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@xxxxxxxxxx> 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@xxxxxxxxxxxxx/ 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
