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

[Valentin Schneider]

On 26/04/23 13:51, Yury Norov wrote:
>> 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...
>

riscv and arm64 rely on drivers/base/arch_numa.c to provide
__node_distance() (cf. CONFIG_GENERIC_ARCH_NUMA).

x86, sparc, powerpc and ia64 define __node_distance()
loongarch and mips define their own node_distance().

So all of those archs will have a usable node_distance(), the others won't
and that means the scheduler can't do anything about it - the scheduler
relies on node_distance() to understand the topolgoy!