Re: [PATCH] topology: make for_each_node_with_cpus() O(N)

[Andrea Righi <arighi@nvidia.com>]

Hi Yury,

On Fri, May 09, 2025 at 12:20:08PM -0400, Yury Norov wrote:
> From: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> 
> for_each_node_with_cpus() calls nr_cpus_node() at every iteration, which
> makes it O(N^2). Kernel tracks such nodes with N_CPU record in node_states
> array. Switching to it makes for_each_node_with_cpus() O(N).

Makes sense to me.

Maybe we should mention that previously we were only considering online
nodes with CPUs assigned. Now, we can include also offline nodes with CPUs
assigned (assuming it's possible)?

Semantically speaking, since the name doesn't include "online", it seems
more logical to ignore the state of the node. And if checking the online
state is required, the user can just use node_online() within the loop.

Thanks,
-Andrea

> 
> Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> ---
>  include/linux/nodemask.h | 1 +
>  include/linux/topology.h | 5 +----
>  2 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h
> index f0ac0633366b..1e2bdda1a0a5 100644
> --- a/include/linux/nodemask.h
> +++ b/include/linux/nodemask.h
> @@ -541,6 +541,7 @@ static __always_inline int node_random(const nodemask_t *maskp)
>  
>  #define for_each_node(node)	   for_each_node_state(node, N_POSSIBLE)
>  #define for_each_online_node(node) for_each_node_state(node, N_ONLINE)
> +#define for_each_node_with_cpus(node)	for_each_node_state(node, N_CPU)
>  
>  /*
>   * For nodemask scratch area.
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 24e715f0f6d2..ffee6b4a071a 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -29,6 +29,7 @@
>  
>  #include <linux/arch_topology.h>
>  #include <linux/cpumask.h>
> +#include <linux/nodemask.h>
>  #include <linux/bitops.h>
>  #include <linux/mmzone.h>
>  #include <linux/smp.h>
> @@ -39,10 +40,6 @@
>  #define nr_cpus_node(node) cpumask_weight(cpumask_of_node(node))
>  #endif
>  
> -#define for_each_node_with_cpus(node)			\
> -	for_each_online_node(node)			\
> -		if (nr_cpus_node(node))
> -
>  int arch_update_cpu_topology(void);
>  
>  /* Conform to ACPI 2.0 SLIT distance definitions */
> -- 
> 2.43.0
>