Re: [PATCH] Documentation: kunit: add tips for running KUnit

[Brendan Higgins]

On Mon, Apr 12, 2021 at 10:27 AM Daniel Latypov <dlatypov@xxxxxxxxxx> wrote:
>
> hOn Fri, Apr 9, 2021 at 9:10 PM David Gow <davidgow@xxxxxxxxxx> wrote:
> >
> > Thanks for writing this: it's good to have these things documented at last!
> >
> > There are definitely a few things this document points out which still
> > need deciding, which does make this document lean a bit into "design
> > discussion" territory in a few of the notes. This doesn't bother me --
> > it's an accurate description of the state of things -- but I wouldn't
> > want this documentation held up too long because of these sorts of
> > TODOs (and can definitely see how having too many of them might
> > discourage KUnit use a bit). Particularly things like the
> > ".kunitconfig" fragment file feature stuff: I feel that's something
> > better discussed on patches adding/using the feature than in the
> > documentation / reviews of the documentation, so I'd rather drop or
> > simplify those '..note:'s than bokeshed about it here (something I'm a
> > little guilty of below).
>
> I don't think we'll actually make progress on any of those in the near
> future though.
> So I figured it'd be best to accurately represent the state of the
> world ~somewhere.
>
> But it did feel a bit strange to do it here, so I'm not against removing it.

I actually like the accurate and upfront way that you spelled these things out.

> > Otherwise, a few minor comments and nitpicks:
> >
> > -- David
> >
> > On Sat, Apr 10, 2021 at 2:01 AM Daniel Latypov <dlatypov@xxxxxxxxxx> wrote:
> > >
> > > This is long overdue.
> > >
> > > There are several things that aren't nailed down (in-tree
> > > .kunitconfig's), or partially broken (GCOV on UML), but having them
> > > documented, warts and all, is better than having nothing.
> > >
> > > This covers a bunch of the more recent features
> > > * kunit_filter_glob
> > > * kunit.py run --kunitconfig
> > > * kunit.py run --alltests
> > > * slightly more detail on building tests as modules
> > > * CONFIG_KUNIT_DEBUGFS
> > >
> > > By my count, the only headline features now not mentioned are the KASAN
> > > integration and KernelCI json output support (kunit.py run --json).
> > >
> > > And then it also discusses how to get code coverage reports under UML
> > > and non-UML since this is a question people have repeatedly asked.
> > >
> > > Non-UML coverage collection is no differnt from normal, but we should
> > > probably explicitly call thsi out.
> >
> > Nit: typos in 'different' and 'this'.
> Fixed.
> >
> > >
> > > As for UML, I was able to get it working again with two small hacks.*
> > > E.g. with CONFIG_KUNIT=y && CONFIG_KUNIT_ALL_TESTS=y
> > >   Overall coverage rate:
> > >     lines......: 15.1% (18294 of 120776 lines)
> > >     functions..: 16.8% (1860 of 11050 functions)
> > >
> > > *Switching to use gcc/gcov-6 and not using uml_abort().
> > > I've documented these hacks in "Notes" but left TODOs for
> > > brendanhiggins@xxxxxxxxxx who tracked down the runtime issue in GCC.
> > > To be clear: these are not issues specific to KUnit, but rather to UML.
> >
> > (We should probably note where uml_abort() needs to be replaced if
> > we're mentioning this, though doing so below in the more detailed
> > section may be more useful.)
>
> Updated to
> *Using gcc/gcov-6 and not using uml_abort() in os_dump_core().
>
> I figured we'd be more precise in the documentation itself.
>
> >
> > >
> > > Signed-off-by: Daniel Latypov <dlatypov@xxxxxxxxxx>
> > > ---
> > >  Documentation/dev-tools/kunit/index.rst       |   1 +
> > >  .../dev-tools/kunit/running_tips.rst          | 278 ++++++++++++++++++
> > >  Documentation/dev-tools/kunit/start.rst       |   2 +
> > >  3 files changed, 281 insertions(+)
> > >  create mode 100644 Documentation/dev-tools/kunit/running_tips.rst
> > >
> > > diff --git a/Documentation/dev-tools/kunit/index.rst b/Documentation/dev-tools/kunit/index.rst
> > > index 848478838347..7f7cf8d2ab20 100644
> > > --- a/Documentation/dev-tools/kunit/index.rst
> > > +++ b/Documentation/dev-tools/kunit/index.rst
> > > @@ -14,6 +14,7 @@ KUnit - Unit Testing for the Linux Kernel
> > >         style
> > >         faq
> > >         tips
> > > +       running_tips
> > >
> > >  What is KUnit?
> > >  ==============
> > > diff --git a/Documentation/dev-tools/kunit/running_tips.rst b/Documentation/dev-tools/kunit/running_tips.rst
> > > new file mode 100644
> > > index 000000000000..d38e665e530f
> > > --- /dev/null
> > > +++ b/Documentation/dev-tools/kunit/running_tips.rst
> > > @@ -0,0 +1,278 @@
> > > +.. SPDX-License-Identifier: GPL-2.0
> > > +
> > > +============================
> > > +Tips For Running KUnit Tests
> > > +============================
> > > +
> > > +Using ``kunit.py run`` ("kunit tool")
> > > +=====================================
> > > +
> > > +Running from any directory
> > > +--------------------------
> > > +
> > > +It can be handy to create a bash function like:
> > > +
> > > +.. code-block:: bash
> > > +
> > > +       function run_kunit() {
> > > +         ( cd "$(git rev-parse --show-toplevel)" && ./tools/testing/kunit/kunit.py run $@ )
> > > +       }
> > > +
> > > +.. note::
> > > +       Early versions of ``kunit.py`` (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
> > > +-------------------------
> > > +
> > > +``kunit.py 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:
> > > +
> > > +.. code-block:: bash
> > > +
> > > +       $ echo -e 'CONFIG_KUNIT=y\nCONFIG_KUNIT_ALL_TESTS=y' > .kunit/.kunitconfig
> > > +       $ ./tools/testing/kunit/kunit.py 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
> > > +-----------------------
> > > +
> > > +``kunit.py 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 own for its tests:
> >
> > Nit: 'one' for its tests (or 'its own' for its tests?)
> Fixed, meant to be "one"
> >
> > > +
> > > +.. code-block:: bash
> > > +
> > > +       $ ./tools/testing/kunit/kunit.py 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.
> > > +
> > > +.. code-block:: bash
> > > +
> > > +       $ ./tools/testing/kunit/kunit.py run --kunitconfig=lib/kunit
> > > +
> > > +.. note::
> > > +       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. But if the tests don't have any dependencies
> > > +       (beyond ``CONFIG_KUNIT``), it's probably not worth writing and
> > > +       maintaining a ``.kunitconfig`` fragment.  Running with
> > > +       ``CONFIG_KUNIT_ALL_TESTS=y`` is probably easier.
> >
> > I think the rule of thumb for checked-in .kunitconfig files should be
> > an explicit endorsement by the maintainer that these are the tests for
> > a particular subsystem.
>
> Hmm, I'm not sure we want to prescribe a granularity here.
> If we had something like a "How-to-Test-Cmd" in MAINTAINERS, I'd feel
> more justified in doing so.
>
> But atm, I feel the line should be "use it if it's useful, check it in
> if you think it's useful to 'enough' other people."

