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.

Source: LWN.net – [$] User-managed concurrency groups