Re: [PATCH v2] slub: keep empty main sheaf as spare in __pcs_replace_empty_main()

[Harry Yoo <harry.yoo@oracle.com>]

On Mon, Dec 15, 2025 at 03:30:48PM +0100, Vlastimil Babka wrote:
> On 12/10/25 01:26, Hao Li wrote:
> > From: Hao Li <haolee.swjtu@gmail.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@suse.cz>
> > Signed-off-by: Hao Li <haolee.swjtu@gmail.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);
> +               }

nit: no braces for single statement?

>                 pcs->main = full;
>                 return pcs;
>         }

Otherwise LGTM, so:
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

-- 
Cheers,
Harry / Hyeonggon