I think MIPS RISC/Os had 'green' threads in the 80's, or I could be mixing it up with Tandem terminology, where Fiber was a constant type, I seem to recall .. either way, the idea of a userspace-managed thread scheme is as old as the hills.

The question has always been: who deals out the work, the OS or the App? and as we can see, the question will continue to be asked, and un-answered, probably ad infinitum ..