I think it is a bigger problem that you can't get a snapshot of all processes atomically. It is easy enough to solve the case of atomically accessing a single process, though annoying.
(Even in a single core world I suspect you'd hit a lot of resistance in any method you could use to take an atomic state, for performance reasons. Even just telling the kernel to "stop doing everything else and give me a copy of all this information" is not going to go over well.)
The fact that you can't get that info easily on Linux makes a lot of tools harder to write than they should be, and often give misleading/incorrect information.
1. https://msdn.microsoft.com/en-us/library/windows/desktop/ms6... 2. https://developer.apple.com/legacy/library/documentation/Dar...
The OSX one, I don't know; I scanned over the docs and it went over my threshold for what I'm willing to poke through for a HN post.
In other words, this is exactly what I left myself an out for, and I see nothing that contradicts what I said for the Windows case.
[1]: https://msdn.microsoft.com/en-us/library/windows/desktop/ms6...