Currently we will use 'cc->nr_freepages >= cc->nr_migratepages' comparison
to ensure that enough freepages are isolated in isolate_freepages(), however
it just decreases the cc->nr_freepages without updating cc->nr_migratepages
in compaction_alloc(), which will waste more CPU cycles and cause too many
freepages to be isolated.
So we should also update the cc->nr_migratepages when allocating or freeing
the freepages to avoid isolating excess freepages. And I can see fewer free
pages are scanned and isolated when running thpcompact on my Arm64 server:
k6.7 k6.7_patched
Ops Compaction pages isolated 120692036.00 118160797.00
Ops Compaction migrate scanned 131210329.00 154093268.00
Ops Compaction free scanned 1090587971.00 1080632536.00
Ops Compact scan efficiency 12.03 14.26
Moreover, I did not see an obvious latency improvements, this is likely because
isolating freepages is not the bottleneck in the thpcompact test case.
k6.7 k6.7_patched
Amean fault-both-1 1089.76 ( 0.00%) 1080.16 * 0.88%*
Amean fault-both-3 1616.48 ( 0.00%) 1636.65 * -1.25%*
Amean fault-both-5 2266.66 ( 0.00%) 2219.20 * 2.09%*
Amean fault-both-7 2909.84 ( 0.00%) 2801.90 * 3.71%*
Amean fault-both-12 4861.26 ( 0.00%) 4733.25 * 2.63%*
Amean fault-both-18 7351.11 ( 0.00%) 6950.51 * 5.45%*
Amean fault-both-24 9059.30 ( 0.00%) 9159.99 * -1.11%*
Amean fault-both-30 10685.68 ( 0.00%) 11399.02 * -6.68%*
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
mm/compaction.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/mm/compaction.c b/mm/compaction.c
index 066b72b3471a..6c84e3a5b32b 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1779,6 +1779,7 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
dst = list_entry(cc->freepages.next, struct folio, lru);
list_del(&dst->lru);
cc->nr_freepages--;
+ cc->nr_migratepages--;
return dst;
}
@@ -1794,6 +1795,7 @@ static void compaction_free(struct folio *dst, unsigned long data)
list_add(&dst->lru, &cc->freepages);
cc->nr_freepages++;
+ cc->nr_migratepages++;
}
/* possible outcome of isolate_migratepages */
--
2.39.3
