Date:   Sat, 2 Oct 2021 11:09:50 -0700
From:   Daniel Latypov <dlatypov@...gle.com>
To:     Marcelo Schmitt <marcelo.schmitt1@...il.com>
Cc:     brendanhiggins@...gle.com, andy.li@...soc.com,
        andersonreisrosa@...il.com, linux-kernel@...r.kernel.org,
        kunit-dev@...glegroups.com
Subject: Re: [PATCH] kunit: mock: add support for function mocks with no parameters

On Mon, Sep 27, 2021 at 9:44 AM Daniel Latypov <dlatypov@...gle.com> wrote:
>
> [2]
> On Sat, Sep 25, 2021 at 12:30 PM Marcelo Schmitt
> <marcelo.schmitt1@...il.com> wrote:
> >
> > Hi Daniel,
> >
> > Thanks for your review.
> >
> > On 09/21, Daniel Latypov wrote:
> > > On Sat, Sep 18, 2021 at 12:44 PM Marcelo Schmitt
> > > <marcelo.schmitt1@...il.com> wrote:
> > > >
> > > > Function mocks defined with DEFINE_FUNCTION_MOCK(...) do not support
> > > > empty parameters list due to strict function prototypes enforcement
> > > > (-Werror=strict-prototypes). Add support for function mocks with no
> > > > parameters by adding checks to declare strict function prototypes when
> > > > an empty param list is provided.
> > > > Further, add an expectation to test that the generated code works.
> > > >
> > > > Co-developed-by: Anderson Reis Rosa <andersonreisrosa@...il.com>
> > > > Signed-off-by: Anderson Reis Rosa <andersonreisrosa@...il.com>
> > > > Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@...il.com>
> > >
> > > Meta: kunit/alpha/master isn't really maintained anymore.
> > > I think David and myself having some commits from this year might give
> > > the wrong impression.
> > > But all of my patches in 2021 were to make it easier to get people to
> > > *move away* from kunit/alpha/master ;)
> >
> > We can't submit it upstream because the mock stuff isn't there yet.
> > By the way, as nothing from mocking is upstream and kunit/alpha/master is
> > being frozen somewhat, what tree/branch should we base our work on if we
> > decide to develop more on the mocking framework?
> > I recall the branch with the POC for mocking was at
> > https://kunit-review.googlesource.com/c/linux/+/1114
> > Should we use this branch to base future work on mocking?
> > Or will the mocking framework be discontinued?
>
> All the mocking stuff is in limbo at the moment.
> The v2 of the class mocking RFC was sent out Oct 2020,
> https://lore.kernel.org/linux-kselftest/20201012222050.999431-1-dlatypov@google.com/
>
> Until we have user interest in mocking support, that RFC will likely
> just sit there.
> Or maybe we scrap it and just introduce the functionality piece by piece.
>
> There's https://kunit.dev/mocking.html which talks about how one can
> implement mocking on their own, or (better yet) write fakes while
> leveraging KUnit.
> If we start seeing adoption of that, we can start factoring out
> functionality into shared code as needed, e.g. support for saying a
> mock should be called N times, then maybe gradually the parameter
> matchers and return values, etc.
>
> I missed it, but I know David and Brendan talked a bit about this in
> their recent LPC talk, https://youtu.be/Y_minEhZNm8?t=15905
>
> >
> > Sorry for asking so many questions. We just want to help to enhance KUnit.
>
> No worries, and we really appreciate it.
>
> > We can work on something else besides mocking if it makes more sense to the
> > project.
>
> Mocking doesn't feel like an area where we can expect to see progress right now.
> In terms of other KUnit features we know would be useful now, I think
> it's mostly just [1] and [2], which hopefully will land in 5.16.

To be clear, if anyone thinks up a useful feature, that'd be great.
I personally am just out of ideas at the moment, and I think so are
Brendan and David.

We'd want to prioritize features that can improve existing tests or
unblock known new tests.
Mocking in the alpha version of KUnit is a case where a feature
sounded really good on paper and had a bunch of bells and whistles
(e.g. strict/nice/naggy mocks support, etc.) but was perhaps
overengineered and thus failed to find a home upstream.

But I just thought of a few more things we could do in the kunit.py script.
I think we have more room for improvement there than in the in-kernel
part of KUnit right now, but I assume it's the more boring part for
most people.

One thing I'd really like to see is getting code coverage to work in
kunit.py while using QEMU.
We have a process for doing so under UML here:
https://lore.kernel.org/linux-kselftest/20210901190623.315736-1-rmoar@google.com/
UML actually uses a different coverage implementation than normal, so
there's a few things that would need to change.

We can build and run against "normal" coverage kernels pretty easily:

$ cat >qemu_coverage_kunitconfig <<EOF
CONFIG_KUNIT=y
CONFIG_KUNIT_EXAMPLE_TEST=y
CONFIG_GCOV_KERNEL=y
CONFIG_DEBUG_FS=y
CONFIG_GCOV_PROFILE_ALL=y
EOF
$ ./tools/testing/kunit/kunit.py run --arch=x86_64
--kunitconfig=qemu_coverage_kunitconfig

The problem is we'd need to copy the coverage data off the VM instead
of just letting it shutdown when tests are done.
If we had a userspace running, we'd basically do something like
$ scp -r user@vm:/sys/kernel/debug/gcov .
<some stuff to get these files in the right spot under .kunit/>
<then we'd run lcov and genhtml, just like we do for UML>

Normal KUnit tests definitely don't want to have to have the overhead
of running a userspace, so the implementation might look like a
"--qemu_coverage" flag, or maybe a set of generic flags that would
give a user enough control over the VM to do this.
Or maybe the right answer is to not involve kunit.py at all.

Not sure if that sounds interesting to you or anyone.

>
> I think right now we probably need more tests written to have a better
> idea of what else we could/should do.
> Partly because of that, David is trying to get the ball rolling on
> testing ext4. We're also hopeful that it'll be easier to add tests if
> adjacent code is already tested (sharing fakes, conventions, ability
> to copy-paste, etc.).
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git/commit/?h=kunit&id=3b29021ddd10cfb6b2565c623595bd3b02036f33
> [2] https://lore.kernel.org/linux-kselftest/20210909001037.2842954-1-dlatypov@google.com/
>
>

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