[$] Sub-schedulers for sched_ext

The extensible scheduler class (sched_ext)
allows the installation of a custom CPU scheduler built as a set of BPF
programs. Its merging for the 6.12 kernel release moved the kernel away
from the “one scheduler fits all” approach that had been taken until then;
now any system can have its own scheduler optimized for its workloads.
Within any given machine, though, it’s still “one scheduler fits all”; only
one scheduler can be loaded for the system as a whole. The sched_ext
sub-scheduler patch series from Tejun Heo aims to change that situation
by allowing multiple CPU schedulers to run on a single system.