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

[Kairui Song]

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.

Using xa_empty rather than zswap_never_enabled is more helpful as it
cover both case where zswap wes never used or the particular range
doesn't have any zswap entry. And it's safe as the swap slot should
be currently pinned by caller with HAS_CACHE.

Sequential SWAP in/out tests with zswap disabled showed a minor
performance gain, SWAP in of zero page with zswap enabled also
showed a performance gain. (swapout is basically unchanged so
only test one case):

Swapout of 2G zero page using brd as SWAP, zswap disabled
(total time, 4 testrun, +0.1%):
Before: 1705013 us 1703119 us 1704335 us 1705848 us.
After:  1703579 us 1710640 us 1703625 us 1708699 us.

Swapin of 2G zero page using brd as SWAP, zswap disabled
(total time, 4 testrun, -3.5%):
Before: 1912312 us 1915692 us 1905837 us 1912706 us.
After:  1845354 us 1849691 us 1845868 us 1841828 us.

Swapin of 2G zero page using brd as SWAP, zswap enabled
(total time, 4 testrun, -3.3%):
Before: 1897994 us 1894681 us 1899982 us 1898333 us
After:  1835894 us 1834113 us 1832047 us 1833125 us

Swapin of 2G random page using brd as SWAP, zswap enabled
(total time, 4 testrun, -0.1%):
Before: 4519747 us 4431078 us 4430185 us 4439999 us
After:  4492176 us 4437796 us 4434612 us 4434289 us

And the performance is very slightly better or unchanged for
build kernel test with zswap enabled or disabled.

Build Linux Kernel with defconfig and -j32 in 1G memory cgroup,
using brd SWAP, zswap disabled (sys time in seconds, 6 testrun, -0.1%):
Before: 1648.83 1653.52 1666.34 1665.95 1663.06 1656.67
After:  1651.36 1661.89 1645.70 1657.45 1662.07 1652.83

Build Linux Kernel with defconfig and -j32 in 2G memory cgroup,
using brd SWAP zswap enabled (sys time in seconds, 6 testrun, -0.3%):
Before: 1240.25 1254.06 1246.77 1265.92 1244.23 1227.74
After:  1226.41 1218.21 1249.12 1249.13 1244.39 1233.01

Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 mm/zswap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/zswap.c b/mm/zswap.c
index 7f00cc918e7c..f6316b66fb23 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1641,6 +1641,9 @@ void zswap_invalidate(swp_entry_t swp)
 	struct xarray *tree = swap_zswap_tree(swp);
 	struct zswap_entry *entry;
 
+	if (xa_empty(tree))
+		return;
+
 	entry = xa_erase(tree, offset);
 	if (entry)
 		zswap_entry_free(entry);
-- 
2.47.0