[PATCH percpu/for-3.17 1/2] percpu: implement percpu_pool

[Tejun Heo]

Hello,

So, I have another case where percpu allocation needs to be performed
from an atomic context (also on the IO path), so I wrote up a simple
percpu alloc cache which is filled asynchronously and replaced
blk-throttle's custom implementation with it.

I still think it's quite unlikely that we implement atomic allocation
directly in the percpu allocator; however, even if that happens
eventually, having percpu_pool instead of multiple custom mechanisms
scattered around the kernel should make it a lot easier to transition
to that.

Thanks.
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