Re: [PATCH] mm/slub: batch-detach node partial slabs

[Harry Yoo <harry@kernel.org>]

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On 5/25/26 12:22 PM, Hao Li wrote:
> get_partial_node_bulk() used to move each selected slab from the node
> partial list to the local pc->slabs list using a remove_partial() and
> list_add() pair. In practice, the loop often detaches several adjacent
> slabs, so this repeatedly manipulates list pointers while holding
> n->list_lock, which causes unnecessary churn.
> 
> Instead, track contiguous runs of matching slabs and move each run with
> list_bulk_move_tail() in one operation.

TIL list_bulk_move_tail() :D

 > This reduces list pointer churn> inside the lock critical section.

Similar to this, can we return all slabs in pc->slabs at once when 
returning those slabs to the list? ... I see Vlastimil removed 'nr of 
empty slabs' check in the other series already.

Now that it inserts slabs to the tail with Vlastimil's patchset, let's 
do a list_splice_tail() instead?

> The mmap2 testcase shows a 5% improvement after applying this patch.
> 
> Signed-off-by: Hao Li <hao.li@linux.dev>
> ---
>   mm/slub.c | 22 ++++++++++++++++++----
>   1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 04692a6f9128..180973a4a3d2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3775,15 +3783,21 @@ static bool get_partial_node_bulk(struct kmem_cache *s,
>   		    && total_free + slab_free > pc->max_objects)
>   			break;
>   
> -		remove_partial(n, slab);
> -
> -		list_add(&slab->slab_list, &pc->slabs);
> +		if (!first)
> +			first = slab;
> +		last = slab;

> +		slab_clear_node_partial(slab);
> +		n->nr_partial--;

Perhaps factor out those two statements into to a common function and 
call it in get_partial_node_bulk() and remove_partial()?
>   		total_free += slab_free;
>   		if (total_free >= pc->max_objects)
>   			break;
>   	}
>   
> +	if (first)
> +		list_bulk_move_tail(&pc->slabs, &first->slab_list,
> +				    &last->slab_list);
> +
>   	spin_unlock_irqrestore(&n->list_lock, flags);
>   	return total_free > 0;
>   }

-- 
Cheers,
Harry / Hyeonggon


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