Date:   Tue, 14 May 2019 01:22:14 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Greg KH <gregkh@...uxfoundation.org>, 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 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.  :-)
> > 
> > 

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