[$] Finding real-world kernel subsystems

The kernel development community talks often about subsystems and subsystem
maintainers, but it is less than entirely clear about what a "subsystem" is in
the first place. People wanting to understand how kernel development works
could benefit from a clearer idea of what actually comprises a subsystem
within the kernel. In an attempt to better understand how kernel
development works, Pia Eichinger and her colleagues spent a lot of time looking
for the actual boundaries; Eichinger presented that work at the 2021
linux.conf.au online gathering.