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

[Dennis Zhou <dennis@kernel.org>]

Hello,

On Thu, Sep 02, 2021 at 08:47:42AM -0700, Linus Torvalds wrote:
> On Tue, Aug 31, 2021 at 2:18 PM Darrick J. Wong <djwong@kernel.org> wrote:
> >
> > As for new features: we now batch inode inactivations in percpu
> > background threads, which sharply decreases frontend thread wait time
> > when performing file deletions and should improve overall directory tree
> > deletion times.
> 
> So no complaints on this one, but I do have a reaction: we have a lot
> of these random CPU hotplug events, and XFS now added another one.
> 
> I don't see that as a problem, but just the _randomness_ of these
> callbacks makes me go "hmm". And that "enum cpuhp_state" thing isn't
> exactly a thing of beauty, and just makes me think there's something
> nasty going on.
> 
> For the new xfs usage, I really get the feeling that it's not that XFS
> actually cares about the CPU states, but that this is literally tied
> to just having percpu state allocated and active, and that maybe it
> would be sensible to have something more specific to that kind of use.
> 
> We have other things that are very similar in nature - like the page
> allocator percpu caches etc, which for very similar reasons want cpu
> dead/online notification.
> 
> 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.
> 
> 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.
> 
> Comments?
> 

I think there are 2 pieces here from percpu's side:
A) Onlining and offlining state related to a percpu alloc.
B) Freeing backing memory of an allocation wrt hot plug.

An RFC was sent out for B) in [1] and you need A) for B).
I can see percpu having a callback model for basic allocations that are
independent, but for anything more complex, that subsystem would need to
register with hotplug anyway. It appears percpu_counter already has hot
plug support. percpu_refcount could be extended as well, but more
complex initialization like the runqueues and slab related allocations
would require work. In short, yes I think A) is doable/reasonable.

Freeing the backing memory for A) seems trickier. We would have to
figure out a clean way to handle onlining/offlining racing with new
percpu allocations (adding or removing pages for the corresponding cpu's
chunk). To support A), init and onlining/offlining can be separate
phases, but for B) init/freeing would have to be rolled into
onlining/offlining.

Without freeing, it's not incorrect for_each_online_cpu() to read a dead
cpu's percpu values, but with freeing it does.

I guess to summarize, A) seems like it might be a good idea with
init/destruction happening at allocation/freeing times. I'm a little
skeptical of B) in terms of complexity. If y'all think it's a good idea
I can look into it again.

[1] https://lore.kernel.org/lkml/20210601065147.53735-1-bharata@linux.ibm.com/

Thanks,
Dennis

> > Lastly, with this release, two new features have graduated to supported
> > status: inode btree counters (for faster mounts), and support for dates
> > beyond Y2038.
> 
> Oh, I had thought Y2038 was already a non-issue for xfs. Silly me.
> 
>               Linus