I think that's fine.

> > > +
> > > +.. note::
> > > +       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.
> > > +
> > > +Running with ``allyesconfig``
> > > +-----------------------------
> > > +
> > > +.. code-block:: bash
> > > +
> > > +       $ ./tools/testing/kunit/kunit.py run --alltests
> > > +
> > > +This will try and use ``allyesconfig``, or rather ``allyesconfig`` with a list
> > Excessively pedantic nit: 'try to use'
> Done.
>
> > > +of UML-incompatible configs turned off. That list is maintained in
> > > +``tools/testing/kunit/configs/broken_on_uml.config``.
> > > +
> > > +.. note::
> > > +       This will take a *lot* longer to run and might be broken from time to
> > > +       time, especially on -next. It's not recommended to use this unless you
> > > +       need to or are morbidly curious.
> >
> > Given that it's been the plan to run this for KernelCI, I'm not sure
> > we should discourage it in general to quite this
> > extent. I think it is broken at the moment, though, so that's
> > nevertheless worth noting.
>
> It was broken for me when I tried as I was writing this up, haven't
> checked again yet.
> I think until KernelCI uses it regularly, it's not going to be as easy
> to keep it working.
>
> So IMO,
> * KernelCI and other automation should try and use it
> * we shouldn't necessarily encourage a human to go and try it at this time
>   * or maybe never: this basically eliminates one of the biggest
> selling points: the fast edit/compile/test cycle that KUnit on UML
> has.
>
> So how about something like:
>
> This will take a *lot* longer to run and might be broken from time to time.
> You'll probably be better off just building and running the tests you
> care about if you need to do so more than once.

Sounds good to me.

