Frequently Asked Questions

How is this different from Autotest, kselftest, and so on?

KUnit is a unit testing framework. Autotest, kselftest (and some others) are not.

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

There are no testing frameworks currently available for the kernel that do not require installing the kernel on a test machine or in a virtual machine. All testing frameworks require tests to be written in userspace and run on the kernel under test. This is true for Autotest, kselftest, and some others, disqualifying any of them from being considered unit testing frameworks.

Does KUnit support running on architectures other than UML?

Yes. KUnit can run on any architecture, though the kunit.py tool can only build and run kernels for some architectures (of which UML is the default).

You can build and run tests without kunit.py at all on any architecture by enabling CONFIG_KUNIT=y and booting the kernel. See Run Tests without kunit_tool for more details.

Alternatively, kunit.py supports many common architectures using cross-compilers and the qemu emulator. This can be done using the --arch parameter when running the tests, and the --cross_compile parameter when building (if the architecture is not supported by the host compiler). See Running tests on QEMU for more details.

When writing tests targeting other architectures, it’s worth keeping the tips on the Writing Tests For Other Architectures page in mind.

What is the difference between a unit test and other kinds of tests?

Most existing tests for the Linux kernel would be categorized as an integration test, or an end-to-end test.

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

  • An integration test tests the interaction between a minimal set of components, usually just two or three. For example, someone might write an integration test to test the interaction between a driver and a piece of hardware, or to test the interaction between the userspace libraries the kernel provides and the kernel itself. However, one of these tests would probably not test the entire kernel along with hardware interactions and interactions with the userspace.

  • An end-to-end test usually tests the entire system from the perspective of the code under test. For example, someone might write an end-to-end test for the kernel by installing a production configuration of the kernel on production hardware with a production userspace and then trying to exercise some behavior that depends on interactions between the hardware, the kernel, and userspace.

KUnit is not working, what should I do?

Unfortunately, there are a number of things which can break, but here are some things to try.

  1. Run ./tools/testing/kunit/kunit.py run with the --raw_output parameter. This might show details or error messages hidden by the kunit_tool parser.

  2. Instead of running kunit.py run, try running kunit.py config, kunit.py build, and kunit.py exec independently. This can help track down where an issue is occurring. (If you think the parser is at fault, you can run it manually against stdin or a file with kunit.py parse.)

  3. Running the UML kernel directly can often reveal issues or error messages, kunit_tool ignores. This should be as simple as runningi the vmlinux binary in the output directory (by default ./.kunit/vmlinux) after building the UML kernel (for example, by using kunit.py build). Note that UML has some unusual requirements (such as the host having a tmpfs filesystem mounted), and has had issues in the past when built statically and the host has KASLR enabled. (On older host kernels, you may need to run setarch `uname -m` -R ./vmlinux to disable KASLR.)

  4. Try running KUnit on a different architecture by using the --arch option. On an x86_64 host, using --arch=x86_64 is a good first step.

  5. Make sure the kernel .config has CONFIG_KUNIT=y and at least one test (e.g. CONFIG_KUNIT_EXAMPLE_TEST=y). kunit_tool will keep its .config around, so you can see what config was used after running kunit.py run. It also preserves any config changes you might make, so you can enable/disable things with make ARCH=um menuconfig or similar, and then re-run kunit_tool.

  6. Try to run make ARCH=um defconfig before running kunit.py run. This may help clean up any residual config items which could be causing problems.

  7. Finally, try running KUnit manually, instead of via kunit.py. KUnit can be built into any kernel, or can be built as a module and loaded at runtime. When tests are built-in, they will execute when the kernel boots, and modules will automatically execute associated tests when loaded. Test results can be collected from /sys/kernel/debug/kunit/<test suite>/results, and can be parsed with kunit.py parse. For more details, see Run Tests without kunit_tool

If none of the above tricks help, you are always welcome to email any issues to kunit-dev@googlegroups.com.