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From: Dennis Zhou <email@example.com>
To: Linus Torvalds <firstname.lastname@example.org>
Cc: "Darrick J. Wong" <email@example.com>,
Thomas Gleixner <firstname.lastname@example.org>,
Dennis Zhou <email@example.com>, Tejun Heo <firstname.lastname@example.org>,
Dave Chinner <email@example.com>,
Linux Kernel Mailing List <firstname.lastname@example.org>,
Eric Sandeen <email@example.com>,
Christoph Hellwig <firstname.lastname@example.org>
Subject: Re: [GIT PULL] xfs: new code for 5.15
Date: Fri, 3 Sep 2021 14:40:58 -0400 [thread overview]
Message-ID: <YTJsOoqaI3FiTkZD@fedora> (raw)
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 <email@example.com> 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.
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  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
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.
> > 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.
next prev parent reply other threads:[~2021-09-03 18:41 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-31 21:18 Darrick J. Wong
2021-09-02 15:47 ` Linus Torvalds
2021-09-02 17:43 ` Darrick J. Wong
2021-09-02 22:35 ` Dave Chinner
2021-09-03 6:26 ` Thomas Gleixner
2021-09-05 0:21 ` Dave Chinner
2021-09-05 23:28 ` Thomas Gleixner
2021-09-06 2:11 ` Randy Dunlap
2021-09-06 9:42 ` Thomas Gleixner
2021-09-02 19:13 ` Thomas Gleixner
2021-09-03 4:29 ` Christoph Hellwig
2021-09-03 18:40 ` Dennis Zhou [this message]
2021-09-02 17:37 ` pr-tracker-bot
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