Re: [GIT PULL] xfs: new code for 5.15

[Christoph Hellwig]

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