[$] Sub-schedulers for sched_ext

The extensible scheduler class (sched_ext)allows the installation of a custom CPU scheduler built as a set of BPFprograms. Its merging for the 6.12 kernel release moved the kernel awayfrom 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"; onlyone scheduler can be loaded for the system as a whole. The sched_extsub-scheduler patch series from Tejun Heo aims to change that situationby allowing multiple CPU schedulers to run on a single system.