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Date: Fri, 16 Feb 2024 14:51:24 +0100
From: Oscar Salvador <osalvador@suse.de>
To: Byungchul Park <byungchul@sk.com>
Cc: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
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	bristot@redhat.com, vschneid@redhat.com,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	kernel_team@skhynix.com, akpm@linux-foundation.org
Subject: Re: [PATCH v3] sched/numa, mm: do not promote folios to nodes not
 set N_MEMORY
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References: <20240216114045.24828-1-byungchul@sk.com>
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On Fri, Feb 16, 2024 at 08:40:45PM +0900, Byungchul Park wrote:
> From 150af2f78e19217a1d03e47e3ee5279684590fb4 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@sk.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

"do not try to promote folios to memoryless nodes"

because AFAICS we are just trying.
Even if should_numa_migrate_memory() returns true, I assume that we will
fail somewhere down the chain e.g: migrate_pages() when we see that this
node does not any memory, right?

> 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.

If you talk about memoryless nodes everybody gets it better IMHO.
"Memoryless nodes do not have any memory to migrate to, so stop trying it."


> Signed-off-by: Byungchul Park <byungchul@sk.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;

"Cannot migrate to memoryless nodes"

seems shorter and more clear.

So, what happens when we return true here? will we fail at
migrate_pages() I guess? That is quite down the road so I guess
this check can save us some time.


-- 
Oscar Salvador
SUSE Labs