Re: [Patch v3] mm/page_alloc: add same penalty is enough to get round-robin order

[Oscar Salvador <osalvador@suse.de>]

On Tue, Apr 12, 2022 at 12:13:19AM +0000, Wei Yang wrote:
> To make node order in round-robin in the same distance group, we add a
> penalty to the first node we got in each round.
> 
> To get a round-robin order in the same distance group, we don't need to
> decrease the penalty since:
> 
>   * find_next_best_node() always iterates node in the same order
>   * distance matters more then penalty in find_next_best_node()
>   * in nodes with the same distance, the first one would be picked up
> 
> So it is fine to increase same penalty when we get the first node in the
> same distance group. Since we just increase a constance of 1 to node
> penalty, it is not necessary to multiply MAX_NODE_LOAD for preference.
> 
> [vbabka@suse.cz: suggests to remove MAX_NODE_LOAD]
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> CC: Vlastimil Babka <vbabka@suse.cz>
> CC: David Hildenbrand <david@redhat.com>
> CC: Oscar Salvador <osalvador@suse.de>

Acked-by: Oscar Salvador <osalvador@suse.de>

> ---
> v3: merge into a single patch
> v2: adjust constant penalty to 1
> ---
>  mm/page_alloc.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5d71b8dcb5f4..0334c06a0a47 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6170,7 +6170,6 @@ int numa_zonelist_order_handler(struct ctl_table *table, int write,
>  }
>  
>  
> -#define MAX_NODE_LOAD (nr_online_nodes)
>  static int node_load[MAX_NUMNODES];
>  
>  /**
> @@ -6217,7 +6216,7 @@ int find_next_best_node(int node, nodemask_t *used_node_mask)
>  			val += PENALTY_FOR_NODE_WITH_CPUS;
>  
>  		/* Slight preference for less loaded node */
> -		val *= (MAX_NODE_LOAD*MAX_NUMNODES);
> +		val *= MAX_NUMNODES;
>  		val += node_load[n];
>  
>  		if (val < min_val) {
> @@ -6283,13 +6282,12 @@ static void build_thisnode_zonelists(pg_data_t *pgdat)
>  static void build_zonelists(pg_data_t *pgdat)
>  {
>  	static int node_order[MAX_NUMNODES];
> -	int node, load, nr_nodes = 0;
> +	int node, nr_nodes = 0;
>  	nodemask_t used_mask = NODE_MASK_NONE;
>  	int local_node, prev_node;
>  
>  	/* NUMA-aware ordering of nodes */
>  	local_node = pgdat->node_id;
> -	load = nr_online_nodes;
>  	prev_node = local_node;
>  
>  	memset(node_order, 0, sizeof(node_order));
> @@ -6301,11 +6299,10 @@ static void build_zonelists(pg_data_t *pgdat)
>  		 */
>  		if (node_distance(local_node, node) !=
>  		    node_distance(local_node, prev_node))
> -			node_load[node] += load;
> +			node_load[node] += 1;
>  
>  		node_order[nr_nodes++] = node;
>  		prev_node = node;
> -		load--;
>  	}
>  
>  	build_zonelists_in_node_order(pgdat, node_order, nr_nodes);
> -- 
> 2.33.1
> 
> 

-- 
Oscar Salvador
SUSE Labs