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Date:   Wed, 8 May 2019 17:58:49 -0700
From:   Frank Rowand <>
To:     Theodore Ts'o <>,
        Greg KH <>,
        Brendan Higgins <>,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Subject: Re: [PATCH v2 00/17] kunit: introduce KUnit, the Linux kernel unit
 testing framework

Hi Ted,

On 5/7/19 10:22 AM, Theodore Ts'o wrote:
> On Tue, May 07, 2019 at 10:01:19AM +0200, Greg KH wrote:
Not very helpful to cut the text here, plus not explicitly indicating that
text was cut (yes, I know the ">>>" will be a clue for the careful reader),
losing the set up for my question.

>>> 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?
> Yes, that's basically right.  You don't *have* to use KUnit.  It's

If KUnit is added to the kernel, and a subsystem that I am submitting
code for has chosen to use KUnit instead of kselftest, then yes, I do
*have* to use KUnit if my submission needs to contain a test for the
code unless I want to convince the maintainer that somehow my case
is special and I prefer to use kselftest instead of KUnittest.

> supposed to be a simple way to run a large number of small tests that
> for specific small components in a system.

kselftest also supports running a subset of tests.  That subset of tests
can also be a large number of small tests.  There is nothing inherent
in KUnit vs kselftest in this regard, as far as I am aware.

> For example, I currently use xfstests using KVM and GCE to test all of
> ext4.  These tests require using multiple 5 GB and 20GB virtual disks,
> and it works by mounting ext4 file systems and exercising ext4 through
> the system call interfaces, using userspace tools such as fsstress,
> fsx, fio, etc.  It requires time overhead to start the VM, create and
> allocate virtual disks, etc.  For example, to run a single 3 seconds
> xfstest (generic/001), it requires full 10 seconds to run it via
> kvm-xfstests.

> KUnit is something else; it's specifically intended to allow you to
> create lightweight tests quickly and easily, and by reducing the
> effort needed to write and run unit tests, hopefully we'll have a lot
> more of them and thus improve kernel quality.

The same is true of kselftest.  You can create lightweight tests in

> As an example, I have a volunteer working on developing KUinit tests
> for ext4.  We're going to start by testing the ext4 extent status
> tree.  The source code is at fs/ext4/extent_status.c; it's
> approximately 1800 LOC.  The Kunit tests for the extent status tree
> will exercise all of the corner cases for the various extent status
> tree functions --- e.g., ext4_es_insert_delayed_block(),
> ext4_es_remove_extent(), ext4_es_cache_extent(), etc.  And it will do
> this in isolation without our needing to create a test file system or
> using a test block device.

> Next we'll test the ext4 block allocator, again in isolation.  To test
> the block allocator we will have to write "mock functions" which
> simulate reading allocation bitmaps from disk.  Again, this will allow
> the test writer to explicitly construct corner cases and validate that
> the block allocator works as expected without having to reverese
> engineer file system data structures which will force a particular
> code path to be executed.

This would be a difference, but mock functions do not exist in KUnit.
The KUnit test will call the real kernel function in the UML kernel.

I think Brendan has indicated a desire to have mock functions in the

Brendan, do I understand that correctly?


> So this is why it's largely irrelevant to me that KUinit uses UML.  In
> fact, it's a feature.  We're not testing device drivers, or the
> scheduler, or anything else architecture-specific.  UML is not about
> virtualization.  What it's about in this context is allowing us to
> start running test code as quickly as possible.  Booting KVM takes
> about 3-4 seconds, and this includes initializing virtio_scsi and
> other device drivers.  If by using UML we can hold the amount of
> unnecessary kernel subsystem initialization down to the absolute
> minimum, and if it means that we can communicating to the test
> framework via a userspace "printf" from UML/KUnit code, as opposed to
> via a virtual serial port to KVM's virtual console, it all makes for
> lighter weight testing.
> Why did I go looking for a volunteer to write KUnit tests for ext4?
> Well, I have a plan to make some changes in restructing how ext4's
> write path works, in order to support things like copy-on-write, a
> more efficient delayed allocation system, etc.  This will require
> making changes to the extent status tree, and by having unit tests for
> the extent status tree, we'll be able to detect any bugs that we might
> accidentally introduce in the es tree far more quickly than if we
> didn't have those tests available.  Google has long found that having
> these sorts of unit tests is a real win for developer velocity for any
> non-trivial code module (or C++ class), even when you take into
> account the time it takes to create the unit tests.
> 					- Ted>
> P.S.  Many thanks to Brendan for finding such a volunteer for me; the
> person in question is a SRE from Switzerland who is interested in
> getting involved with kernel testing, and this is going to be their
> 20% project.  :-)

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