Subject: Re: [PATCH v2 7/8] lib: add test for for_each_numa_{cpu,hop_mask}()

From: Yury Norov <[email-masked]>
Date: Wed, 26 Apr 2023 13:51:36 -0700

> I realized I only wrote half the relevant code - comparing node IDs is
> meaningless, I meant to compare distances as we walk through the
> CPUs... I tested the below against a few NUMA topologies and it seems to be
> sane:
> 
> @@ -756,12 +773,23 @@ static void __init test_for_each_numa(void)
>  {
>  	unsigned int cpu, node;
>  
> -	for (node = 0; node < sched_domains_numa_levels; node++) {
> -		unsigned int hop, c = 0;
> +	for_each_node(node) {
> +		unsigned int start_cpu, prev_dist, hop = 0;
> +
> +		cpu = cpumask_first(cpumask_of_node(node));
> +		prev_dist = node_distance(node, node);
> +		start_cpu = cpu;
>  
>  		rcu_read_lock();
> -		for_each_numa_cpu(cpu, hop, node, cpu_online_mask)
> -			expect_eq_uint(cpumask_local_spread(c++, node), cpu);
> +
> +		/* Assert distance is monotonically increasing */
> +		for_each_numa_cpu(cpu, hop, node, cpu_online_mask) {
> +			unsigned int dist = node_distance(cpu_to_node(cpu), cpu_to_node(start_cpu));

Interestingly, node_distance() is an arch-specific function. Generic
implementation is quite useless:

 #define node_distance(from,to)  ((from) == (to) ? LOCAL_DISTANCE : REMOTE_DISTANCE)

Particularly, arm64 takes the above. With node_distance() implemented
like that, we can barely test something...

Taking that into the account, I think it's better to test iterator against
cpumask_local_spread(), like in v2. I'll add a comment about that in v3.

> +
> +			expect_ge_uint(dist, prev_dist);
> +			prev_dist = dist;
> +		}
> +
>  		rcu_read_unlock();
>  	}
>  }