On Thu, Sep 02, 2021 at 09:13:24PM +0200, Thomas Gleixner wrote:

quoted

I'm only throwing this out as a reaction to this - I'm not sure
another interface would be good or worthwhile, but that "enum
cpuhp_state" is ugly enough that I thought I'd rope in Thomas for CPU
hotplug, and the percpu memory allocation people for comments.

It's not only about memory. 

quoted

IOW, just _maybe_ we would want to have some kind of callback model
for "percpu_alloc()" and it being explicitly about allocations
becoming available or going away, rather than about CPU state.

The per cpu storage in XFS does not go away. It contains a llist head
and the queued work items need to be moved from the dead CPU to an alive
CPU and exposed to a work queue for processing. Similar to what we do
with timers, hrtimers and other stuff.

If there are callbacks which are doing pretty much the same thing, then
I'm all for a generic infrastructure for these.

In the block layer we've added a new per-cpu bio list, for which
the dead callback literally does nothing but free some memory.
For that case a simple callback would be neat, but I'm not sure how
common that is.