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

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Date:   Thu, 19 Oct 2023 16:21:44 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     akpm@...ux-foundation.org, rppt@...nel.org, vbabka@...e.cz,
        mhocko@...e.com, 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

Hi David,

On 2023/10/19 16:07, David Hildenbrand wrote:
> 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.

Yes, the find_next_best_node() will be called by build_all_zonelists().

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

Thanks.

>