[PATCH] mm: vmpressure: scale window size based on machine memory

[Taha Sezer <tahasezer.is@gmail.com>]

The vmpressure window size is currently a compile-time constant of
512 pages (2 MB with 4 KB pages), regardless of the machine size.
On large machines with hundreds of gigabytes of RAM, this small window
leads to excessive false-positive pressure notifications due to the
higher absolute reclaim activity.  Conversely, the fixed size is
already appropriate for small machines.

Scale the window size logarithmically with total memory at boot time,
following the same approach used by calculate_normal_threshold() in
mm/vmstat.c.  Total memory is converted to 128 MB units and fls() is
used for cheap logarithmic scaling.  The multiplier is clamped between
4 and 64, yielding a window range of 128 pages (512 KB) to 2048 pages
(8 MB).

This resolves the TODO that has been in the code since the original
vmpressure implementation.

Signed-off-by: Taha Sezer <tahasezer.is@gmail.com>
---
 include/linux/vmpressure.h |  2 +-
 mm/vmpressure.c            | 60 ++++++++++++++++++++++++++++++++++++--
 2 files changed, 58 insertions(+), 4 deletions(-)

diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
index b4d13457b..09111f5bd 100644
--- a/include/linux/vmpressure.h
+++ b/include/linux/vmpressure.h
@@ -51,7 +51,7 @@ extern struct vmpressure *memcg_to_vmpressure(struct mem_cgroup *memcg);
 extern struct mem_cgroup *vmpressure_to_memcg(struct vmpressure *vmpr);
 
 /* Shared with the v1 vmpressure block in mm/memcontrol-v1.c. */
-extern const unsigned long vmpressure_win;
+extern unsigned long vmpressure_win;
 extern enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
 						    unsigned long reclaimed);
 
diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 9629240d7..f88bafb71 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -13,6 +13,7 @@
  * (tree=false) socket-pressure path that runs on cgroup v2.
  */
 
+#include <linux/bitops.h>
 #include <linux/cgroup.h>
 #include <linux/log2.h>
 #include <linux/mm.h>
@@ -31,10 +32,63 @@
  * As the vmscan reclaimer logic works with chunks which are multiple of
  * SWAP_CLUSTER_MAX, it makes sense to use it for the window size as well.
  *
- * TODO: Make the window size depend on machine size, as we do for vmstat
- * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
+ * The window size scales logarithmically with total memory, following the
+ * same approach as calculate_normal_threshold() in mm/vmstat.c.  On small
+ * machines the window stays small for responsiveness; on large machines it
+ * grows to reduce false positives from the higher absolute reclaim activity.
+ *
+ * Sample window sizes (SWAP_CLUSTER_MAX = 32, PAGE_SIZE = 4K):
+ *
+ *   RAM          fls(mem)   multiplier   window (pages)   window (bytes)
+ *   -----------------------------------------------------------------
+ *   <= 4 GB      0-4        4            128              512 KB
+ *   8 GB         6          6            192              768 KB
+ *   16 GB        7          7            224              896 KB
+ *   32 GB        8          8            256              1 MB
+ *   64 GB        9          9            288              1.1 MB
+ *   128 GB       10         10           320              1.3 MB
+ *   256 GB       11         11           352              1.4 MB
+ *   512 GB       12         12           384              1.5 MB
+ *   1 TB         13         13           416              1.6 MB
  */
-const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
+unsigned long __read_mostly vmpressure_win = SWAP_CLUSTER_MAX * 16;
+
+/*
+ * Initialize vmpressure window size based on machine memory.
+ *
+ * Use fls() for cheap logarithmic scaling, following the same approach
+ * as calculate_normal_threshold() in mm/vmstat.c.  Memory is measured
+ * in 128 MB units so that the window starts growing once total RAM
+ * exceeds a few GB.
+ */
+static int __init vmpressure_win_init(void)
+{
+	unsigned long mem;
+	int multiplier;
+
+	/*
+	 * Convert total pages to 128 MB units, matching the vmstat
+	 * convention: mem = totalram >> (27 - PAGE_SHIFT).
+	 */
+	mem = totalram_pages() >> (27 - PAGE_SHIFT);
+
+	/*
+	 * fls(0) == 0, so for machines with < 128 MB the multiplier
+	 * is clamped to 4, preserving the original 128-page minimum
+	 * (SWAP_CLUSTER_MAX * 4).  The maximum multiplier is clamped
+	 * to 64, yielding a 2048-page (8 MB) ceiling which prevents
+	 * excessively delayed notifications on very large machines.
+	 */
+	multiplier = clamp(fls(mem), 4, 64);
+
+	vmpressure_win = SWAP_CLUSTER_MAX * (unsigned long)multiplier;
+
+	pr_info("vmpressure: window size set to %lu pages (%lu KB)\n",
+		vmpressure_win, vmpressure_win << (PAGE_SHIFT - 10));
+
+	return 0;
+}
+core_initcall(vmpressure_win_init);
 
 /*
  * These thresholds are used when we account memory pressure through
-- 
2.53.0