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

From: Frank Rowand
Date: Mon May 06 2019 - 23:15:05 EST

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,
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.

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?


> ## More information on KUnit
> There is a bunch of documentation near the end of this patch set that
> describes how to use KUnit and best practices for writing unit tests.
> For convenience I am hosting the compiled docs here:
> Additionally for convenience, I have applied these patches to a branch:
> The repo may be cloned with:
> git clone
> This patchset is on the kunit/rfc/v5.1-rc7/v1 branch.
> ## Changes Since Last Version
> None. I just rebased the last patchset on v5.1-rc7.