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From: Brendan Higgins <>
To: Knut Omang <>
Cc: Greg KH <>,
	Kees Cook <>,,,,,,,,,,
	Linux Kernel Mailing List <>,,,
	Julia Lawall <>,
	Joel Stanley <>,,,,,,,,
Subject: Re: [RFC v2 00/14] kunit: introduce KUnit, the Linux kernel unit testing framework
Date: Mon, 26 Nov 2018 17:41:19 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Fri, Nov 23, 2018 at 9:15 PM Knut Omang <> wrote:
> On Tue, 2018-10-23 at 16:57 -0700, Brendan Higgins wrote:
> Brendan, I regret you weren't at this year's testing and fuzzing workshop at
> LPC last week so we could have continued our discussions from last year there!

Likewise! Unfortunately, I could not make it. So it goes.

> I hope we can work on this for a while longer before anything gets merged.
> Maybe it can be topic for a longer session in a future test related workshop?

I don't see why we cannot just discuss it here as we are already
doing. Besides, you are mostly interested in out of tree testing,
right? I don't see how this precludes anything that you are trying to
do with KTF.

I think the best way to develop something like what I am trying to do
with KUnit is gradually, in tree, and with the active input and
participation of the Linux kernel community.

> Links to more info about KTF:
> ------
> Git repo:
> Formatted docs:
> LWN mention from my presentation at LPC'17:
> Oracle blog post:
> OSS'18 presentation slides:
> In the documentation (see
> we present some more motivation for choices made with KTF.
> As described in that introduction, we believe in a more pragmatic approach
> to unit testing for the kernel than the classical "mock everything" approach,
> except for typical heavily algorithmic components that has interfaces simple to mock,
> such as container implementations, or components like page table traversal
> algorithms or memory allocators, where the benefit of being able to "listen"
> on the mock interfaces needed pays handsomely off.

I am not advocating that we mock everything. Using as much real code
dependencies as possible for code under test is a pretty common
position, and one which I adhere to myself.

> We also used strategies to compile kernel code in user mode,
> for parts of the code which seemed easy enough to mock interfaces for.
> I also looked at UML back then, but dismissed it in favor of the
> more lightweight approach of just compiling the code under test
> directly in user mode, with a minimal partly hand crafted, flat mock layer.

Is this new? When I tried your code out, I had to install the kernel
objects into my host kernel. Indeed, your documentation references
having to install kernel modules on the host:

> > 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.
> I am curious, with the intention of only running in user mode anyway,

I made it possible to "port" KUnit to other architectures.
Nevertheless, I believe all unit tests should be able to run without
depending on hardware or some special test harness. If I see a unit
test, I should not need to know anything about it just to run it.
Since there is no way to have all possible hardware configurations a
priori, all tests must be able to be run in a place that doesn't
depend in hardware; hence they should all be runnable as just normal
plane old user space programs with no dependency on a host kernel or
host hardware.

> why not try to build upon Googletest/Googlemock (or a similar C unit
> test framework if C is desired), instead of "reinventing"
> specific kernel macros for the tests?

I would love to reuse Googletest/Googlemock if it were possible; I
have used it a lot on other projects that I have worked on and think
it is great, but I need something I can check into the Linux kernel;
this requirement rules out Googletest/Googlemock since it depends on
C++. There are existing frameworks for C, true, but we then need to
check that into the Linux kernel or have the kernel depend on that; to
me that seemed like a lot more work than just reimplementing what we
need, which isn't much. Most of the hard parts are specific to the
kernel anyway.

> > 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.
> I think it is clearly a trade-off here: Tests run in an isolated, mocked
> environment are subject to fewer external components. But the more complex
> the mock environment gets, the more likely it also is to be a source of errors,
> nondeterminism and capacity limits itself, also the mock code would typically be
> less well tested than the mocked parts of the kernel, so it is by no means any
> silver bullet, precise testing with a real kernel on real hardware is still
> often necessary and desired.

I think you are misunderstand me. By isolation, I just mean no code
under test should depend on anything outside of the control of the
test environment. As I mention above, reusing real code for test
dependencies is highly encouraged.

As for running against hardware, yes, we need tests for that too, but
that falls under integration testing; it is possible to use what I
have here as a basis for that, but for right now, I just want to focus
on the problem of unit testing: I think this patchset is large enough
as it is.

> If the code under test is fairly standalone and complex enough, building a mock
> environment for it and test it independently may be worth it, but pragmatically,
> if the same functionality can be relatively easily exercised within the kernel,
> that would be my first choice.
> I like to think about all sorts of testing and assertion making as adding more
> redundancy. When errors surface you can never be sure whether it is a problem
> with the test, the test framework, the environment, or an actual error, and
> all places have to be fixed before the test can pass.

Yep, I totally agree, but this is why I think test isolation is so
important. If one test, or one piece of code is running that doesn't
need to be, it makes debugging tests that much more complicated.


  reply	other threads:[~2018-11-27  1:41 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-23 23:57 Brendan Higgins
2018-10-23 23:57 ` [RFC v2 01/14] kunit: test: add KUnit test runner core Brendan Higgins
2018-11-02 18:44   ` Shuah Khan
2018-11-07  1:28     ` Brendan Higgins
2018-11-07 20:02       ` Shuah Khan
2018-10-23 23:57 ` [RFC v2 02/14] kunit: test: add test resource management API Brendan Higgins
2018-10-23 23:57 ` [RFC v2 03/14] kunit: test: add string_stream a std::stream like string builder Brendan Higgins
2018-10-23 23:57 ` [RFC v2 04/14] kunit: test: add test_stream a std::stream like logger Brendan Higgins
2018-10-23 23:57 ` [RFC v2 05/14] kunit: test: add the concept of expectations Brendan Higgins
2018-10-23 23:57 ` [RFC v2 06/14] arch: um: enable running kunit from User Mode Linux Brendan Higgins
2018-10-23 23:57 ` [RFC v2 07/14] kunit: test: add initial tests Brendan Higgins
2018-10-23 23:57 ` [RFC v2 08/14] arch: um: add shim to trap to allow installing a fault catcher for tests Brendan Higgins
2018-10-23 23:57 ` [RFC v2 09/14] kunit: test: add the concept of assertions Brendan Higgins
2018-10-23 23:57 ` [RFC v2 10/14] kunit: add Python libraries for handing KUnit config and kernel Brendan Higgins
2018-10-23 23:57 ` [RFC v2 11/14] kunit: add KUnit wrapper script and simple output parser Brendan Higgins
2018-10-23 23:57 ` [RFC v2 12/14] improve output from python wrapper Brendan Higgins
2018-10-23 23:57 ` [RFC v2 13/14] Documentation: kunit: add documentation for KUnit Brendan Higgins
2018-10-23 23:57 ` [RFC v2 14/14] MAINTAINERS: add entry for KUnit the unit testing framework Brendan Higgins
2018-10-24  9:14 ` [RFC v2 00/14] kunit: introduce KUnit, the Linux kernel " Daniel Vetter
2018-10-25 21:25   ` Brendan Higgins
2018-10-25 17:40 ` Shuah Khan
2018-11-02 18:23 ` Shuah Khan
2018-11-07  1:17   ` Brendan Higgins
2018-11-07 17:46     ` Frank Rowand
2018-11-13 10:10       ` Brendan Higgins
2018-11-24  5:15 ` Knut Omang
2018-11-27  1:41   ` Brendan Higgins [this message]
2018-11-28 19:54     ` Knut Omang
2018-11-28 20:50       ` shuah
2018-11-30  0:59         ` Luis Chamberlain

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