Re: [PATCH v2 00/17] kunit: introduce KUnit, the Linux kernel unit testing framework

[shuah]

On 5/7/19 2:01 AM, Greg KH wrote:
> On Mon, May 06, 2019 at 08:14:12PM -0700, Frank Rowand wrote:
> > On 5/1/19 4:01 PM, Brendan Higgins wrote:
> > > ## TLDR
> > >
> > > I rebased the last patchset on 5.1-rc7 in hopes that we can get this in
> > > 5.2.
> > >
> > > Shuah, I think you, Greg KH, and myself talked off thread, and we agreed
> > > we would merge through your tree when the time came? Am I remembering
> > > correctly?
> > >
> > > ## Background
> > >
> > > This patch set proposes KUnit, a lightweight unit testing and mocking
> > > framework for the Linux kernel.
> > >
> > > Unlike Autotest and kselftest, KUnit is a true unit testing framework;
> > > it does not require installing the kernel on a test machine or in a VM
> > > and does not require tests to be written in userspace running on a host
> > > kernel. Additionally, KUnit is fast: From invocation to completion KUnit
> > > can run several dozen tests in under a second. Currently, the entire
> > > KUnit test suite for KUnit runs in under a second from the initial
> > > invocation (build time excluded).
> > >
> > > KUnit is heavily inspired by JUnit, Python's unittest.mock, and
> > > Googletest/Googlemock for C++. KUnit provides facilities for defining
> > > unit test cases, grouping related test cases into test suites, providing
> > > common infrastructure for running tests, mocking, spying, and much more.
> >
> > As a result of the emails replying to this patch thread, I am now
> > starting to look at kselftest.  My level of understanding is based
> > on some slide presentations, an LWN article, https://kselftest.wiki.kernel.org/
> > and a _tiny_ bit of looking at kselftest code.
> >
> > tl;dr; I don't really understand kselftest yet.
> >
> >
> > (1) why KUnit exists
> >
> > > ## What's so special about unit testing?
> > >
> > > A unit test is supposed to test a single unit of code in isolation,
> > > hence the name. There should be no dependencies outside the control of
> > > the test; this means no external dependencies, which makes tests orders
> > > of magnitudes faster. Likewise, since there are no external dependencies,
> > > there are no hoops to jump through to run the tests. Additionally, this
> > > makes unit tests deterministic: a failing unit test always indicates a
> > > problem. Finally, because unit tests necessarily have finer granularity,
> > > they are able to test all code paths easily solving the classic problem
> > > of difficulty in exercising error handling code.
> >
> > (2) KUnit is not meant to replace kselftest
> >
> > > ## Is KUnit trying to replace other testing frameworks for the kernel?
> > >
> > > No. Most existing tests for the Linux kernel are end-to-end tests, which
> > > have their place. A well tested system has lots of unit tests, a
> > > reasonable number of integration tests, and some end-to-end tests. KUnit
> > > is just trying to address the unit test space which is currently not
> > > being addressed.
> >
> > My understanding is that the intent of KUnit is to avoid booting a kernel on
> > real hardware or in a virtual machine.  That seems to be a matter of semantics
> > to me because isn't invoking a UML Linux just running the Linux kernel in
> > a different form of virtualization?
> >
> > So I do not understand why KUnit is an improvement over kselftest.

They are in two different categories. Kselftest falls into black box
regression test suite which is a collection of user-space tests with a
few kernel test modules back-ending the tests in some cases.

Kselftest can be used by both kernel developers and users and provides
a good way to regression test releases in test rings.

KUnit is a white box category and is a better fit as unit test framework
for development and provides a in-kernel testing. I wouldn't view them
one replacing the other. They just provide coverage for different areas
of testing.

I wouldn't view KUnit as something that would be easily run in test 
rings for example.

Brendan, does that sound about right?

> > It seems to me that KUnit is just another piece of infrastructure that I
> > am going to have to be familiar with as a kernel developer.  More overhead,
> > more information to stuff into my tiny little brain.
> >
> > I would guess that some developers will focus on just one of the two test
> > environments (and some will focus on both), splitting the development
> > resources instead of pooling them on a common infrastructure.

> > What am I missing?
>
> kselftest provides no in-kernel framework for testing kernel code
> specifically.  That should be what kunit provides, an "easy" way to
> write in-kernel tests for things.
>
> Brendan, did I get it right?
thanks,
-- Shuah