Subject: Re: [PATCH 1/2] sched/topology: Introduce sched_numa_hop_mask()

From: Tariq Toukan <[email-masked]>
Date: Wed, 10 Aug 2022 15:57:54 +0300

On 8/10/2022 3:42 PM, Tariq Toukan wrote:
> 
> 
> On 8/10/2022 1:51 PM, Valentin Schneider wrote:
>> Tariq has pointed out that drivers allocating IRQ vectors would benefit
>> from having smarter NUMA-awareness - cpumask_local_spread() only knows
>> about the local node and everything outside is in the same bucket.
>>
>> sched_domains_numa_masks is pretty much what we want to hand out (a 
>> cpumask
>> of CPUs reachable within a given distance budget), introduce
>> sched_numa_hop_mask() to export those cpumasks. Add in an iteration 
>> helper
>> to iterate over CPUs at an incremental distance from a given node.
>>
>> Link: http://lore.kernel.org/r/[email-masked]
>> Signed-off-by: Valentin Schneider <[email-masked]>
>> ---
>>   include/linux/topology.h | 12 ++++++++++++
>>   kernel/sched/topology.c  | 28 ++++++++++++++++++++++++++++
>>   2 files changed, 40 insertions(+)
>>
>> diff --git a/include/linux/topology.h b/include/linux/topology.h
>> index 4564faafd0e1..d66e3cf40823 100644
>> --- a/include/linux/topology.h
>> +++ b/include/linux/topology.h
>> @@ -245,5 +245,17 @@ static inline const struct cpumask 
>> *cpu_cpu_mask(int cpu)
>>       return cpumask_of_node(cpu_to_node(cpu));
>>   }
>> +#ifdef CONFIG_NUMA
>> +extern const struct cpumask *sched_numa_hop_mask(int node, int hops);
>> +#else
>> +static inline const struct cpumask *sched_numa_hop_mask(int node, int 
>> hops)
>> +{
>> +    return -ENOTSUPP;
> 
> missing ERR_PTR()
> 
>> +}
>> +#endif    /* CONFIG_NUMA */
>> +
>> +#define for_each_numa_hop_mask(node, hops, mask)            \
>> +    for (mask = sched_numa_hop_mask(node, hops); 
>> !IS_ERR_OR_NULL(mask); \
>> +         mask = sched_numa_hop_mask(node, ++hops))
>>   #endif /* _LINUX_TOPOLOGY_H */
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index 8739c2a5a54e..f0236a0ae65c 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
>> @@ -2067,6 +2067,34 @@ int sched_numa_find_closest(const struct 
>> cpumask *cpus, int cpu)
>>       return found;
>>   }
>> +/**
>> + * sched_numa_hop_mask() - Get the cpumask of CPUs at most @hops hops 
>> away.
>> + * @node: The node to count hops from.
>> + * @hops: Include CPUs up to that many hops away. 0 means local node.

AFAIU, here you work with a specific level/num of hops, description is 
not accurate.

>> + *
>> + * Requires rcu_lock to be held. Returned cpumask is only valid 
>> within that
>> + * read-side section, copy it if required beyond that.
>> + *
>> + * Note that not all hops are equal in size; see sched_init_numa() 
>> for how
>> + * distances and masks are handled.
>> + *
>> + * Also note that this is a reflection of sched_domains_numa_masks, 
>> which may change
>> + * during the lifetime of the system (offline nodes are taken out of 
>> the masks).
>> + */
>> +const struct cpumask *sched_numa_hop_mask(int node, int hops)
>> +{
>> +    struct cpumask ***masks = rcu_dereference(sched_domains_numa_masks);
>> +
>> +    if (node >= nr_node_ids || hops >= sched_domains_numa_levels)
>> +        return ERR_PTR(-EINVAL);
>> +
>> +    if (!masks)
>> +        return NULL;
>> +
>> +    return masks[hops][node];
>> +}
>> +EXPORT_SYMBOL_GPL(sched_numa_hop_mask);
>> +
>>   #endif /* CONFIG_NUMA */
>>   static int __sdt_alloc(const struct cpumask *cpu_map)