[$] User-managed concurrency groups

The kernel's thread model is relatively straightforward and performsreasonably well, but that's not enough for all users. Specifically, thereare use cases out there that benefit from a lightweight threading modelthat gives user space control over scheduling decisions. Back in May 2021,Peter Oskolkov posted a patch set implementing an abstraction known as user-managedconcurrency groups, or UMCG. Several revisions later, many observersstill lack a clear idea of what this patch is supposed to do, much lesswhether it is a good idea for the kernel. Things have taken a turn,though, with Peter Zijlstra's reimplementationof UMCG.