linux-kernel - Re: [PATCH v4 5/7] sched/topology: Introduce sched_numa_hop

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Date:   Tue, 27 Sep 2022 17:45:15 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Tony Luck <tony.luck@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Gal Pressman <gal@...dia.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>
Subject: Re: [PATCH v4 5/7] sched/topology: Introduce sched_numa_hop_mask()

On 25/09/22 11:05, Yury Norov wrote:
> On Fri, Sep 23, 2022 at 04:55:40PM +0100, Valentin Schneider wrote:
>> +const struct cpumask *sched_numa_hop_mask(int node, int hops)
>> +{
>> +	struct cpumask ***masks = rcu_dereference(sched_domains_numa_masks);
>> +
>> +	if (node == NUMA_NO_NODE && !hops)
>> +		return cpu_online_mask;
>> +
>> +	if (node >= nr_node_ids || hops >= sched_domains_numa_levels)
>> +		return ERR_PTR(-EINVAL);
>
> This looks like a sanity check. If so, it should go before the snippet
> above, so that client code would behave consistently.
>

nr_node_ids is unsigned, so -1 >= nr_node_ids is true.

>> +
>> +	if (!masks)
>> +		return NULL;
>
> In (node == NUMA_NO_NODE && !hops) case you return online cpus. Here
> you return NULL just to convert it to cpu_online_mask in the caller.
> This looks inconsistent. So, together with the above comment, this
> makes me feel that you'd do it like this:
>
>  const struct cpumask *sched_numa_hop_mask(int node, int hops)
>  {
>       struct cpumask ***masks;
>
>       if (node >= nr_node_ids || hops >= sched_domains_numa_levels)
>         {
>  #ifdef CONFIG_SCHED_DEBUG
>                 pr_err(...);
>  #endif
>               return ERR_PTR(-EINVAL);
>         }
>
>       if (node == NUMA_NO_NODE && !hops)
>               return cpu_online_mask; /* or NULL */
>
>         masks = rcu_dereference(sched_domains_numa_masks);
>       if (!masks)
>               return cpu_online_mask; /* or NULL */
>
>       return masks[hops][node];
>  }

If we're being pedantic, sched_numa_hop_mask() shouldn't return
cpu_online_mask in those cases, but that was the least horrible
option I found to get something sensible for the NUMA_NO_NODE /
!CONFIG_NUMA case. I might be able to better handle this with your
suggestion of having a mask iterator.