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

Jonathan Cameron
2025-10-02T16:24:20Z

On Thu, 14 Aug 2025 15:32:57 +0000
Raghavendra K T <raghavendra.kt@amd.com> wrote:

> 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).

Explain why those values were chosen.

>
> 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>

Various minor comments inline.

Thanks,

Jonathan

> ---
> 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. */

Name it scan_period_ms and you can drop the unit comment + that info is
conveyed everywhere it is used. You do that for the global so why not here as well.

> + 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)
> }
> }

> +/* 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)

As below. I'd make total a local variable for now.

> +{
> + unsigned int scan_period;
> + unsigned long now;
> + unsigned long old_scan_delta;

Might as well combine these two lines.

> +
> + 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;

Given this never changes in this patch I'd drop the parameter
from kscand_update_mmslot_info() and bring it back when you have code
that passes in non 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))

If now is only used once, seems better just to put jiffies in the call
and drop local variable.

> + 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);