linux-kernel - Re: [PATCH v2] slub: keep empty main sheaf as spare in __pcs_replace_empty

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Message-ID: <a231264a-2da5-4468-a276-777fc0241246@suse.cz>
Date: Mon, 15 Dec 2025 15:30:48 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Hao Li <haolee.swjtu@...il.com>, akpm@...ux-foundation.org,
 harry.yoo@...cle.com, cl@...two.org, rientjes@...gle.com,
 roman.gushchin@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] slub: keep empty main sheaf as spare in
 __pcs_replace_empty_main()

On 12/10/25 01:26, Hao Li wrote:
> From: Hao Li <haolee.swjtu@...il.com>
> 
> When __pcs_replace_empty_main() fails to obtain a full sheaf directly
> from the barn, it may either:
> 
>   - Refill an empty sheaf obtained via barn_get_empty_sheaf(), or
>   - Allocate a brand new full sheaf via alloc_full_sheaf().
> 
> After reacquiring the per-CPU lock, if pcs->main is still empty and
> pcs->spare is NULL, the current code donates the empty main sheaf to
> the barn via barn_put_empty_sheaf() and installs the full sheaf as
> pcs->main, leaving pcs->spare unpopulated.
> 
> Instead, keep the existing empty main sheaf locally as the spare:
> 
>   pcs->spare = pcs->main;
>   pcs->main = full;
> 
> This populates pcs->spare earlier, which can reduce future barn traffic.
> 
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Hao Li <haolee.swjtu@...il.com>
> ---
> 
> The Gmail account(haoli.tcs) I used to send v1 of the patch has been
> restricted from sending emails for unknown reasons, so I'm sending v2
> from this address instead. Thanks.
> 
>  mm/slub.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index a0b905c2a557..a3e73ebb0cc8 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5077,6 +5077,11 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
>  	 */
>  
>  	if (pcs->main->size == 0) {
> +		if (!pcs->spare) {
> +			pcs->spare = pcs->main;
> +			pcs->main = full;
> +			return pcs;
> +		}
>  		barn_put_empty_sheaf(barn, pcs->main);
>  		pcs->main = full;
>  		return pcs;

Thanks, LGTM. We can make it smaller though. Adding to slab/for-next
adjusted like this:

diff --git a/mm/slub.c b/mm/slub.c
index f21b2f0c6f5a..ad71f01571f0 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5052,7 +5052,11 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
         */
 
        if (pcs->main->size == 0) {
-               barn_put_empty_sheaf(barn, pcs->main);
+               if (!pcs->spare) {
+                       pcs->spare = pcs->main;
+               } else {
+                       barn_put_empty_sheaf(barn, pcs->main);
+               }
                pcs->main = full;
                return pcs;
        }