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

Jonathan Cameron
2025-10-02T16:03:45Z

On Thu, 14 Aug 2025 15:32:55 +0000
Raghavendra K T <raghavendra.kt@amd.com> wrote:

> 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>
A few superficial things inline.

> ---
> 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;
> +
Unrelated change. If you want it make sure to push back to earlier patch.

> /* 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);

Unusual line wrapping. I'd go nearer to 80 chars and put at least
some paramters on same line.

> +}

> 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;

Similar comment to earlier on not expecting side effects of a thread to stop
if the start failed.

> +
> return 0;
>
> +err_kmigrated:
> + stop_kmigrated();
> +
> err_kscand:
> stop_kscand();
> kscand_destroy();