUnit - Unit Testing for the Linux Kernel

What is KUnit?

KUnit is a lightweight unit testing and mocking framework for the Linux kernel. These tests are able to be run locally on a developer’s workstation without a VM or special hardware.

KUnit is heavily inspired by JUnit, Python’s unittest.mock, and Googletest/Googlemock for C++; it provides many of the same features in a similar manner (mocking, spying, assertions, etc) in a way that makes sense with the Linux kernel’s flavor of C.

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Why KUnit?

Aside from KUnit there is no true unit testing framework for the Linux kernel. Autotest and kselftest are sometimes cited as unit testing frameworks; however, they are not by most reasonable definitions of unit tests.

A unit test is supposed to test a single unit of code in isolation, hence the name. A unit test should be the finest granularity of testing and as such should allow all possible code paths to be tested in the code under test; this is only possible if the code under test is very small and does not have any external dependencies outside of the test’s control like hardware.

Outside of KUnit, there are no testing frameworks currently available for the kernel that do not require installing the kernel on a test machine or in a VM and all require tests to be written in userspace running on the kernel; this is true for Autotest, and kselftest, disqualifying any of them from being considered unit testing frameworks.

KUnit addresses the problem of being able to run tests without needing a virtual machine or actual hardware with User Mode Linux. User Mode Linux is a Linux architecture, like ARM or x86; however, unlike other architectures it compiles to a standalone program that can be run like any other program directly inside of a host operating system; to be clear, it does not require any virtualization support; it is just a regular program.

KUnit is fast. Excluding build time, from invocation to completion KUnit can run several dozen tests in only 10 to 20 seconds; this might not sound like a big deal to some people, but having such fast and easy to run tests fundamentally changes the way you go about testing and even writing code in the first place. Linus himself said in his git talk at Google:

“… a lot of people seem to think that performance is about doing the same thing, just doing it faster, and that is not true. That is not what performance is all about. If you can do something really fast, really well, people will start using it differently.”

In this context Linus was talking about branching and merging, but this point also applies to testing. If your tests are slow, unreliable, are difficult to write, and require a special setup or special hardware to run, then you wait a lot longer to write tests, and you wait a lot longer to run tests; this means that tests are likely to break, unlikely to test a lot of things, and are unlikely to be rerun once they pass. If your tests are really fast, you run them all the time, everytime you make a change, and everytime someone sends you some code. Why trust that someone ran all their tests correctly on every change when you can just run them yourself in less time than it takes to read his / her test log?

How do I use it?