Re: [PATCH] sched/numa: skip VMA scanning on memory pinned to one NUMA node via cpuset.mems

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 11, 2025 at 09:04:47AM -0700, Libo Chen wrote:
> When the memory of the current task is pinned to one NUMA node by cgroup,
> there is no point in continuing the rest of VMA scanning and hinting page
> faults as they will just be overhead. With this change, there will be no
> more unnecessary PTE updates or page faults in this scenario.

Its been a while since I looked at all this, but if we don't scan these
pages, then it will not account for these pages, and the pinned memory
will not become an attractor for the tasks that use this memory, right?

> Signed-off-by: Libo Chen <libo.chen@oracle.com>
> ---
>  kernel/sched/fair.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c798d27952431..ec4749a7be33a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3315,6 +3315,13 @@ static void task_numa_work(struct callback_head *work)
>  	if (p->flags & PF_EXITING)
>  		return;
>  
> +	/*
> +	 * Memory is pinned to only one NUMA node via cpuset.mems, naturally
> +	 * no page can be migrated.
> +	 */
> +	if (nodes_weight(cpuset_current_mems_allowed) == 1)
> +		return;
> +
>  	if (!mm->numa_next_scan) {
>  		mm->numa_next_scan = now +
>  			msecs_to_jiffies(sysctl_numa_balancing_scan_delay);
> -- 
> 2.43.5
>