> On Tue, May 07, 2019 at 10:01:19AM +0200, Greg KH wrote: > > > 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 > supposed to be a simple way to run a large number of small tests that > for specific small components in a system. > > 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. > > 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. > > 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. :-) Thanks Ted, I really appreciate it! Since Ted provided such an awesome detailed response, I don't think I really need to go into any detail; nevertheless, I think that Greg and Shuah have the right idea; in particular, Shuah provides a good summary. Thanks everyone!
