How to allocate per-cpu data for online CPUs only (and safely)?

[Martin Peschke <mp3@de.ibm.com>]

I am trying to optimize the memory footprint of some code of mine.
It's been using per-cpu data and alloc_percpu() so far. The latter has 
the disadvantage of getting hold of memory for CPU's which aren't there 
(yet).

I could imagine using CPU-hotplug notifications as triggers for 
additional allocations or for cleaning up unneeded memory. But 
alloc_percpu() appears to conflict with that idea.

I was briefly tempted to derive some code from alloc_percpu() more to my 
liking, until I was scared off by this comment in alloc_percpu():

         /*
          * Cannot use for_each_online_cpu since a cpu may come online
          * and we have no way of figuring out how to fix the array
          * that we have allocated then....
          */

Well, and then there is kernel/profile.c, for example, which boldly 
ignors alloc_percpu()'s qualms and allocates and releases per-cpu data 
as needed.

Is that the way to go?
If so, why alloc_percpu()'s reservations?
Or, does that comment imply that the exploiter isn't expected to take 
care of CPU hotplug events?
Am I missing anything?

Thank you.

Martin