linux-kernel - Re: [PATCH 1/2] mm/page_alloc: add same penalty is enough to get round-robin order

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Date:   Tue, 5 Apr 2022 19:11:12 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Wei Yang <richard.weiyang@...il.com>, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/page_alloc: add same penalty is enough to get
 round-robin order

On 1/23/22 02:35, 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.

With that logic I'm not even sure if we need nr_online_nodes as penalty or
it could be just 1. Would you know?

> 
> Signed-off-by: Wei Yang <richard.weiyang@...il.com>
> ---
>  mm/page_alloc.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c5952749ad40..f27afd517652 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6245,13 +6245,12 @@ 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));
> @@ -6263,11 +6262,10 @@ 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] += nr_online_nodes;
>  
>  		node_order[nr_nodes++] = node;
>  		prev_node = node;
> -		load--;
>  	}
>  
>  	build_zonelists_in_node_order(pgdat, node_order, nr_nodes);