[RFC PATCH V3 11/17] mm/kscand: Implement migration failure feedback

Raghavendra K T
2025-08-14T15:35:36Z

Before this, scanning kthread continues to scan even after
migration fails. To control migration, scanning is slowed down
based on the failure/success ratio obtained from migration
thread.

Decaying failure ratio is maintained for 1024 migration window.
The ratio further contributes to approximately 10% scaling of
scan_period.

Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
---
mm/kscand.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 55 insertions(+)

diff --git a/mm/kscand.c b/mm/kscand.c
index bf975e82357d..41321d373be7 100644
--- a/mm/kscand.c
+++ b/mm/kscand.c
@@ -146,6 +146,8 @@ struct kmigrated_mm_slot {
spinlock_t migrate_lock;
/* Head of per mm migration list */
struct list_head migrate_head;
+ /* Indicates weighted success, failure */
+ int msuccess, mfailed, fratio;
};

/* System wide list of mms that maintain migration list */
@@ -812,13 +814,45 @@ static void kscand_collect_mm_slot(struct kscand_mm_slot *mm_slot)
}
}

+static int kmigrated_get_mstat_fratio(struct mm_struct *mm)
+{
+ int fratio = 0;
+ struct kmigrated_mm_slot *mm_slot = NULL;
+ struct mm_slot *slot;
+
+ guard(spinlock)(&kscand_migrate_lock);
+
+ slot = mm_slot_lookup(kmigrated_slots_hash, mm);
+ mm_slot = mm_slot_entry(slot, struct kmigrated_mm_slot, mm_slot);
+
+ if (mm_slot)
+ fratio = mm_slot->fratio;
+
+ return fratio;
+}
+
+static void update_mstat_ratio(struct kmigrated_mm_slot *mm_slot,
+ int msuccess, int mfailed)
+{
+ mm_slot->msuccess = (mm_slot->msuccess >> 2) + msuccess;
+ mm_slot->mfailed = (mm_slot->mfailed >> 2) + mfailed;
+ mm_slot->fratio = mm_slot->mfailed * 100;
+ mm_slot->fratio /= (mm_slot->msuccess + mm_slot->mfailed);
+}
+
+#define MSTAT_UPDATE_FREQ 1024
+
static void kmigrated_migrate_mm(struct kmigrated_mm_slot *mm_slot)
{
+ int mfailed = 0;
+ int msuccess = 0;
+ int mstat_counter;
int ret = 0, dest = -1;
struct mm_slot *slot;
struct mm_struct *mm;
struct kscand_migrate_info *info, *tmp;

+ mstat_counter = MSTAT_UPDATE_FREQ;
spin_lock(&mm_slot->migrate_lock);

slot = &mm_slot->mm_slot;
@@ -842,11 +876,23 @@ static void kmigrated_migrate_mm(struct kmigrated_mm_slot *mm_slot)
}

ret = kmigrated_promote_folio(info, mm, dest);
+ mstat_counter--;
+
+ /* TBD: encode migrated count here, currently assume folio_nr_pages */
+ if (!ret)
+ msuccess++;
+ else
+ mfailed++;

kfree(info);

cond_resched();
spin_lock(&mm_slot->migrate_lock);
+ if (!mstat_counter) {
+ update_mstat_ratio(mm_slot, msuccess, mfailed);
+ msuccess = mfailed = 0;
+ mstat_counter = MSTAT_UPDATE_FREQ;
+ }
}
}
clean_list_handled:
@@ -882,6 +928,12 @@ static void kmigrated_migrate_folio(void)
}
}

+/* Get scan_period based on migration failure statistics */
+static int kscand_mstat_scan_period(unsigned int scan_period, int fratio)
+{
+ return scan_period * (1 + fratio / 10);
+}
+
/*
* This is the normal change percentage when old and new delta remain same.
* i.e., either both positive or both zero.
@@ -928,6 +980,7 @@ static void kmigrated_migrate_folio(void)
static inline void kscand_update_mmslot_info(struct kscand_mm_slot *mm_slot,
unsigned long total, int target_node)
{
+ int fratio;
unsigned int scan_period;
unsigned long now;
unsigned long scan_size;
@@ -967,6 +1020,8 @@ static inline void kscand_update_mmslot_info(struct kscand_mm_slot *mm_slot,
}

scan_period = clamp(scan_period, KSCAND_SCAN_PERIOD_MIN, KSCAND_SCAN_PERIOD_MAX);
+ fratio = kmigrated_get_mstat_fratio((&mm_slot->slot)->mm);
+ scan_period = kscand_mstat_scan_period(scan_period, fratio);
scan_size = clamp(scan_size, KSCAND_SCAN_SIZE_MIN, KSCAND_SCAN_SIZE_MAX);

now = jiffies;
--
2.34.1