[RFC PATCH V3 07/17] mm: Add throttling of mm scanning using scan_period

Raghavendra K T
2025-08-14T15:34:54Z

Before this patch, scanning of tasks' mm is done continuously and also
at the same rate.

Improve that by adding a throttling logic:
1) If there were useful pages found during last scan and current scan,
decrease the scan_period (to increase scan rate) by TUNE_PERCENT (15%).

2) If there were no useful pages found in last scan, and there are
candidate migration pages in the current scan decrease the scan_period
aggressively by 2 power SCAN_CHANGE_SCALE (2^3 = 8 now).

Vice versa is done for the reverse case.
Scan period is clamped between MIN (600ms) and MAX (5sec).

Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
---
mm/kscand.c | 110 +++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 109 insertions(+), 1 deletion(-)

diff --git a/mm/kscand.c b/mm/kscand.c
index 5cd2764114df..843069048c61 100644
--- a/mm/kscand.c
+++ b/mm/kscand.c
@@ -20,6 +20,7 @@
#include <linux/string.h>
#include <linux/delay.h>
#include <linux/cleanup.h>
+#include <linux/minmax.h>

#include <asm/pgalloc.h>
#include "internal.h"
@@ -33,6 +34,16 @@ static DEFINE_MUTEX(kscand_mutex);
#define KSCAND_SCAN_SIZE (1 * 1024 * 1024 * 1024UL)
static unsigned long kscand_scan_size __read_mostly = KSCAND_SCAN_SIZE;

+/*
+ * Scan period for each mm.
+ * Min: 600ms default: 2sec Max: 5sec
+ */
+#define KSCAND_SCAN_PERIOD_MAX 5000U
+#define KSCAND_SCAN_PERIOD_MIN 600U
+#define KSCAND_SCAN_PERIOD 2000U
+
+static unsigned int kscand_mm_scan_period_ms __read_mostly = KSCAND_SCAN_PERIOD;
+
/* How long to pause between two scan cycles */
static unsigned int kscand_scan_sleep_ms __read_mostly = 20;

@@ -78,6 +89,11 @@ static struct kmem_cache *kmigrated_slot_cache __read_mostly;
/* Per mm information collected to control VMA scanning */
struct kscand_mm_slot {
struct mm_slot slot;
+ /* Unit: ms. Determines how aften mm scan should happen. */
+ unsigned int scan_period;
+ unsigned long next_scan;
+ /* Tracks how many useful pages obtained for migration in the last scan */
+ unsigned long scan_delta;
long address;
bool is_scanned;
};
@@ -715,13 +731,92 @@ static void kmigrated_migrate_folio(void)
}
}

