/proc virtualization

When we run a fully featured container, we need a containerized view of the proc filesystem. Current upstream kernel can do that for the process tree and a few other things, but that is just not enough. For running top, for instance, one needs to know not only that, but also how much cpu time each container used in total, how much of that is system time, for how long that container was out of the CPU, etc. That information is available - or will be (some if it is patches pending) in different cgroups.

What are the best ways to achieve this? What problems will we face? Is there any value in keep trying to push this functionality into the kernel, or would a userspace implementation suffice ?