[PATCH v2] mm: compaction: update the cc->nr_migratepages when allocating or freeing the freepages

[Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>]

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>
---
Changes from v1:
 - Rebased on the latest mm-unstable branch.
---
 mm/compaction.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index 055687e6bd17..363c3e49fe80 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1882,6 +1882,7 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
 	if (order)
 		prep_compound_page(&dst->page, order);
 	cc->nr_freepages -= 1 << order;
+	cc->nr_migratepages -= 1 << order;
 	return page_rmappable_folio(&dst->page);
 }
 
@@ -1904,6 +1905,7 @@ static void compaction_free(struct folio *dst, unsigned long data)
 	list_add(&dst->lru, &cc->freepages[order].pages);
 	cc->freepages[order].nr_pages++;
 	cc->nr_freepages += 1 << order;
+	cc->nr_migratepages += 1 << order;
 }
 
 /* possible outcome of isolate_migratepages */
-- 
2.39.3