[$] User-managed concurrency groups

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