Re: [PATCH v3 8/9] sched/topology: Introduce for_each_numa_hop_cpu()

[Valentin Schneider <vschneid@redhat.com>]

On 05/09/22 12:46, Tariq Toukan wrote:
> On 8/25/2022 9:12 PM, Valentin Schneider wrote:
>> The recently introduced sched_numa_hop_mask() exposes cpumasks of CPUs
>> reachable within a given distance budget, but this means each successive
>> cpumask is a superset of the previous one.
>>
>> Code wanting to allocate one item per CPU (e.g. IRQs) at increasing
>> distances would thus need to allocate a temporary cpumask to note which
>> CPUs have already been visited. This can be prevented by leveraging
>> for_each_cpu_andnot() - package all that logic into one ugl^D fancy macro.
>>
>> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
>> ---
>>   include/linux/topology.h | 37 +++++++++++++++++++++++++++++++++++++
>>   1 file changed, 37 insertions(+)
>>
>> diff --git a/include/linux/topology.h b/include/linux/topology.h
>> index 13b82b83e547..6c671dc3252c 100644
>> --- a/include/linux/topology.h
>> +++ b/include/linux/topology.h
>> @@ -254,5 +254,42 @@ static inline const struct cpumask *sched_numa_hop_mask(int node, int hops)
>>   }
>>   #endif	/* CONFIG_NUMA */
>>
>> +/**
>> + * for_each_numa_hop_cpu - iterate over CPUs by increasing NUMA distance,
>> + *                         starting from a given node.
>> + * @cpu: the iteration variable.
>> + * @node: the NUMA node to start the search from.
>> + *
>> + * Requires rcu_lock to be held.
>> + * Careful: this is a double loop, 'break' won't work as expected.
>> + *
>> + *
>> + * Implementation notes:
>> + *
>> + * Providing it is valid, the mask returned by
>> + *  sched_numa_hop_mask(node, hops+1)
>> + * is a superset of the one returned by
>> + *   sched_numa_hop_mask(node, hops)
>> + * which may not be that useful for drivers that try to spread things out and
>> + * want to visit a CPU not more than once.
>> + *
>> + * To accommodate for that, we use for_each_cpu_andnot() to iterate over the cpus
>> + * of sched_numa_hop_mask(node, hops+1) with the CPUs of
>> + * sched_numa_hop_mask(node, hops) removed, IOW we only iterate over CPUs
>> + * a given distance away (rather than *up to* a given distance).
>> + *
>> + * hops=0 forces us to play silly games: we pass cpu_none_mask to
>> + * for_each_cpu_andnot(), which turns it into for_each_cpu().
>> + */
>> +#define for_each_numa_hop_cpu(cpu, node)				       \
>> +	for (struct { const struct cpumask *curr, *prev; int hops; } __v =     \
>> +		     { sched_numa_hop_mask(node, 0), NULL, 0 };		       \
>> +	     !IS_ERR_OR_NULL(__v.curr);					       \
>> +	     __v.hops++,                                                       \
>> +	     __v.prev = __v.curr,					       \
>> +	     __v.curr = sched_numa_hop_mask(node, __v.hops))                   \
>> +		for_each_cpu_andnot(cpu,				       \
>> +				    __v.curr,				       \
>> +				    __v.hops ? __v.prev : cpu_none_mask)
>>
>
> Hiding two nested loops together in one for_each_* macro leads to
> unexpected behavior for the standard usage of 'break/continue'.
>
> for_each_numa_hop_cpu(cpu, node) {
>      if (condition)
>          break; <== will terminate the inner loop only, but it's
> invisible to the human developer/reviewer.
> }
>
> These bugs will not be easy to spot in code review.
>

Yeah, it's not ideal, but I *did* attach a comment warning about
it. Spreading this pattern for sure isn't great, but there *is* a precedent
with for_each_process_thread().

>>   #endif /* _LINUX_TOPOLOGY_H */