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

[Michal Clapinski <mclapinski@google.com>]

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@google.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