linux-kernel - Re: [RFC PATCH v3 1/7] slub: Keep track of whether slub is on the per-node partial list

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Date:   Fri, 27 Oct 2023 17:09:41 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     chengming.zhou@...ux.dev, cl@...ux.com, penberg@...nel.org
Cc:     rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
        42.hyeyoo@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [RFC PATCH v3 1/7] slub: Keep track of whether slub is on the
 per-node partial list

On 10/24/23 11:33, chengming.zhou@...ux.dev wrote:
> From: Chengming Zhou <zhouchengming@...edance.com>
> 
> Now we rely on the "frozen" bit to see if we should manipulate the
> slab->slab_list, which will be changed in the following patch.
> 
> Instead we introduce another way to keep track of whether slub is on
> the per-node partial list, here we reuse the PG_workingset bit.
> 
> We use __set_bit and __clear_bit directly instead of the atomic version
> for better performance and it's safe since it's protected by the slub
> node list_lock.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  mm/slab.h | 19 +++++++++++++++++++
>  mm/slub.c |  3 +++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 8cd3294fedf5..50522b688cfb 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h

I think these helper might just go to mm/slub.c as nothing else would use them.

> @@ -193,6 +193,25 @@ static inline void __slab_clear_pfmemalloc(struct slab *slab)
>  	__folio_clear_active(slab_folio(slab));
>  }
>  
> +/*
> + * Slub reuse PG_workingset bit to keep track of whether it's on

"SLUB reuses" ...

Looks fine otherwise!

> + * the per-node partial list.
> + */
> +static inline bool slab_test_node_partial(const struct slab *slab)
> +{
> +	return folio_test_workingset((struct folio *)slab_folio(slab));
> +}
> +
> +static inline void slab_set_node_partial(struct slab *slab)
> +{
> +	__set_bit(PG_workingset, folio_flags(slab_folio(slab), 0));
> +}
> +
> +static inline void slab_clear_node_partial(struct slab *slab)
> +{
> +	__clear_bit(PG_workingset, folio_flags(slab_folio(slab), 0));
> +}
> +
>  static inline void *slab_address(const struct slab *slab)
>  {
>  	return folio_address(slab_folio(slab));
> diff --git a/mm/slub.c b/mm/slub.c
> index 63d281dfacdb..3fad4edca34b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2127,6 +2127,7 @@ __add_partial(struct kmem_cache_node *n, struct slab *slab, int tail)
>  		list_add_tail(&slab->slab_list, &n->partial);
>  	else
>  		list_add(&slab->slab_list, &n->partial);
> +	slab_set_node_partial(slab);
>  }
>  
>  static inline void add_partial(struct kmem_cache_node *n,
> @@ -2141,6 +2142,7 @@ static inline void remove_partial(struct kmem_cache_node *n,
>  {
>  	lockdep_assert_held(&n->list_lock);
>  	list_del(&slab->slab_list);
> +	slab_clear_node_partial(slab);
>  	n->nr_partial--;
>  }
>  
> @@ -4831,6 +4833,7 @@ static int __kmem_cache_do_shrink(struct kmem_cache *s)
>  
>  			if (free == slab->objects) {
>  				list_move(&slab->slab_list, &discard);
> +				slab_clear_node_partial(slab);
>  				n->nr_partial--;
>  				dec_slabs_node(s, node, slab->objects);
>  			} else if (free <= SHRINK_PROMOTE_MAX)