bsd variants and sun os used to have n:m APIs but they got deprecated and removed long time ago. It's hard to provide a consistent behavior that works for everyone.
I said in a sibling comment that scheduler activations may have been a flawed idea, but I don’t think the space of user-space scheduling APIs is fully explored. Google’s ghOSt work is an example of that. If io-uring is proof of anything, it’s that there’s still fundamental changes we can make in how we schedule work with the kernel.
