linux-kernel - Re: [PATCH 2/6] sched/topology: Introduce for_each_numa

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Message-ID: <Z6jq4RTT7ynoM2vO@thinkpad>
Date: Sun, 9 Feb 2025 12:50:25 -0500
From: Yury Norov <yury.norov@...il.com>
To: Andrea Righi <arighi@...dia.com>
Cc: Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>, Ian May <ianm@...dia.com>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] sched/topology: Introduce for_each_numa_node()
 iterator

On Fri, Feb 07, 2025 at 09:40:49PM +0100, Andrea Righi wrote:
> Introduce the new helper for_each_numa_node() to iterate over node IDs
> in order of increasing NUMA distance from a given starting node.
> 
> This iterator is similar to for_each_numa_hop_mask(), but instead of
> providing a cpumask at each iteration, it provides a node ID.
> 
> Example usage:
> 
>   nodemask_t unvisited = NODE_MASK_ALL;
>   int node, start = cpu_to_node(smp_processor_id());
> 
>   node = start;
>   for_each_numa_node(node, unvisited, N_ONLINE)
>   	pr_info("node (%d, %d) -> %d\n",
>   		 start, node, node_distance(start, node));
> 
> On a system with equidistant nodes:
> 
>  $ numactl -H
>  ...
>  node distances:
>  node     0    1    2    3
>     0:   10   20   20   20
>     1:   20   10   20   20
>     2:   20   20   10   20
>     3:   20   20   20   10
> 
> Output of the example above (on node 0):
> 
> [    7.367022] node (0, 0) -> 10
> [    7.367151] node (0, 1) -> 20
> [    7.367186] node (0, 2) -> 20
> [    7.367247] node (0, 3) -> 20
> 
> On a system with non-equidistant nodes (simulated using virtme-ng):

Great to see virtme-ng maturing!
 
>  $ numactl -H
>  ...
>  node distances:
>  node     0    1    2    3
>     0:   10   51   31   41
>     1:   51   10   21   61
>     2:   31   21   10   11
>     3:   41   61   11   10
> 
> Output of the example above (on node 0):
> 
>  [    8.953644] node (0, 0) -> 10
>  [    8.953712] node (0, 2) -> 31
>  [    8.953764] node (0, 3) -> 41
>  [    8.953817] node (0, 1) -> 51
> 
> Cc: Yury Norov <yury.norov@...il.com>
> Signed-off-by: Andrea Righi <arighi@...dia.com>
> ---
>  include/linux/topology.h | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 52f5850730b3e..09c18ee8be0eb 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -261,6 +261,34 @@ sched_numa_hop_mask(unsigned int node, unsigned int hops)
>  }
>  #endif	/* CONFIG_NUMA */
>  
> +/**
> + * for_each_numa_node - iterate over nodes at increasing distances from a
> + *			given starting node.

Nit: in increasing distance order, starting from a given node

> + * @node: the iteration variable and the starting node.
> + * @unvisited: a nodemask to keep track of the unvisited nodes.
> + * @state: state of NUMA nodes to iterate.
> + *
> + * This macro iterates over NUMA node IDs in increasing distance from the
> + * starting @node and yields MAX_NUMNODES when all the nodes have been
> + * visited.

Please also mention that the unvisited nodemask will be empty when it finish.

> + *
> + * The difference between for_each_node() and for_each_numa_node() is that
> + * the former allows to iterate over nodes in numerical order, whereas the
> + * latter iterates over nodes in increasing order of distance.
> + *
> + * This complexity of this iterator is O(N^2), where N represents the
> + * number of nodes, as each iteration involves scanning all nodes to
> + * find the one with the shortest distance.
> + *
> + * Requires rcu_lock to be held.
> + */
> +#define for_each_numa_node(node, unvisited, state)				\
> +	for (int start = (node),						\
> +	     node = numa_nearest_nodemask((start), (state), &(unvisited));	\
> +	     node < MAX_NUMNODES;						\
> +	     node_clear(node, (unvisited)),					\
> +	     node = numa_nearest_nodemask((start), (state), &(unvisited)))
> +
>  /**
>   * for_each_numa_hop_mask - iterate over cpumasks of increasing NUMA distance
>   *                          from a given node.
> -- 
> 2.48.1