Re: [RFC v4 00/17] kunit: introduce KUnit, the Linux kernel unit testing framework

From: Brendan Higgins
Date: Thu Mar 21 2019 - 12:55:28 EST

On Thu, Mar 21, 2019 at 8:56 AM Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:
> On 2019-03-20 11:23 p.m., Knut Omang wrote:
> > Testing drivers, hardware and firmware within production kernels was the use
> > case that inspired KTF (Kernel Test Framework). Currently KTF is available as a
> > standalone git repository. That's been the most efficient form for us so far,
> > as we typically want tests to be developed once but deployed on many different
> > kernel versions simultaneously, as part of continuous integration.
> Interesting. It seems like it's really in direct competition with KUnit.

I won't speak for Knut, but I don't think we are in competition. I see
KTF as a novel way to do a kind of white box end-to-end testing for
the Linux kernel, which is a valuable thing, especially in some
circumstances. I could see KTF having a lot of value for someone who
wants to maintain out of tree drivers, in particular.

Nevertheless, I don't really see KTF as a real unit testing framework
for a number of different reasons; you pointed out some below, but I
think the main one being that it requires booting a real kernel on
actual hardware; I imagine it could be made to work on a VM, but that
isn't really the point; it fundamentally depends on having part of the
test, or at least driving the test from userspace on top of the kernel
under test. Knut, myself, and others, had a previous discussion to
this effect here:

> I didn't really go into it in too much detail but these are my thoughts:
> From a developer perspective I think KTF not being in the kernel tree is
> a huge negative. I want minimal effort to include my tests in a patch
> series and minimal effort for other developers to be able to use them.
> Needing to submit these tests to another project or use another project
> to run them is too much friction.
> Also I think the goal of having tests that run on any kernel version is
> a pipe dream. You'd absolutely need a way to encode which kernel
> versions a test is expected to pass on because the tests might not make
> sense until a feature is finished in upstream. And this makes it even
> harder to develop these tests because, when we write them, we might not
> even know which kernel version the feature will be added to. Similarly,
> if a feature is removed or substantially changed, someone will have to
> do a patch to disable the test for subsequent kernel versions and create
> a new test for changed features. So, IMO, tests absolutely have to be
> part of the kernel tree so they can be changed with the respective
> features they test.
> Kunit's ability to run without having to build and run the entire kernel
> is also a huge plus. (Assuming there's a way to get around the build
> dependency issues). Because of this, it can be very quick to run these
> tests which makes development a *lot* easier seeing you don't have to
> reboot a machine every time you want to test a fix.

I will reply to your comments directly on your original email. I don't
want to hijack this thread, in case we want to discuss the topic of
KUnit vs. KTF further.