Re: [RFC PATCH V3 08/17] mm: Add throttling of mm scanning using scan_size

[Jonathan Cameron]

On Thu, 14 Aug 2025 15:32:58 +0000
Raghavendra K T <raghavendra.kt@xxxxxxx> wrote:

> Before this patch, scanning is done on entire virtual address space
> of all the tasks. Now the scan size is shrunk or expanded based on the
> useful pages found in the last scan.
> 
> This helps to quickly get out of unnecessary scanning thus burning
> lesser CPU.
> 
> Drawback: If a useful chunk is at the other end of the VMA space, it
> will delay scanning and migration.
> 
> Shrink/expand algorithm for scan_size:
> X : Number of useful pages in the last scan.
> Y : Number of useful pages found in current scan.
> Initial scan_size is 1GB
>  case 1: (X = 0, Y = 0)
>   Decrease scan_size by 2
>  case 2: (X = 0, Y > 0)
>   Aggressively change to MAX (4GB)
>  case 3: (X > 0, Y = 0 )
>    No change
>  case 4: (X > 0, Y > 0)
>    Increase scan_size by 2
> 
> Scan size is clamped between MIN (256MB) and MAX (4GB)).
> TBD:  Tuning based on real workloads

Seems like a reasonable thing to do, but as you say tuning
data needed to justify how aggressive this should be and
those size limits.

Trivial stuff inline.

> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxx>
> ---
>  mm/kscand.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/mm/kscand.c b/mm/kscand.c
> index 843069048c61..39a7fcef7de8 100644
> --- a/mm/kscand.c
> +++ b/mm/kscand.c
> @@ -28,10 +28,15 @@
>  
>  static struct task_struct *kscand_thread __read_mostly;
>  static DEFINE_MUTEX(kscand_mutex);
> +

Push that into earlier patch to cut down on churn / noise.


>  /*
>   * Total VMA size to cover during scan.
> + * Min: 256MB default: 1GB max: 4GB
>   */
> +#define KSCAND_SCAN_SIZE_MIN	(256 * 1024 * 1024UL)
> +#define KSCAND_SCAN_SIZE_MAX	(4 * 1024 * 1024 * 1024UL)
>  #define KSCAND_SCAN_SIZE	(1 * 1024 * 1024 * 1024UL)
> +

Likewise.

>  static unsigned long kscand_scan_size __read_mostly = KSCAND_SCAN_SIZE;
>  
>  /*
> @@ -94,6 +99,8 @@ struct kscand_mm_slot {
>  	unsigned long next_scan;
>  	/* Tracks how many useful pages obtained for migration in the last scan */
>  	unsigned long scan_delta;
> +	/* Determines how much VMA address space to be covered in the scanning */
> +	unsigned long scan_size;
>  	long address;
>  	bool is_scanned;
>  };

>  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 scan_size;

Combining a few of these or assigning at declaration will reduce the code size a bit
which is always nice to have if it doesn't hurt readability.

>  	unsigned long old_scan_delta;
>  
> +	scan_size = mm_slot->scan_size;
>  	scan_period = mm_slot->scan_period;
>  	old_scan_delta = mm_slot->scan_delta;