Unit Testing in the Linux Kernel

Brendan Higgins recently proposed adding unit tests to the Linux kernel, supplementing other development infrastructure such as perf, autotest and kselftest. The whole issue of testing is very dear to kernel developers' hearts, because Linux sits at the core of the system and often has a very strong stability/security requirement. Hosts of automated tests regularly churn through kernel source code, reporting any oddities to the mailing list.

Unit tests, Brendan said, specialize in testing standalone code snippets. It was not necessary to run a whole kernel, or even to compile the kernel source tree, in order to perform unit tests. The code to be tested could be completely extracted from the tree and tested independently. Among other benefits, this meant that dozens of unit tests could be performed in less than a second, he explained.

Giving credit where credit was due, Brendan identified JUnit, Python's unittest.mock and Googletest/Googlemock for C++ as the inspirations for this new KUnit testing idea.

Brendan also pointed out that since all code being unit-tested is standalone and has no dependencies, this meant the tests also were deterministic. Unlike on a running Linux system, where any number of pieces of the running system might be responsible for a given problem, unit tests would identify problem code with repeatable certainty.

Daniel Vetter replied extremely enthusiastically to Brendan's work. In particular, he said, "Having proper and standardized infrastructure for kernel unit tests sounds terrific. In other words: I want." He added that he and some others already had been working on a much more specialized set of unit tests for the Direct Rendering Manager (DRM) driver. Brendan's approach, he said, would be much more convenient than his own more localized efforts.

Dan Williams was also very excited about Brendan's work, and he said he had been doing a half-way job of unit tests on the libnvdimm (non-volatile device) project code. He felt Brendan's work was much more general-purpose, and he wanted to convert his own tests to use KUnit.

Tim Bird replied to Brendan's initial email as well, saying he thought unit tests could be useful, but he wanted to make sure the behaviors were correct. In particular, he wanted clarification on just how it was possible to test standalone code. If the code were to be compiled independently, would it then run on the local system? What if the local system had a different hardware architecture from the system for which the code was intended? Also, who would maintain unit tests, and where would the tests live, within the source tree? Would they clutter up the directory being tested, or would they live far away in a special directory reserved for test code? And finally, would test code be easier to write than the code being tested? In other words, could new developers cut their teeth on a project by writing test code, as a gateway to helping work on a given driver or subsystem? Or would unit tests have to be written by people who had total expertise in the area already?

Brendan attempted to address each of those issues in turn. To start, he confirmed that the test code was indeed extracted and compiled on the local system. Eventually, he said, each test would compile into its own completely independent test binary, although for the moment, they were all lumped together into a single user-mode-linux (UML) binary.

In terms of cross-compiling test code for other architectures, Brendan felt this would be hard to maintain and had decided not to support it. Tests would run locally and would not depend on architecture-specific characteristics.

In terms of where the unit tests would live, Brendan said they would be in the same directory as the code being tested. So every directory would have its own set of unit tests readily available and visible. The same person maintaining the code being tested would maintain the tests themselves. The unit tests, essentially, would become an additional element of every project. That maintainer would then presumably require that all patches to that driver or subsystem pass all the unit tests before they could be accepted into the tree.

In terms of who was qualified to write unit tests for a given project, Brendan explained:

In order to write a unit test, the person who writes the test must understand what the code they are testing is supposed to do. To some extent that will probably require someone with some expertise to ensure that the test makes sense, and indeed a change that breaks a test should be accompanied by an update to the test. On the other hand, I think understanding what pre-existing code does and is supposed to do is much easier than writing new code from scratch, and probably doesn't require too much expertise.

Brendan added that unit tests would probably reduce, rather than increase, a maintainer's workload. In spite of representing more code overall:

Code with unit tests is usually cleaner, the tests tell me exactly what the code is supposed to do, and I can run the tests (or ideally have an automated service run the tests) that tell me that the code actually does what the tests say it should. Even when it comes to writing code, I find that writing code with unit tests ends up saving me time.

Overall, Brendan was very pleased by all the positive interest, and said he planned to do additional releases to address the various technical suggestions that came up during the course of discussion. No voices really were raised in opposition to any of Brendan's ideas. It appears that unit tests may soon become a standard part of many drivers and subsystems.

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Zack Brown is a tech journalist at Linux Journal and Linux Magazine, and is a former author of the "Kernel Traffic" weekly newsletter and the "Learn Plover" stenographic typing tutorials. He first installed Slackware Linux in 1993 on his 386 with 8 megs of RAM and had his mind permanently blown by the Open Source community. He is the inventor of the Crumble pure strategy board game, which you can make yourself with a few pieces of cardboard. He also enjoys writing fiction, attempting animation, reforming Labanotation, designing and sewing his own clothes, learning French and spending time with friends'n'family.

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