linux-kernel - [PATCH v3 2/2] mm/compaction: make proactive compaction high watermark configurable via sysctl

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Message-ID: <20250127215020.4023545-3-mclapinski@google.com>
Date: Mon, 27 Jan 2025 22:50:20 +0100
From: Michal Clapinski <mclapinski@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>, 
	Pasha Tatashin <tatashin@...gle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Michal Clapinski <mclapinski@...gle.com>
Subject: [PATCH v3 2/2] mm/compaction: make proactive compaction high
 watermark configurable via sysctl

Currently, the difference between the high and low watermarks for
proactive compaction is hardcoded to 10. This hardcoded difference is
too large for free page reporting to work well.

Add a new sysctl, `compaction_proactiveness_leeway`, to control the
difference between the high and low watermarks.

Signed-off-by: Michal Clapinski <mclapinski@...gle.com>
---
 Documentation/admin-guide/sysctl/vm.rst | 17 +++++++++++++++++
 mm/compaction.c                         | 12 +++++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index f48eaa98d22d2..ec6343ee4248d 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -27,6 +27,7 @@ Currently, these files are in /proc/sys/vm:
 - admin_reserve_kbytes
 - compact_memory
 - compaction_proactiveness
+- compaction_proactiveness_leeway
 - compact_unevictable_allowed
 - dirty_background_bytes
 - dirty_background_ratio
@@ -133,6 +134,22 @@ proactive compaction is not being effective.
 Be careful when setting it to extreme values like 100, as that may
 cause excessive background compaction activity.
 
+compaction_proactiveness_leeway
+===============================
+
+This tunable controls the difference between high and low watermarks for
+proactive compaction. This tunable takes a value in the range [0, 100] with
+a default value of 10. Higher values will result in proactive compaction
+triggering less often but doing more work when it does trigger.
+
+Proactive compaction triggers when fragmentation score (lower is better) gets
+larger than high watermark. Compaction stops when the score gets smaller or
+equal to low watermark (or when no progress is being made).
+The watermarks are calculated as follows:
+
+low_wmark = 100 - compaction_proactiveness;
+high_wmark = low_wmark + compaction_proactiveness_leeway;
+
 compact_unevictable_allowed
 ===========================
 
diff --git a/mm/compaction.c b/mm/compaction.c
index 29524242a16ef..fd546b797e544 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1921,6 +1921,7 @@ static int sysctl_compact_unevictable_allowed __read_mostly = CONFIG_COMPACT_UNE
  * background. It takes values in the range [0, 100].
  */
 static unsigned int __read_mostly sysctl_compaction_proactiveness = 20;
+static unsigned int __read_mostly sysctl_compaction_proactiveness_leeway = 10;
 static int sysctl_extfrag_threshold = 500;
 static int __read_mostly sysctl_compact_memory;
 
@@ -2254,7 +2255,7 @@ static unsigned int fragmentation_score_wmark(bool low)
 	 * close to 100 (maximum).
 	 */
 	wmark_low = 100U - sysctl_compaction_proactiveness;
-	return low ? wmark_low : min(wmark_low + 10, 100U);
+	return low ? wmark_low : min(wmark_low + sysctl_compaction_proactiveness_leeway, 100U);
 }
 
 static bool should_proactive_compact_node(pg_data_t *pgdat)
@@ -3314,6 +3315,15 @@ static struct ctl_table vm_compaction[] = {
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_ONE_HUNDRED,
 	},
+	{
+		.procname	= "compaction_proactiveness_leeway",
+		.data		= &sysctl_compaction_proactiveness_leeway,
+		.maxlen		= sizeof(sysctl_compaction_proactiveness_leeway),
+		.mode		= 0644,
+		.proc_handler	= compaction_proactiveness_sysctl_handler,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE_HUNDRED,
+	},
 	{
 		.procname	= "extfrag_threshold",
 		.data		= &sysctl_extfrag_threshold,
-- 
2.48.1.262.g85cc9f2d1e-goog