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

[Tariq Toukan <ttoukan.linux@gmail.com>]

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.

>   #endif /* _LINUX_TOPOLOGY_H */