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. The format is "<suite_glob>[.test_glob]".

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 can filter down to just the “write” tests via:

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

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
$ ./tools/testing/kunit/ run --kunitconfig=.kunit/ --kunitconfig=tools/testing/kunit/configs/coverage_uml.config
# 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

Alternatively, LLVM-based toolchains can also be used:

# Build with LLVM and append coverage options to the current config
$ ./tools/testing/kunit/ run --make_options LLVM=1 --kunitconfig=.kunit/ --kunitconfig=tools/testing/kunit/configs/coverage_uml.config
$ llvm-profdata merge -sparse default.profraw -o default.profdata
$ llvm-cov export --format=lcov .kunit/vmlinux -instr-profile default.profdata >
# The file is in lcov-compatible format and it can be used to e.g. generate HTML report
$ genhtml -o /tmp/coverage_html

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

Test Attributes and Filtering

Test suites and cases can be marked with test attributes, such as speed of test. These attributes will later be printed in test output and can be used to filter test execution.

Marking Test Attributes

Tests are marked with an attribute by including a kunit_attributes object in the test definition.

Test cases can be marked using the KUNIT_CASE_ATTR(test_name, attributes) macro to define the test case instead of KUNIT_CASE(test_name).

static const struct kunit_attributes example_attr = {
        .speed = KUNIT_VERY_SLOW,

static struct kunit_case example_test_cases[] = {
        KUNIT_CASE_ATTR(example_test, example_attr),


To mark a test case as slow, you can also use KUNIT_CASE_SLOW(test_name). This is a helpful macro as the slow attribute is the most commonly used.

Test suites can be marked with an attribute by setting the “attr” field in the suite definition.

static const struct kunit_attributes example_attr = {
        .speed = KUNIT_VERY_SLOW,

static struct kunit_suite example_test_suite = {
        .attr = example_attr,


Not all attributes need to be set in a kunit_attributes object. Unset attributes will remain uninitialized and act as though the attribute is set to 0 or NULL. Thus, if an attribute is set to 0, it is treated as unset. These unset attributes will not be reported and may act as a default value for filtering purposes.

Reporting Attributes

When a user runs tests, attributes will be present in the raw kernel output (in KTAP format). Note that attributes will be hidden by default in output for all passing tests but the raw kernel output can be accessed using the --raw_output flag. This is an example of how test attributes for test cases will be formatted in kernel output:

# example_test.speed: slow
ok 1 example_test

This is an example of how test attributes for test suites will be formatted in kernel output:

  KTAP version 2
  # Subtest: example_suite
  # module: kunit_example_test
ok 1 example_suite

Additionally, users can output a full attribute report of tests with their attributes, using the command line flag --list_tests_attr: run "example" --list_tests_attr


This report can be accessed when running KUnit manually by passing in the module_param kunit.action=list_attr.


Users can filter tests using the --filter command line flag when running tests. As an example: run --filter speed=slow

You can also use the following operations on filters: “<”, “>”, “<=”, “>=”, “!=”, and “=”. Example: run --filter "speed>slow"

This example will run all tests with speeds faster than slow. Note that the characters < and > are often interpreted by the shell, so they may need to be quoted or escaped, as above.

Additionally, you can use multiple filters at once. Simply separate filters using commas. Example: run --filter "speed>slow, module=kunit_example_test"


You can use this filtering feature when running KUnit manually by passing the filter as a module param: kunit.filter="speed>slow, speed<=normal".

Filtered tests will not run or show up in the test output. You can use the --filter_action=skip flag to skip filtered tests instead. These tests will be shown in the test output in the test but will not run. To use this feature when running KUnit manually, use the module param kunit.filter_action=skip.

Rules of Filtering Procedure

Since both suites and test cases can have attributes, there may be conflicts between attributes during filtering. The process of filtering follows these rules:

  • Filtering always operates at a per-test level.

  • If a test has an attribute set, then the test’s value is filtered on.

  • Otherwise, the value falls back to the suite’s value.

  • If neither are set, the attribute has a global “default” value, which is used.

List of Current Attributes


This attribute indicates the speed of a test’s execution (how slow or fast the test is).

This attribute is saved as an enum with the following categories: “normal”, “slow”, or “very_slow”. The assumed default speed for tests is “normal”. This indicates that the test takes a relatively trivial amount of time (less than 1 second), regardless of the machine it is running on. Any test slower than this could be marked as “slow” or “very_slow”.

The macro KUNIT_CASE_SLOW(test_name) can be easily used to set the speed of a test case to “slow”.


This attribute indicates the name of the module associated with the test.

This attribute is automatically saved as a string and is printed for each suite. Tests can also be filtered using this attribute.


This attribute indicates whether the test uses init data or functions.

This attribute is automatically saved as a boolean and tests can also be filtered using this attribute.