Re: [PATCH v4 2/9] sched/topology: Extract "imb_numa_nr" calculation into a separate helper

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

Hi Prateek,

On 16.03.26 04:41, K Prateek Nayak wrote:
> Hello Dietmar,
> 
> On 3/16/2026 5:48 AM, Dietmar Eggemann wrote:

[...]

> Indeed! "imb_numa_nr" only makes sense when looking at NUMA domains
> and having it assigned to 1 for lower domains is harmless
> (but wasteful indeed). I'm 99% sure we can simply do:
> 
>   (Only build tested)
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 43150591914b..e9068a809dbc 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2623,9 +2623,6 @@ static void adjust_numa_imbalance(struct sched_domain *sd_llc)
>  	else
>  		imb = nr_llcs;
>  
> -	imb = max(1U, imb);
> -	sd_llc->parent->imb_numa_nr = imb;
> -
>  	/*
>  	 * Set span based on the first NUMA domain.
>  	 *
> @@ -2639,10 +2636,14 @@ static void adjust_numa_imbalance(struct sched_domain *sd_llc)
>  	while (parent && !(parent->flags & SD_NUMA))
>  		parent = parent->parent;
>  
> -	imb_span = parent ? parent->span_weight : sd_llc->parent->span_weight;
> +	/* No NUMA domain to adjust imbalance for! */
> +	if (!parent)
> +		return;
> +
> +	imb = max(1U, imb);
> +	imb_span = parent->span_weight;
>  
>  	/* Update the upper remainder of the topology */
> -	parent = sd_llc->parent;
>  	while (parent) {
>  		int factor = max(1U, (parent->span_weight / imb_span));
>  
> ---
> 
> If we have NUMA domains, we definitely have NODE and NODE sets neither
> SD_SHARE_LLC, nor SD_NUMA so likely sd->parent is PKG / NODE domain and
> NUMA has to start at sd->parent->parent and it has to break at the first
> SD_NUMA domains.
> 
> If it doesn't exist, we don't have any NUMA domains and nothing to worry
> about, and if we do, the final loop will adjust the NUMA imbalance.
> 
> Thoughts? Again, this commit was kept 1:1 with the previous loop but we
> can always improve :-)
Ah, I see!

This would work, IMHO.

Tested on qemu-system-aarch64 w/

  -smp 8,sockets=2,clusters=2,cores=2,threads=1

Are you aware of a setup in which PKG would survive between MC and
lowest NUMA?