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

[Chris Li]

On Wed, Jan 17, 2024 at 11:05 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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.

> 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.

>
> 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.

Chris