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

[Christopher Li <chrisl@kernel.org>]

On Wed, Jan 17, 2024 at 10:02 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> That's a long CC list for sure :)
>
> On Wed, Jan 17, 2024 at 7:06 PM Chris Li <chrisl@kernel.org> wrote:
> >
> > The RB tree shows some contribution to the swap fault
> > long tail latency due to two factors:
> > 1) RB tree requires re-balance from time to time.
> > 2) The zswap RB tree has a tree level spin lock protecting
> > the tree access.
> >
> > The swap cache is using xarray. The break down the swap
> > cache access does not have the similar long time as zswap
> > RB tree.
>
> I think the comparison to the swap cache may not be valid as the swap
> cache has many trees per swapfile, while zswap has a single tree.

Yes, good point. I think we can bench mark the xarray zswap vs the RB
tree zswap, that would be more of a direct comparison.

> > Moving the zswap entry to xarray enable read side
> > take read RCU lock only.
>
> Nice.
>
> >
> > The first patch adds the xarray alongside the RB tree.
> > There is some debug check asserting the xarray agrees with
> > the RB tree results.
> >
> > The second patch removes the zwap RB tree.
>
> The breakdown looks like something that would be a development step,
> but for patch submission I think it makes more sense to have a single
> patch replacing the rbtree with an xarray.

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 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.

Chris