Date:   Tue, 7 May 2019 10:01:19 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Brendan Higgins <brendanhiggins@...gle.com>, keescook@...gle.com,
        kieran.bingham@...asonboard.com, mcgrof@...nel.org,
        robh@...nel.org, sboyd@...nel.org, shuah@...nel.org,
        devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        kunit-dev@...glegroups.com, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-um@...ts.infradead.org,
        Alexander.Levin@...rosoft.com, Tim.Bird@...y.com,
        amir73il@...il.com, dan.carpenter@...cle.com,
        dan.j.williams@...el.com, daniel@...ll.ch, jdike@...toit.com,
        joel@....id.au, julia.lawall@...6.fr, khilman@...libre.com,
        knut.omang@...cle.com, logang@...tatee.com, mpe@...erman.id.au,
        pmladek@...e.com, richard@....at, rientjes@...gle.com,
        rostedt@...dmis.org, wfg@...ux.intel.com
Subject: Re: [PATCH v2 00/17] kunit: introduce KUnit, the Linux kernel unit
 testing framework

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.
> 
> 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,

greg k-h

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