[RFC PATCH V3 05/17] mm: Create a separate kthread for migration

Raghavendra K T
2025-08-14T15:34:31Z

Having independent thread helps in:
- Alleviating the need for multiple scanning threads
- Aids to control batch migration (TBD)
- Migration throttling (TBD)

Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
---
mm/kscand.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 74 insertions(+)

diff --git a/mm/kscand.c b/mm/kscand.c
index 7552ce32beea..55efd0a6e5ba 100644
--- a/mm/kscand.c
+++ b/mm/kscand.c
@@ -4,6 +4,7 @@
#include <linux/sched.h>
#include <linux/sched/mm.h>
#include <linux/mmu_notifier.h>
+#include <linux/migrate.h>
#include <linux/rmap.h>
#include <linux/pagewalk.h>
#include <linux/page_ext.h>
@@ -41,6 +42,15 @@ static unsigned long kscand_mms_to_scan __read_mostly = KSCAND_MMS_TO_SCAN;

bool kscand_scan_enabled = true;
static bool need_wakeup;
+static bool migrated_need_wakeup;
+
+/* How long to pause between two migration cycles */
+static unsigned int kmigrate_sleep_ms __read_mostly = 20;
+
+static struct task_struct *kmigrated_thread __read_mostly;
+static DEFINE_MUTEX(kmigrated_mutex);
+static DECLARE_WAIT_QUEUE_HEAD(kmigrated_wait);
+static unsigned long kmigrated_sleep_expire;

static unsigned long kscand_sleep_expire;

@@ -79,6 +89,7 @@ struct kscand_scanctrl {
};

struct kscand_scanctrl kscand_scanctrl;
+
/* Per folio information used for migration */
struct kscand_migrate_info {
struct list_head migrate_node;
@@ -134,6 +145,19 @@ static inline bool is_valid_folio(struct folio *folio)
return true;
}

+static inline void kmigrated_wait_work(void)
+{
+ const unsigned long migrate_sleep_jiffies =
+ msecs_to_jiffies(kmigrate_sleep_ms);
+
+ if (!migrate_sleep_jiffies)
+ return;
+
+ kmigrated_sleep_expire = jiffies + migrate_sleep_jiffies;
+ wait_event_timeout(kmigrated_wait,
+ true,
+ migrate_sleep_jiffies);
+}

static bool folio_idle_clear_pte_refs_one(struct folio *folio,
struct vm_area_struct *vma,
@@ -537,6 +561,49 @@ static int stop_kscand(void)
return 0;
}

+static int kmigrated(void *arg)
+{
+ while (true) {
+ WRITE_ONCE(migrated_need_wakeup, false);
+ if (unlikely(kthread_should_stop()))
+ break;
+ msleep(20);
+ kmigrated_wait_work();
+ }
+ return 0;
+}
+
+static int start_kmigrated(void)
+{
+ struct task_struct *kthread;
+
+ guard(mutex)(&kmigrated_mutex);
+
+ /* Someone already succeeded in starting daemon */
+ if (kmigrated_thread)
+ return 0;
+
+ kthread = kthread_run(kmigrated, NULL, "kmigrated");
+ if (IS_ERR(kmigrated_thread)) {
+ pr_err("kmigrated: kthread_run(kmigrated) failed\n");
+ return PTR_ERR(kthread);
+ }
+
+ kmigrated_thread = kthread;
+ pr_info("kmigrated: Successfully started kmigrated");
+
+ wake_up_interruptible(&kmigrated_wait);
+
+ return 0;
+}
+
+static int stop_kmigrated(void)
+{
+ guard(mutex)(&kmigrated_mutex);
+ kthread_stop(kmigrated_thread);
+ return 0;
+}
+
static inline void init_list(void)
{
INIT_LIST_HEAD(&kscand_scanctrl.scan_list);
@@ -559,8 +626,15 @@ static int __init kscand_init(void)
if (err)
goto err_kscand;

+ err = start_kmigrated();
+ if (err)
+ goto err_kmigrated;
+
return 0;

+err_kmigrated:
+ stop_kmigrated();
+
err_kscand:
stop_kscand();
kscand_destroy();
--
2.34.1