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

Brendan Higgins
2019-05-14T08:22:22Z

On Wed, May 08, 2019 at 05:58:49PM -0700, Frank Rowand wrote:
> 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
> kselftest.
>
>
> > 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
> future.
>
> Brendan, do I understand that correctly?

Oh, sorry, I missed this comment from earlier.

Yes, you are correct. Function mocking is a feature I will be
introducing in a follow up patchset (assuming this one gets merged of
course ;-) ).

Cheers!

> -Frank
>
> > 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. :-)
> >
> >