Re: [PATCH v2 1/2] sched/numa: skip VMA scanning on memory pinned to one NUMA node via cpuset.mems

[Mel Gorman]

On Wed, Mar 26, 2025 at 05:23:51PM -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.
> 
> We have seen up to a 6x improvement on a typical java workload running on
> VMs with memory and CPU pinned to one NUMA node via cpuset in a two-socket
> AARCH64 system. With the same pinning, on a 18-cores-per-socket Intel
> platform, we have seen 20% improvment in a microbench that creates a
> 30-vCPU selftest KVM guest with 4GB memory, where each vCPU reads 4KB
> pages in a fixed number of loops.
> 
> Signed-off-by: Libo Chen <libo.chen@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e43993a4e5807..6f405e00c9c7e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3329,6 +3329,13 @@ 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;
> +

Check cpusets_enabled() first?

-- 
Mel Gorman
SUSE Labs