OK, so the point of clamping all sockets simultaneously is to be able
to power down the electronics surrounding the sockets as well as the
sockets themselves?  

yup; memory can go to self refresh etc etc

If all you cared about was the individual sockets,
I don't see why you couldn't power the sockets down individually rather
than in sync with each other.

the hardware that this driver supports does not support powering down sockets individually.

(since the memory controllers are part of the "socket"... it would increase latency
etc etc, and likely wreak havoc with the cache coherency protocols)

I think I know, but I feel the need to ask anyway.  Why not tell
RCU about the clamping?

I don't mind telling RCU, but what cannot happen is a bunch of CPU time suddenly getting used
(since that is the opposite of what is needed at the specific point in time of going idle)