linux-kernel - Re: [PATCH v2] mm/slub: Simplify get_partial

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Date: Tue, 12 Mar 2024 23:22:54 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: sxwjean@...com, Christoph Lameter <cl@...ux.com>,
 Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>,
 Joonsoo Kim <iamjoonsoo.kim@....com>,
 Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Xiongwei Song <xiongwei.song@...driver.com>
Subject: Re: [PATCH v2] mm/slub: Simplify get_partial_node()

On 2024/3/12 22:05, sxwjean@...com wrote:
> From: Xiongwei Song <xiongwei.song@...driver.com>
> 
> Remove the check of !kmem_cache_has_cpu_partial() because it is always
> false, we've known this by calling kmem_cache_debug() before calling
> remove_partial(), so we can remove the check.
> 
> Meanwhile, redo filling cpu partial and add comment to improve the
> readability.
> 
> Signed-off-by: Xiongwei Song <xiongwei.song@...driver.com>
> ---
> Changes in v2:
>  - Use "#if IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL)" to instead 
>    "if (IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL))" to fix build error.
>    (Thanks Chengming Zhou)
>  - Add __maybe_unused for partial_slabs to prevent compiler warning.
> 
> v1: 
>  https://lore.kernel.org/linux-kernel/20240311132720.37741-1-sxwjean@me.com/T/
> ---
>  mm/slub.c | 24 +++++++++++++-----------
>  1 file changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index a3ab096c38c0..ab526960ee5b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2588,7 +2588,7 @@ static struct slab *get_partial_node(struct kmem_cache *s,
>  {
>  	struct slab *slab, *slab2, *partial = NULL;
>  	unsigned long flags;
> -	unsigned int partial_slabs = 0;
> +	unsigned int __maybe_unused partial_slabs = 0;
>  
>  	/*
>  	 * Racy check. If we mistakenly see no partial slabs then we
> @@ -2620,19 +2620,21 @@ static struct slab *get_partial_node(struct kmem_cache *s,
>  		if (!partial) {
>  			partial = slab;
>  			stat(s, ALLOC_FROM_PARTIAL);
> -		} else {
> -			put_cpu_partial(s, slab, 0);
> -			stat(s, CPU_PARTIAL_NODE);
> -			partial_slabs++;
> +
> +			/* Fill cpu partial if needed from next iteration, or break */
> +			if (IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL))
> +				continue;
> +			else
> +				break;
>  		}
> -#ifdef CONFIG_SLUB_CPU_PARTIAL
> -		if (!kmem_cache_has_cpu_partial(s)
> -			|| partial_slabs > s->cpu_partial_slabs / 2)
> +
> +#if IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL)

Hmm, these two CONFIG_SLUB_CPU_PARTIAL look verbose to me :(

How about using just one, maybe like this?

diff --git a/mm/slub.c b/mm/slub.c
index 2ef88bbf56a3..a018c715b648 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2620,19 +2620,16 @@ static struct slab *get_partial_node(struct kmem_cache *s,
 		if (!partial) {
 			partial = slab;
 			stat(s, ALLOC_FROM_PARTIAL);
+#ifdef CONFIG_SLUB_CPU_PARTIAL
 		} else {
 			put_cpu_partial(s, slab, 0);
 			stat(s, CPU_PARTIAL_NODE);
-			partial_slabs++;
-		}
-#ifdef CONFIG_SLUB_CPU_PARTIAL
-		if (!kmem_cache_has_cpu_partial(s)
-			|| partial_slabs > s->cpu_partial_slabs / 2)
-			break;
+			if (++partial_slabs > s->cpu_partial_slabs / 2)
+				break;
 #else
-		break;
+			break;
 #endif
-
+		}
 	}
 	spin_unlock_irqrestore(&n->list_lock, flags);
 	return partial;


> +		put_cpu_partial(s, slab, 0);
> +		stat(s, CPU_PARTIAL_NODE);
> +
> +		if (++partial_slabs > s->cpu_partial_slabs/2)
>  			break;
> -#else
> -		break;
>  #endif
> -
>  	}
>  	spin_unlock_irqrestore(&n->list_lock, flags);
>  	return partial;