linux-kernel - Re: [PATCH 0/2] mm/zswap: optimize the scalability of zswap rb-tree

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Date: Wed, 17 Jan 2024 15:47:45 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chris Li <chrisl@...nel.org>
Cc: Chengming Zhou <zhouchengming@...edance.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org, 
	Nhat Pham <nphamcs@...il.com>
Subject: Re: [PATCH 0/2] mm/zswap: optimize the scalability of zswap rb-tree

> Currently the xarray patch should have everything it takes to use RCU
> read lock. However taking out the tree spinlock is more work than
> previously. If we are going to remove the tree spinlock, I think we
> should revert back to doing a zswap tree lookup and return the zswap
> entry with reference increased. The tree mapping can still decouple
> from the zswap entry reference count drop to zero.  Anyway, my V1 of
> the xarray patch will not include removing the tree spinlock.

Interesting. What do you mean by removing the tree spinlock? My
assumption was that the xarray reduces lock contention because we do
not need a lock to do lookups, but we still need the lock otherwise.
Did you have something in mind to completely remove the tree lock?

>
> > The reason why I think we should wait for the xarray patch(es) is
> > there is a chance we may see less improvements from splitting the tree
> > if it was an xarray. If we merge this series first, there is no way to
> > know.
> >
> > Chris, do you intend to send the xarray patch(es) anytime soon?
>
> Thanks for the heads up. Let me send it out now.

Awesome, thanks! I assume Chengming can test whether this series
provides the same benefits with the xarray.