linux-kernel - Re: [PATCH v3] sched/numa, mm: do not promote folios to nodes not set N

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Date: Mon, 19 Feb 2024 10:30:18 +0100
From: David Hildenbrand <david@...hat.com>
To: Byungchul Park <byungchul@...com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 kernel_team@...ynix.com, akpm@...ux-foundation.org
Subject: Re: [PATCH v3] sched/numa, mm: do not promote folios to nodes not set
 N_MEMORY

On 16.02.24 12:40, Byungchul Park wrote:
> Changes from v2:
> 	1. Rewrite the comment in code and the commit message becasue it
> 	   turns out that this patch is not the real fix for the oops
> 	   descriped. The real fix goes in another patch below:
> 
> 	   https://lore.kernel.org/lkml/20240216111502.79759-1-byungchul@sk.com/
> 
> Changes from v1:
> 	1. Trim the verbose oops in the commit message. (feedbacked by
> 	   Phil Auld)
> 	2. Rewrite a comment in code. (feedbacked by Phil Auld)
> 
> --->8---
>  From 150af2f78e19217a1d03e47e3ee5279684590fb4 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@...com>
> Date: Fri, 16 Feb 2024 20:18:10 +0900
> Subject: [PATCH v3] sched/numa, mm: do not promote folios to nodes not set N_MEMORY
> 
> A numa node might not have its local memory but CPUs. Promoting a folio
> to the node's local memory is nonsense. So avoid nodes not set N_MEMORY
> from getting promoted.

So there is no bug/panic that can be triggered and this is not a "fix" 
but an optimization?

> 
> Signed-off-by: Byungchul Park <byungchul@...com>
> ---
>   kernel/sched/fair.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d7a3c63a2171..7ed9ef3c0134 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1828,6 +1828,13 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
>   	int dst_nid = cpu_to_node(dst_cpu);
>   	int last_cpupid, this_cpupid;
>   
> +	/*
> +	 * A node of dst_nid might not have its local memory. Promoting
> +	 * a folio to the node is meaningless.
> +	 */
> +	if (!node_state(dst_nid, N_MEMORY))
> +		return false;
> +
>   	/*
>   	 * The pages in slow memory node should be migrated according
>   	 * to hot/cold instead of private/shared.

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

-- 
Cheers,

David / dhildenb