linux-kernel - Re: [PATCH v2 1/2] mm: page_alloc: skip memoryless nodes entirely

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Message-ID: <cd210991-5038-4ad3-ac03-abb6761c67bd@redhat.com>
Date:   Thu, 19 Oct 2023 10:07:39 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Qi Zheng <zhengqi.arch@...edance.com>, akpm@...ux-foundation.org,
        rppt@...nel.org, vbabka@...e.cz, mhocko@...e.com
Cc:     willy@...radead.org, mgorman@...hsingularity.net, mingo@...nel.org,
        aneesh.kumar@...ux.ibm.com, ying.huang@...el.com,
        hannes@...xchg.org, osalvador@...e.de,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/2] mm: page_alloc: skip memoryless nodes entirely

On 19.10.23 09:36, Qi Zheng wrote:
> In find_next_best_node(), We skipped the memoryless nodes
> when building the zonelists of other normal nodes (N_NORMAL),
> but did not skip the memoryless node itself when building
> the zonelist. This will cause it to be traversed at runtime.
> 
> For example, say we have node0 and node1, node0 is memoryless
> node, then the fallback order of node0 and node1 as follows:
> 
> [    0.153005] Fallback order for Node 0: 0 1
> [    0.153564] Fallback order for Node 1: 1
> 
> After this patch, we skip memoryless node0 entirely, then
> the fallback order of node0 and node1 as follows:
> 
> [    0.155236] Fallback order for Node 0: 1
> [    0.155806] Fallback order for Node 1: 1
> 
> So it becomes completely invisible, which will reduce runtime
> overhead.
> 
> And in this way, we will not try to allocate pages from memoryless
> node0, then the panic mentioned in [1] will also be fixed. Even though
> this problem has been solved by dropping the NODE_MIN_SIZE constrain
> in x86 [2], it would be better to fix it in core MM as well.
> 
> [1]. https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
> [2]. https://lore.kernel.org/all/20231017062215.171670-1-rppt@kernel.org/
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
>   mm/page_alloc.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ee392a324802..e978272699d3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5052,8 +5052,11 @@ int find_next_best_node(int node, nodemask_t *used_node_mask)
>   	int min_val = INT_MAX;
>   	int best_node = NUMA_NO_NODE;
>   
> -	/* Use the local node if we haven't already */
> -	if (!node_isset(node, *used_node_mask)) {
> +	/*
> +	 * Use the local node if we haven't already. But for memoryless local
> +	 * node, we should skip it and fallback to other nodes.
> +	 */
> +	if (!node_isset(node, *used_node_mask) && node_state(node, N_MEMORY)) {
>   		node_set(node, *used_node_mask);
>   		return node;
>   	}

Makes sense to me; I suspect that online_pages() will just to the right 
thing and call build_all_zonelists() to fix it up.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb