I think he's asking if implementing CFS in userspace, with no other changes, wouldn't in itself cause more context switches. I don't understand how your comment addresses that point. Sure you could implement something different, but you could do that without the userspace component too, and adding the userspace component is pretty expensive.
Nope, you want information to go from userspace to scheduling decisions. Where the code resides is implementation. Having userspace tell the scheduler what it needs avoids the round trip if it applies to more than one scheduling decision.
