Documentation: kunit: Add some troubleshooting tips to the FAQ

Add an FAQ entry to the KUnit documentation with some tips for
troubleshooting KUnit and kunit_tool.

These suggestions largely came from an email thread:
https://lore.kernel.org/linux-kselftest/41db8bbd-3ba0-8bde-7352-083bf4b947ff@intel.com/T/#m23213d4e156db6d59b0b460a9014950f5ff6eb03

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/faq.rst

@@ -61,3 +61,43 @@ test, or an end-to-end test.
   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.