Linux kernel is the most important part of Linux operating system. Everything else depends on it, all applications must interact with it. Software bugs which are located there, in the most low-level areas of the system, are most difficult and most time-consuming to debug and fix.
That is why one should pay special attention while developing features in the kernel. How does it look in practice? Read this fascinating post on #embeddedbits blog to learn from people who deal first-hand with kernel testing: