linux-kernel - Re: [RFC PATCH V3 09/17] mm: Add initial scan delay

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Message-ID: <20251003104141.00001744@huawei.com>
Date: Fri, 3 Oct 2025 10:41:41 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: Raghavendra K T <raghavendra.kt@....com>
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Subject: Re: [RFC PATCH V3 09/17] mm: Add initial scan delay

On Thu, 14 Aug 2025 15:32:59 +0000
Raghavendra K T <raghavendra.kt@....com> wrote:

>  This is  to prevent unnecessary scanning of short lived tasks
> to reduce CPU burning.
Add a tiny bit on why reusing the existing sysctl value.
Also useful to add a comment here on current default of that
so when considering this algorithm we know what it is.


> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@....com>
> ---
>  mm/kscand.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/kscand.c b/mm/kscand.c
> index 39a7fcef7de8..880c3693866d 100644
> --- a/mm/kscand.c
> +++ b/mm/kscand.c
> @@ -28,6 +28,7 @@
>  
>  static struct task_struct *kscand_thread __read_mostly;
>  static DEFINE_MUTEX(kscand_mutex);
> +extern unsigned int sysctl_numa_balancing_scan_delay;

Maybe we need to move the declaration from kernel/sched/sched.h
I'd rather see that than a somewhat hidden away extern declaration.

>  
>  /*
>   * Total VMA size to cover during scan.
> @@ -1010,6 +1011,7 @@ void __kscand_enter(struct mm_struct *mm)
>  {
>  	struct kscand_mm_slot *kscand_slot;
>  	struct mm_slot *slot;
> +	unsigned long now;
>  	int wakeup;
>  
>  	/* __kscand_exit() must not run from under us */
> @@ -1020,10 +1022,12 @@ void __kscand_enter(struct mm_struct *mm)
>  	if (!kscand_slot)
>  		return;
>  
> +	now = jiffies;
>  	kscand_slot->address = 0;
>  	kscand_slot->scan_period = kscand_mm_scan_period_ms;
>  	kscand_slot->scan_size = kscand_scan_size;
> -	kscand_slot->next_scan = 0;
> +	kscand_slot->next_scan = now +
> +			2 * msecs_to_jiffies(sysctl_numa_balancing_scan_delay);

I think one tab beyond line above is enough for indent here.

>  	kscand_slot->scan_delta = 0;
>  
>  	slot = &kscand_slot->slot;