The BPF extensible scheduler class

It was only a matter of time before somebody found a way to inject BPF into
the CPU scheduler. This patch
series, posted by Tejun Heo and containing work by David Vernet, Josh
Don, and Barret Rhoden, does exactly that. The cover letter covers the
motivation behind this work in detail:

One of our main goals was to lower the barrier to entry for
experimenting with the scheduler. sched_ext provides ergonomic
callbacks and helpers to ease common operations such as managing
idle CPUs, scheduling tasks on arbitrary CPUs, handling preemptions
from other scheduling classes, and more. While sched_ext does
require some ramp-up, the complexity is self-contained, and the
learning curve gradual. Developers can ramp up by first
implementing simple policies such as global FIFO in only tens of
lines of code, and then continue to learn the APIs and building
blocks available with sched_ext as they build more featureful and
complex schedulers.

There is a bit more documentation in this
patch.