Tips For Running KUnit Tests

Using run (“kunit tool”)

Running from any directory

It can be handy to create a bash function like:

function run_kunit() {
  ( cd "$(git rev-parse --show-toplevel)" && ./tools/testing/kunit/ run $@ )


Early versions of (before 5.6) didn’t work unless run from the kernel root, hence the use of a subshell and cd.

Running a subset of tests run accepts an optional glob argument to filter tests. Currently this only matches against suite names, but this may change in the future.

Say that we wanted to run the sysctl tests, we could do so via:

$ echo -e 'CONFIG_KUNIT=y\nCONFIG_KUNIT_ALL_TESTS=y' > .kunit/.kunitconfig
$ ./tools/testing/kunit/ run 'sysctl*'

We’re paying the cost of building more tests than we need this way, but it’s easier than fiddling with .kunitconfig files or commenting out kunit_suite’s.

However, if we wanted to define a set of tests in a less ad hoc way, the next tip is useful.

Defining a set of tests run (along with build, and config) supports a --kunitconfig flag. So if you have a set of tests that you want to run on a regular basis (especially if they have other dependencies), you can create a specific .kunitconfig for them.

E.g. kunit has one for its tests:

$ ./tools/testing/kunit/ run --kunitconfig=lib/kunit/.kunitconfig

Alternatively, if you’re following the convention of naming your file .kunitconfig, you can just pass in the dir, e.g.

$ ./tools/testing/kunit/ run --kunitconfig=lib/kunit


This is a relatively new feature (5.12+) so we don’t have any conventions yet about on what files should be checked in versus just kept around locally. It’s up to you and your maintainer to decide if a config is useful enough to submit (and therefore have to maintain).


Having .kunitconfig fragments in a parent and child directory is iffy. There’s discussion about adding an “import” statement in these files to make it possible to have a top-level config run tests from all child directories. But that would mean .kunitconfig files are no longer just simple .config fragments.

One alternative would be to have kunit tool recursively combine configs automagically, but tests could theoretically depend on incompatible options, so handling that would be tricky.

Setting kernel commandline parameters

You can use --kernel_args to pass arbitrary kernel arguments, e.g.

$ ./tools/testing/kunit/ run --kernel_args=param=42 --kernel_args=param2=false

Generating code coverage reports under UML


TODO( There are various issues with UML and versions of gcc 7 and up. You’re likely to run into missing .gcda files or compile errors.

This is different from the “normal” way of getting coverage information that is documented in Using gcov with the Linux kernel.

Instead of enabling CONFIG_GCOV_KERNEL=y, we can set these options:


Putting it together into a copy-pastable sequence of commands:

# Append coverage options to the current config
$ echo -e "CONFIG_DEBUG_KERNEL=y\nCONFIG_DEBUG_INFO=y\nCONFIG_GCOV=y" >> .kunit/.kunitconfig
$ ./tools/testing/kunit/ run
# Extract the coverage information from the build dir (.kunit/)
$ lcov -t "my_kunit_tests" -o -c -d .kunit/

# From here on, it's the same process as with CONFIG_GCOV_KERNEL=y
# E.g. can generate an HTML report in a tmp dir like so:
$ genhtml -o /tmp/coverage_html

If your installed version of gcc doesn’t work, you can tweak the steps:

$ ./tools/testing/kunit/ run --make_options=CC=/usr/bin/gcc-6
$ lcov -t "my_kunit_tests" -o -c -d .kunit/ --gcov-tool=/usr/bin/gcov-6

Running tests manually

Running tests without using run is also an important use case. Currently it’s your only option if you want to test on architectures other than UML.

As running the tests under UML is fairly straightforward (configure and compile the kernel, run the ./linux binary), this section will focus on testing non-UML architectures.

Running built-in tests

When setting tests to =y, the tests will run as part of boot and print results to dmesg in TAP format. So you just need to add your tests to your .config, build and boot your kernel as normal.

So if we compiled our kernel with:


Then we’d see output like this in dmesg signaling the test ran and passed:

TAP version 14
    # Subtest: example
    # example_simple_test: initializing
    ok 1 - example_simple_test
ok 1 - example

Running tests as modules

Depending on the tests, you can build them as loadable modules.

For example, we’d change the config options from before to


Then after booting into our kernel, we can run the test via

$ modprobe kunit-example-test

This will then cause it to print TAP output to stdout.


The modprobe will not have a non-zero exit code if any test failed (as of 5.13). But parse would, see below.


You can set CONFIG_KUNIT=m as well, however, some features will not work and thus some tests might break. Ideally tests would specify they depend on KUNIT=y in their Kconfig’s, but this is an edge case most test authors won’t think about. As of 5.13, the only difference is that current->kunit_test will not exist.

Pretty-printing results

You can use parse to parse dmesg for test output and print out results in the same familiar format that run does.

$ ./tools/testing/kunit/ parse /var/log/dmesg

Retrieving per suite results

Regardless of how you’re running your tests, you can enable CONFIG_KUNIT_DEBUGFS to expose per-suite TAP-formatted results:


The results for each suite will be exposed under /sys/kernel/debug/kunit/<suite>/results. So using our example config:

$ modprobe kunit-example-test > /dev/null
$ cat /sys/kernel/debug/kunit/example/results
... <TAP output> ...

# After removing the module, the corresponding files will go away
$ modprobe -r kunit-example-test
$ cat /sys/kernel/debug/kunit/example/results
/sys/kernel/debug/kunit/example/results: No such file or directory

Generating code coverage reports

See Using gcov with the Linux kernel for details on how to do this.

The only vaguely KUnit-specific advice here is that you probably want to build your tests as modules. That way you can isolate the coverage from tests from other code executed during boot, e.g.

# Reset coverage counters before running the test.
$ echo 0 > /sys/kernel/debug/gcov/reset
$ modprobe kunit-example-test