+/*
+ * This is the normal change percentage when old and new delta remain same.
+ * i.e., either both positive or both zero.
+ */
+#define SCAN_PERIOD_TUNE_PERCENT 15
+
+/* This is to change the scan_period aggressively when deltas are different */
+#define SCAN_PERIOD_CHANGE_SCALE 3
+/*
+ * XXX: Hack to prevent unmigrated pages coming again and again while scanning.
+ * Actual fix needs to identify the type of unmigrated pages OR consider migration
+ * failures in next scan.
+ */
+#define KSCAND_IGNORE_SCAN_THR 256
+
+/* Maintains stability of scan_period by decaying last time accessed pages */
+#define SCAN_DECAY_SHIFT 4
+/*
+ * X : Number of useful pages in the last scan.
+ * Y : Number of useful pages found in current scan.
+ * Tuning scan_period:
+ * Initial scan_period is 2s.
+ * case 1: (X = 0, Y = 0)
+ * Increase scan_period by SCAN_PERIOD_TUNE_PERCENT.
+ * case 2: (X = 0, Y > 0)
+ * Decrease scan_period by (2 << SCAN_PERIOD_CHANGE_SCALE).
+ * case 3: (X > 0, Y = 0 )
+ * Increase scan_period by (2 << SCAN_PERIOD_CHANGE_SCALE).
+ * case 4: (X > 0, Y > 0)
+ * Decrease scan_period by SCAN_PERIOD_TUNE_PERCENT.
+ */
+static inline void kscand_update_mmslot_info(struct kscand_mm_slot *mm_slot,
+ unsigned long total)
+{
+ unsigned int scan_period;
+ unsigned long now;
+ unsigned long old_scan_delta;
+
+ scan_period = mm_slot->scan_period;
+ old_scan_delta = mm_slot->scan_delta;
+
+ /* decay old value */
+ total = (old_scan_delta >> SCAN_DECAY_SHIFT) + total;
+
+ /* XXX: Hack to get rid of continuously failing/unmigrateable pages */
+ if (total < KSCAND_IGNORE_SCAN_THR)
+ total = 0;
+
+ /*
+ * case 1: old_scan_delta and new delta are similar, (slow) TUNE_PERCENT used.
+ * case 2: old_scan_delta and new delta are different. (fast) CHANGE_SCALE used.
+ * TBD:
+ * 1. Further tune scan_period based on delta between last and current scan delta.
+ * 2. Optimize calculation
+ */
+ if (!old_scan_delta && !total) {
+ scan_period = (100 + SCAN_PERIOD_TUNE_PERCENT) * scan_period;
+ scan_period /= 100;
+ } else if (old_scan_delta && total) {
+ scan_period = (100 - SCAN_PERIOD_TUNE_PERCENT) * scan_period;
+ scan_period /= 100;
+ } else if (old_scan_delta && !total) {
+ scan_period = scan_period << SCAN_PERIOD_CHANGE_SCALE;
+ } else {
+ scan_period = scan_period >> SCAN_PERIOD_CHANGE_SCALE;
+ }
+
+ scan_period = clamp(scan_period, KSCAND_SCAN_PERIOD_MIN, KSCAND_SCAN_PERIOD_MAX);
+
+ now = jiffies;
+ mm_slot->next_scan = now + msecs_to_jiffies(scan_period);
+ mm_slot->scan_period = scan_period;
+ mm_slot->scan_delta = total;
+}
+
static unsigned long kscand_scan_mm_slot(void)
{
bool next_mm = false;
bool update_mmslot_info = false;

+ unsigned int mm_slot_scan_period;
+ unsigned long now;
+ unsigned long mm_slot_next_scan;
unsigned long vma_scanned_size = 0;
unsigned long address;
+ unsigned long total = 0;

struct mm_slot *slot;
struct mm_struct *mm;
@@ -746,6 +841,8 @@ static unsigned long kscand_scan_mm_slot(void)

mm = slot->mm;
mm_slot->is_scanned = true;
+ mm_slot_next_scan = mm_slot->next_scan;
+ mm_slot_scan_period = mm_slot->scan_period;
spin_unlock(&kscand_mm_lock);

if (unlikely(!mmap_read_trylock(mm)))
@@ -756,6 +853,11 @@ static unsigned long kscand_scan_mm_slot(void)
goto outerloop;
}

+ now = jiffies;
+
+ if (mm_slot_next_scan && time_before(now, mm_slot_next_scan))
+ goto outerloop;
+
VMA_ITERATOR(vmi, mm, address);

kmigrated_mm_slot = kmigrated_get_mm_slot(mm, false);
@@ -786,8 +888,10 @@ static unsigned long kscand_scan_mm_slot(void)

update_mmslot_info = true;

- if (update_mmslot_info)
+ if (update_mmslot_info) {
mm_slot->address = address;
+ kscand_update_mmslot_info(mm_slot, total);
+ }

outerloop:
/* exit_mmap will destroy ptes after this */
@@ -889,6 +993,10 @@ void __kscand_enter(struct mm_struct *mm)
return;

kscand_slot->address = 0;
+ kscand_slot->scan_period = kscand_mm_scan_period_ms;
+ kscand_slot->next_scan = 0;
+ kscand_slot->scan_delta = 0;
+
slot = &kscand_slot->slot;

spin_lock(&kscand_mm_lock);
--
2.34.1