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Subject: Re: [PATCH v2 00/17] kunit: introduce KUnit, the Linux kernel unit
 testing framework
To:     Greg KH <gregkh@linuxfoundation.org>,
        Frank Rowand <frowand.list@gmail.com>
Cc:     Brendan Higgins <brendanhiggins@google.com>, keescook@google.com,
        kieran.bingham@ideasonboard.com, mcgrof@kernel.org,
        robh@kernel.org, sboyd@kernel.org, devicetree@vger.kernel.org,
        dri-devel@lists.freedesktop.org, kunit-dev@googlegroups.com,
        linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-um@lists.infradead.org, Alexander.Levin@microsoft.com,
        Tim.Bird@sony.com, amir73il@gmail.com, dan.carpenter@oracle.com,
        dan.j.williams@intel.com, daniel@ffwll.ch, jdike@addtoit.com,
        joel@jms.id.au, julia.lawall@lip6.fr, khilman@baylibre.com,
        knut.omang@oracle.com, logang@deltatee.com, mpe@ellerman.id.au,
        pmladek@suse.com, richard@nod.at, rientjes@google.com,
        rostedt@goodmis.org, wfg@linux.intel.com, shuah <shuah@kernel.org>
References: <20190501230126.229218-1-brendanhiggins@google.com>
 <54940124-50df-16ec-1a32-ad794ee05da7@gmail.com>
 <20190507080119.GB28121@kroah.com>
From:   shuah <shuah@kernel.org>
Message-ID: <1b1efa91-0523-21a9-e541-fdc3612bd117@kernel.org>
Date:   Tue, 7 May 2019 09:23:31 -0600
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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