On 8/9/2022 3:52 PM, Valentin Schneider wrote:

On 09/08/22 13:18, Tariq Toukan wrote:

quoted

On 8/9/2022 1:02 PM, Valentin Schneider wrote:

quoted

Are there cases where we can't figure this out in advance? From what I grok
out of the two callsites you patched, all vectors will be used unless some
error happens, so compressing the CPUs in a single cpumask seemed
sufficient.

All vectors will be initialized to support the maximum number of traffic
rings. However, the actual number of traffic rings can be controlled and
set to a lower number N_actual < N. In this case, we'll be using only
N_actual instances and we want them to be the first/closest.

Ok, that makes sense, thank you.

In that case I wonder if we'd want a public-facing iterator for
sched_domains_numa_masks[%i][node], rather than copy a portion of
it. Something like the below (naming and implementation haven't been
thought about too much).

   const struct cpumask *sched_numa_level_mask(int node, int level)
   {
           struct cpumask ***masks = rcu_dereference(sched_domains_numa_masks);

           if (node >= nr_node_ids || level >= sched_domains_numa_levels)
                   return NULL;

           if (!masks)
                   return NULL;

           return masks[level][node];
   }
   EXPORT_SYMBOL_GPL(sched_numa_level_mask);

The above can be kept static, and expose only the foo() function below, 
similar to my sched_cpus_set_spread().

LGTM.
How do you suggest to proceed?
You want to formalize it? Or should I take it from here?

   #define for_each_numa_level_mask(node, lvl, mask)	    \
           for (mask = sched_numa_level_mask(node, lvl); mask;	\
                mask = sched_numa_level_mask(node, ++lvl))

   void foo(int node, int cpus[], int ncpus)
   {
           const struct cpumask *mask;
           int lvl = 0;
           int i = 0;
           int cpu;

           rcu_read_lock();
           for_each_numa_level_mask(node, lvl, mask) {
                   for_each_cpu(cpu, mask) {
                           cpus[i] = cpu;
                           if (++i == ncpus)
                                   goto done;
                   }
           }
   done:
           rcu_read_unlock();
   }