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.