The extensible scheduler class

I'm not really sure what I feel about pluggable schedulers (it sounds like a good idea superficially, but I don't know the domain enough to emit judgement), but I can't really see why Zijlstra sees the BPF as a worsening over other proposals. IMO that's a net improvement if only because we don't need yet another unstable idiosyncratic API that negates any advantage of the approach in the first place. Even if the idea is bad, BPF sounds like the right way to implement it to me.