netdev - Re: [PATCH net-next V4 1/3] sched/topology: Add NUMA-based CPUs spread API

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Date:   Tue, 09 Aug 2022 13:52:13 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Tariq Toukan <ttoukan.linux@...il.com>,
        Tariq Toukan <tariqt@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Saeed Mahameed <saeedm@...dia.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        Gal Pressman <gal@...dia.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next V4 1/3] sched/topology: Add NUMA-based CPUs
 spread API

On 09/08/22 13:18, Tariq Toukan wrote:
> On 8/9/2022 1:02 PM, Valentin Schneider wrote:
>>
>> 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);

  #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();
  }