File pages use a set of radix tags (DIRTY, TOWRITE, WRITEBACK, etc.)
to
accelerate finding the pages with a specific tag in the radix tree
during inode writeback.A A But for anonymous pages in the swap cache,
there is no inode writeback.A A So there is no need to find the
pages with some writeback tags in the radix tree.A A It is not
necessary
to touch radix tree writeback tags for pages in the swap cache.

With this patch, the swap out bandwidth improved 22.3% (from ~1.2GB/s
to
~ 1.48GBps) in the vm-scalability swap-w-seq test case with 8
processes.
The test is done on a Xeon E5 v3 system.A A The swap device used is a
RAM
simulated PMEM (persistent memory) device.A A The improvement comes
from
the reduced contention on the swap cache radix tree lock.A A To test
sequential swapping out, the test case uses 8 processes, which
sequentially allocate and write to the anonymous pages until RAM and
part of the swap device is used up.

Details of comparison is as follow,

baseA A A A A A A A A A A A A base+patch
---------------- --------------------------
A A A A A A A A A %stddevA A A A A %changeA A A A A A A A A %stddev
A A A A A A A A A A A A A \A A A A A A A A A A |A A A A A A A A A A A A A A A A \
A A A 1207402 A+-A A 7%A A A A A +22.3%A A A A 1476578 A+-A A 6%A A vmstat.swap.so
A A A 2506952 A+-A A 2%A A A A A +28.1%A A A A 3212076 A+-A A 7%A A vm-
scalability.throughput
A A A A A 10.86 A+- 12%A A A A A -23.4%A A A A A A A 8.31 A+- 16%A A perf-profile.cycles-
pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_sw
ap.shrink_page_list
A A A A A 10.82 A+- 13%A A A A A -33.1%A A A A A A A 7.24 A+- 14%A A perf-profile.cycles-
pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_in
active_list.shrink_zone_memcg
A A A A A 10.36 A+- 11%A A A A -100.0%A A A A A A A 0.00 A+- -1%A A perf-profile.cycles-
pp._raw_spin_lock_irqsave.__test_set_page_writeback.bdev_write_page._
_swap_writepage.swap_writepage
A A A A A 10.52 A+- 12%A A A A -100.0%A A A A A A A 0.00 A+- -1%A A perf-profile.cycles-
pp._raw_spin_lock_irqsave.test_clear_page_writeback.end_page_writebac
k.page_endio.pmem_rw_page

Cc: Hugh Dickins <hughd@google.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <redacted>
Cc: Mel Gorman <redacted>
Cc: Tejun Heo <tj@kernel.org>
Cc: Wu Fengguang <redacted>
Cc: Dave Hansen <redacted>
Signed-off-by: "Huang, Ying" <redacted>
---
A mm/page-writeback.c | 6 ++++--
A 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 82e7252..599d2f9 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c

@@ -2728,7 +2728,8 @@ int test_clear_page_writeback(struct page

*page)
A 	int ret;
A 
A 	lock_page_memcg(page);
-	if (mapping) {
+	/* Pages in swap cache don't use writeback tags */
+	if (mapping && !PageSwapCache(page)) {