linux-kernel - Re: [PATCH 3/4] mm/slub: simplify get_partial

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Date: Tue, 2 Apr 2024 11:41:58 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: xiongwei.song@...driver.com, rientjes@...gle.com, cl@...ux.com,
 penberg@...nel.org, iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
 roman.gushchin@...ux.dev, 42.hyeyoo@...il.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, chengming.zhou@...ux.dev
Subject: Re: [PATCH 3/4] mm/slub: simplify get_partial_node()

On 3/31/24 4:19 AM, xiongwei.song@...driver.com wrote:
> From: Xiongwei Song <xiongwei.song@...driver.com>
> 
> The break conditions can be more readable and simple.
> 
> We can check if we need to fill cpu partial after getting the first
> partial slab. If kmem_cache_has_cpu_partial() returns true, we fill
> cpu partial from next iteration, or break up the loop.
> 
> Then we can remove the preprocessor condition of
> CONFIG_SLUB_CPU_PARTIAL. Use dummy slub_get_cpu_partial() to make
> compiler silent.
> 
> Signed-off-by: Xiongwei Song <xiongwei.song@...driver.com>
> ---
>  mm/slub.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 590cc953895d..ec91c7435d4e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2614,18 +2614,20 @@ 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 (kmem_cache_has_cpu_partial(s))

That kinda puts back the check removed in patch 1, although only in the
first iteration. Still not ideal.

> +				continue;
> +			else
> +				break;
>  		}
> -#ifdef CONFIG_SLUB_CPU_PARTIAL
> -		if (partial_slabs > s->cpu_partial_slabs / 2)
> -			break;
> -#else
> -		break;
> -#endif

I'd suggest intead of the changes done in this patch, only change this part
above to:

	if ((slub_get_cpu_partial(s) == 0) ||
	    (partial_slabs > slub_get_cpu_partial(s) / 2))
		break;

That gets rid of the #ifdef and also fixes a weird corner case that if we
set cpu_partial_slabs to 0 from sysfs, we still allocate at least one here.

It could be tempting to use >= instead of > to achieve the same effect but
that would have unintended performance effects that would best be evaluated
separately.

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