> > > +
> > > +Generating code coverage reports under UML
> > > +------------------------------------------
> > > +
> > > +.. note::
> > > +       TODO(brendanhiggins@xxxxxxxxxx): 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. We know one `faulty GCC commit
> > > +       <https://github.com/gcc-mirror/gcc/commit/8c9434c2f9358b8b8bad2c1990edf10a21645f9d>`_
> > > +       but not how we'd go about getting this fixed. The compile errors still
> > > +       need some investigation.
> > > +
> > > +.. note::
> > > +       TODO(brendanhiggins@xxxxxxxxxx): for recent versions of Linux
> > > +       (5.10-5.12, maybe earlier), there's a bug with gcov counters not being
> > > +       flushed in UML. This translates to very low (<1%) reported coverage. This is
> > > +       related to the above issue and can be worked around by replacing the
> > > +       one call to ``uml_abort()`` with a plain ``exit()``.
> >
> > Can we be more specific than 'the one call' here? I know there is only
> > one call, but maybe noting that it's in arch/um/os-Linux/util.c will
> > make this clearer.
>
> Yeah, here's only one call, so I thought leaving it more vague in case
> the file gets renamed or w/e would be safer.
>
> But yeah, if adding something more here makes it more clear, I can do that.
> Hmm, looks like the function that calls it, os_dump_core(void) is
> currently unique.
> Thoughts on referring to that instead of the filename (I'm not sure
> that either is meaningfully less likely to change)?
>
> >
> > > +
> > > +
> > > +This is different from the "normal" way of getting coverage information that is
> > > +documented in Documentation/dev-tools/gcov.rst.
> > > +
> > > +Instead of enabling ``CONFIG_GCOV_KERNEL=y``, we can set these options:
> > > +
> > > +.. code-block:: none
> > > +
> > > +       CONFIG_DEBUG_KERNEL=y
> > > +       CONFIG_DEBUG_INFO=y
> > > +       CONFIG_GCOV=y
> > > +
> > > +
> > > +Putting it together into a copy-pastable sequence of commands:
> > > +
> > > +.. code-block:: bash
> > > +
> > > +       # 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/kunit.py run
> > > +       # Extract the coverage information from the build dir (.kunit/)
> > > +       $ lcov -t "my_kunit_tests" -o coverage.info -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 coverage.info
> > > +
> > > +
> > > +If your installed version of gcc doesn't work, you can tweak the steps:
> > > +
> > > +.. code-block:: bash
> > > +
> > > +       # need to edit tools/testing/kunit/kunit_kernel.py to call make with 'CC=/usr/bin/gcc-6'
> > > +       $ $EDITOR tools/testing/kunit/kunit_kernel.py
> > > +
> > > +       $ lcov -t "my_kunit_tests" -o coverage.info -c -d .kunit/ --gcov-tool=/usr/bin/gcov-6
> > > +
> > > +
> > > +Running tests manually
> > > +======================
> > > +
> > > +Running tests without using ``kunit.py 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:
> > > +
> > > +.. code-block:: none
> > > +
> > > +       CONFIG_KUNIT=y
> > > +       CONFIG_KUNIT_EXAMPLE_TEST=y
> > > +
> > > +Then we'd see output like this in dmesg signaling the test ran and passed:
> > > +
> > > +.. code-block:: none
> > > +
> > > +       TAP version 14
> > > +       1..1
> > > +           # Subtest: example
> > > +           1..1
> > > +           # 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
> > > +
> > > +.. code-block:: none
> > > +
> > > +       CONFIG_KUNIT=y
> > > +       CONFIG_KUNIT_EXAMPLE_TEST=m
> > > +
> > > +Then after booting into our kernel, we can run the test via
> > > +
> > > +.. code-block:: none
> > > +
> > > +       $ modprobe kunit-example-test
> > > +
> > > +This will then cause it to print TAP output to stdout.
> > > +
> > > +.. note::
> > > +       The ``modprobe`` will *not* have a non-zero exit code if any test
> > > +       failed (as of 5.13). But ``kunit.py parse`` would, see below.
> > > +
> > > +.. note::
> > > +       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 ``kunit.py parse`` to parse dmesg for test output and print out
> > > +results in the same familiar format that ``kunit.py run`` does.
> >
> > This also should work for the debugfs files below, so maybe reword
> > this to either mention that or not explicitly mention dmesg above.
>
> This won't work, actually :/
> `kunit.py parse` expects a TAP version header, which doesn't get shown
> in debugfs.
>
> It'll just print out the "no tests run!" message.

Oh, I forgot about that. We need to fix that. Whoops.

> > > +
> > > +.. code-block:: bash
> > > +
> > > +       $ ./tools/testing/kunit/kunit.py 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:
> > > +
> > > +.. code-block:: none
> > > +
> > > +       CONFIG_KUNIT=y
> > > +       CONFIG_KUNIT_EXAMPLE_TEST=m
> > > +       CONFIG_KUNIT_DEBUGFS=y
> > > +
> > > +The results for each suite will be exposed under
> > > +``/sys/kernel/debug/kunit/<suite>/results``.
> > > +So using our example config:
> > > +
> > > +.. code-block:: bash
> > > +
> > > +       $ 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 Documentation/dev-tools/gcov.rst 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.
> > > +
> > > +.. code-block:: bash
> > > +
> > > +       # Reset coverage counters before running the test.
> > > +       $ echo 0 > /sys/kernel/debug/gcov/reset
> > > +       $ modprobe kunit-example-test
> > > diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst
> > > index 0e65cabe08eb..aa56d7ca6bfb 100644
> > > --- a/Documentation/dev-tools/kunit/start.rst
> > > +++ b/Documentation/dev-tools/kunit/start.rst
> > > @@ -236,5 +236,7 @@ Next Steps
> > >  ==========
> > >  *   Check out the :doc:`tips` page for tips on
> > >      writing idiomatic KUnit tests.
> > > +*   Check out the :doc:`running_tips` page for tips on
> > > +    how to make running KUnit tests easier.
> > >  *   Optional: see the :doc:`usage` page for a more
> > >      in-depth explanation of KUnit.
> > >
> > > base-commit: de2fcb3e62013738f22bbb42cbd757d9a242574e
> > > --
> > > 2.31.1.295.g9ea45b61b8-goog
> > >