Re: [PATCH] mm/zswap: avoid touching XArray for unnecessary invalidation

[Kairui Song]

Johannes Weiner <hannes@xxxxxxxxxxx> 于 2024年10月12日周六 02:28写道：
>
> On Fri, Oct 11, 2024 at 10:53:31AM -0700, Yosry Ahmed wrote:
> > On Fri, Oct 11, 2024 at 10:20 AM Kairui Song <ryncsn@xxxxxxxxx> wrote:
> > >
> > > From: Kairui Song <kasong@xxxxxxxxxxx>
> > >
> > > zswap_invalidation simply calls xa_erase, which acquires the Xarray
> > > lock first, then does a look up. This has a higher overhead even if
> > > zswap is not used or the tree is empty.
> > >
> > > So instead, do a very lightweight xa_empty check first, if there is
> > > nothing to erase, don't touch the lock or the tree.
>
> Great idea!
>
> > XA_STATE(xas, ..);
> >
> > rcu_read_lock();
> > entry = xas_load(&xas);
> > if (entry) {
> >     xas_lock(&xas);
> >     WARN_ON_ONCE(xas_reload(&xas) != entry);
> >     xas_store(&xas, NULL);
> >     xas_unlock(&xas);
> > }
> > rcu_read_unlock():
>
> This does the optimization more reliably, and I think we should go
> with this version.

Hi Yosry and Johannes,

This is a good idea. But xa_empty is just much lighweighter, it's just
a inlined ( == NULL ) check, so unsurprising it has better performance
than xas_load.

And surprisingly it's faster than zswap_never_enabled. So I think it
could be doable to introduce something like zswap_may_have_swpentry as
Yosry suggested.

So how about a combined version with xas_load and xa_empty? Check
xa_empty first as a faster path, then xas_load, then xas_store.

Here is the benchmark result (time of swapin 2G zero pages in us):

Before:   1908944 1905870 1905322 1905627 1901667
xa_empty: 1835343 1827367 1828402 1831841 1832719
z.._enabled: 1838428 1831162 1838205 1837287 1840980
xas_load: 1874606 1878971 1870182 1875852 1873403
combined: 1845309 1832919 1831904 1836455 1842570

`combined` is xa_empty + xas_load.