Re: [PATCH 01/10] sched/topology: introduce for_each_numa_hop_node() / sched_numa_hop_node()

[Yury Norov]

On Fri, Dec 20, 2024 at 04:11:33PM +0100, Andrea Righi wrote:
> Introduce for_each_numa_hop_node() and sched_numa_hop_node() to iterate
> over node IDs in order of increasing NUMA distance from a given starting
> node.
> 
> These iterator functions are similar to for_each_numa_hop_mask() and
> sched_numa_hop_mask(), but instead of providing a cpumask at each
> iteration, they provide a node ID.
> 
> Example usage:
> 
>   nodemask_t hop_nodes = NODE_MASK_NONE;
>   int start = cpu_to_node(smp_processor_id());
> 
>   for_each_numa_hop_node(node, start, hop_nodes, N_ONLINE)
>   	pr_info("node (%d, %d) -> \n",
>   		 start, node, node_distance(start, node);

This iterates nodes, not hops. The hop is a set of equidistant nodes,
and the iterator (the first argument) should be a nodemask. I'm OK with
that as soon as you find it practical. But then you shouldn't mention
hops in the patch.

Also, can you check that it works correctly against a configuration with
equidistant nodes?

> Simulating the following NUMA topology in virtme-ng:
> 
>  $ numactl -H
>  available: 4 nodes (0-3)
>  node 0 cpus: 0 1
>  node 0 size: 1006 MB
>  node 0 free: 928 MB
>  node 1 cpus: 2 3
>  node 1 size: 1007 MB
>  node 1 free: 986 MB
>  node 2 cpus: 4 5
>  node 2 size: 889 MB
>  node 2 free: 862 MB
>  node 3 cpus: 6 7
>  node 3 size: 1006 MB
>  node 3 free: 983 MB
>  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
> 
> The output of the example above (on node 0) is the following:
> 
>  [   84.953644] node (0, 0) -> 10
>  [   84.953712] node (0, 2) -> 31
>  [   84.953764] node (0, 3) -> 41
>  [   84.953817] node (0, 1) -> 51
> 
> Cc: Yury Norov <yury.norov@xxxxxxxxx>
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
> ---
>  include/linux/topology.h | 28 ++++++++++++++++++++++-
>  kernel/sched/topology.c  | 49 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 76 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 52f5850730b3..d9014d90580d 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -248,12 +248,18 @@ static inline const struct cpumask *cpu_cpu_mask(int cpu)
>  #ifdef CONFIG_NUMA
>  int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node);
>  extern const struct cpumask *sched_numa_hop_mask(unsigned int node, unsigned int hops);
> -#else
> +extern int sched_numa_hop_node(nodemask_t *hop_nodes, int start, unsigned int state);
> +#else /* !CONFIG_NUMA */
>  static __always_inline int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
>  {
>  	return cpumask_nth_and(cpu, cpus, cpu_online_mask);
>  }
>  
> +static inline int sched_numa_hop_node(nodemask_t *hop_nodes, int start, unsigned int state)
> +{
> +	return NUMA_NO_NODE;
> +}
> +
>  static inline const struct cpumask *
>  sched_numa_hop_mask(unsigned int node, unsigned int hops)
>  {
> @@ -261,6 +267,26 @@ sched_numa_hop_mask(unsigned int node, unsigned int hops)
>  }
>  #endif	/* CONFIG_NUMA */
>  
> +/**
> + * for_each_numa_hop_node - iterate over NUMA nodes at increasing hop distances
> + *                          from a given starting node.
> + * @__node: the iteration variable, representing the current NUMA node.
> + * @__start: the NUMA node to start the iteration from.
> + * @__hop_nodes: a nodemask_t to track the visited nodes.
> + * @__state: state of NUMA nodes to iterate.
> + *
> + * Requires rcu_lock to be held.
> + *
> + * This macro iterates over NUMA nodes in increasing distance from
> + * @start_node.
> + *
> + * Yields NUMA_NO_NODE when all the nodes have been visited.
> + */
> +#define for_each_numa_hop_node(__node, __start, __hop_nodes, __state)		\

As soon as this is not the hops iterator, the proper name would be just

        for_each_numa_node()

And because the 'numa' prefix here doesn't look like a prefix, I think
it would be nice to comment what this 'numa' means and what's the
difference between this and for_each_node() iterator, especially in
terms of complexity.

Also you don't need underscores in macro declarations unless
absolutely necessary.

> +	for (int __node = __start;						\

The __node declared in for() masks out the __node provided in the
macro.

> +	     __node != NUMA_NO_NODE;						\
> +	     __node = sched_numa_hop_node(&(__hop_nodes), __start, __state))
> +
>  /**
>   * for_each_numa_hop_mask - iterate over cpumasks of increasing NUMA distance
>   *                          from a given node.
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 9748a4c8d668..8e77c235ad9a 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2185,6 +2185,55 @@ int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
>  }
>  EXPORT_SYMBOL_GPL(sched_numa_find_nth_cpu);
>  
> +/**
> + * sched_numa_hop_node - Find the NUMA node at the closest hop distance
> + *                       from node @start.
> + *
> + * @hop_nodes: a pointer to a nodemask_t representing the visited nodes.
> + * @start: the NUMA node to start the hop search from.
> + * @state: the node state to filter nodes by.
> + *
> + * This function iterates over all NUMA nodes in the given state and
> + * calculates the hop distance to the starting node. It returns the NUMA
> + * node that is the closest in terms of hop distance
> + * that has not already been considered (not set in @hop_nodes). If the
> + * node is found, it is marked as considered in the @hop_nodes bitmask.
> + *
> + * The function checks if the node is not the start node and ensures it is
> + * not already part of the hop_nodes set. It then computes the distance to
> + * the start node using the node_distance() function. The closest node is
> + * chosen based on the minimum distance.
> + *
> + * Returns the NUMA node ID closest in terms of hop distance from the
> + * @start node, or NUMA_NO_NODE if no node is found (or all nodes have been
> + * visited).

for_each_node_state() returns MAX_NUMNODES when it finishes
traversing. I think you should do the same here. 

> + */
> +int sched_numa_hop_node(nodemask_t *hop_nodes, int start, unsigned int state)
> +{
> +	int dist, n, min_node, min_dist;
> +
> +	if (state >= NR_NODE_STATES)
> +		return NUMA_NO_NODE;

 -EINVAL. But, do we need to check the parameter at all?

> +
> +	min_node = NUMA_NO_NODE;
> +	min_dist = INT_MAX;
> +
> +	for_each_node_state(n, state) {
> +		if (n == start || node_isset(n, *hop_nodes))
> +			continue;
> +		dist = node_distance(start, n);
> +		if (dist < min_dist) {
> +			min_dist = dist;
> +			min_node = n;
> +		}
> +	}

This is a version of numa_nearest_node(). The only difference is that
you add 'hop_nodes' mask, which in fact is 'visited' nodes.

I think it should be like:

 int numa_nearest_unvisited_node(nodemask_t *visited, int start, unsigned int state)
 {
        for_each_node_andnot(node_states[state], visited) (
                ...
        }
 }

Thanks,
Yury

> +	if (min_node != NUMA_NO_NODE)
> +		node_set(min_node, *hop_nodes);
> +
> +	return min_node;
> +}
> +EXPORT_SYMBOL_GPL(sched_numa_hop_node);
> +
>  /**
>   * sched_numa_hop_mask() - Get the cpumask of CPUs at most @hops hops away from
>   *                         @node
> -- 
> 2.47.1