Re: [PATCH 0/2] RFC: zswap tree use xarray instead of RB tree

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Jan 17, 2024 at 11:28 PM Chris Li <chrisl@kernel.org> wrote:
>
> On Wed, Jan 17, 2024 at 11:05 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > The name changes from Chris to Christopher are confusing :D
> >
> > >
> > > I think it makes the review easier. The code adding and removing does
> > > not have much overlap. Combining it to a single patch does not save
> > > patch size. Having the assert check would be useful for some bisecting
> > > to narrow down which step causing the problem. I am fine with squash
> > > it to one patch as well.
> >
> > I think having two patches is unnecessarily noisy, and we add some
> > debug code in this patch that we remove in the next patch anyway.
> > Let's see what others think, but personally I prefer a single patch.
> >
> > > >
> > > > >
> > > > > I expect to merge the zswap rb tree spin lock with the xarray
> > > > > lock in the follow up changes.
> > > >
> > > > Shouldn't this simply be changing uses of tree->lock to use
> > > > xa_{lock/unlock}? We also need to make sure we don't try to lock the
> > > > tree when operating on the xarray if the caller is already holding the
> > > > lock, but this seems to be straightforward enough to be done as part
> > > > of this patch or this series at least.
> > > >
> > > > Am I missing something?
> > >
> > > Currently the zswap entry refcount is protected by the zswap tree spin
> > > lock as well. Can't remove the tree spin lock without changing the
> > > refcount code. I think the zswap search entry should just return the
> > > entry with refcount atomic increase, inside the RCU read() or xarray
> > > lock. The previous zswap code does the find_and_get entry() which is
> > > closer to what I want.
> >
> > I think this can be done in an RCU read section surrounding xa_load()
>
> xa_load() already has RCU read lock inside. If you do that you might
> just as well use some XAS API to work with the lock directly.

RCU reda locks are nestable, some users of xa_load() do some in an RCU
read section, also for refcounting purposes. Also, I thought the point
was avoiding the lock in this path.

>
> > and the refcount increment. Didn't look closely to check how much
> > complexity this adds to manage refcounts with RCU, but I think there
> > should be a lot of examples all around the kernel.
>
> The complexity is not adding the refcount inside xa_load(). It is on
> the zswap code that calls zswap_search() and zswap_{insert,erase}().
> As far as I can tell, those codes need some tricky changes to go along
> with the refcount change.

I don't think it should be very tricky.
https://docs.kernel.org/RCU/rcuref.html may have relevant examples,
and there should be examples all over the code.

>
> >
> > IIUC, there are no performance benefits from this conversion until we
> > remove the tree spinlock, right?
>
> The original intent is helping the long tail case. RB tree has worse
> long tails than xarray. I expect it will help the page fault long tail
> even without removing the tree spinlock.

I think it would be better if we can remove the tree spinlock as part
of this change.