linux-kernel - [PATCH 2/2] mm: compaction: update the cc->nr_migratepages when allocating or freeing the freepages

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Date: Mon, 22 Jan 2024 21:01:54 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: akpm@...ux-foundation.org
Cc: mgorman@...hsingularity.net,
	vbabka@...e.cz,
	baolin.wang@...ux.alibaba.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] mm: compaction: update the cc->nr_migratepages when allocating or freeing the freepages

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@...ux.alibaba.com>
---
 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