Documentation: kunit: Add some troubleshooting tips to the FAQ

  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 isn't working, what should I do?

======================================

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

things to try.

1. Try running ``./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 running ``./vmlinux`` after

   building the UML kernel (e.g., 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. 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.

5. 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.

6. Finally, try running KUnit outside UML. KUnit and KUnit tests can run be

   built into any kernel, or can be built as a module and loaded at runtime.

   Doing so should allow you to determine if UML is causing the issue you're

   seeing. 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 "KUnit on

   non-UML architectures" in :doc:`usage`.

If none of the above tricks help, you are always welcome to email any issues to

kunit-dev@googlegroups.com.