Re: [Regression] mm:slab/sheaves: severe performance regression in cross-CPU slab allocation

[Harry Yoo]

On Thu, Feb 26, 2026 at 07:02:11PM +0100, Vlastimil Babka (SUSE) wrote:
> On 2/25/26 10:31, Ming Lei wrote:
> > Hi Vlastimil,
> > 
> > On Wed, Feb 25, 2026 at 09:45:03AM +0100, Vlastimil Babka (SUSE) wrote:
> >> On 2/24/26 21:27, Vlastimil Babka wrote:
> >> > 
> >> > It made sense to me not to refill sheaves when we can't reclaim, but I
> >> > didn't anticipate this interaction with mempools. We could change them
> >> > but there might be others using a similar pattern. Maybe it would be for
> >> > the best to just drop that heuristic from __pcs_replace_empty_main()
> >> > (but carefully as some deadlock avoidance depends on it, we might need
> >> > to e.g. replace it with gfpflags_allow_spinning()). I'll send a patch
> >> > tomorrow to test this theory, unless someone beats me to it (feel free to).
> >> Could you try this then, please? Thanks!
> > 
> > Thanks for working on this issue!
> > 
> > Unfortunately the patch doesn't make a difference on IOPS in the perf test,
> > follows the collected perf profile on linus tree(basically 7.0-rc1 with your patch):
> 
> what about this patch in addition to the previous one? Thanks.
> 
> ----8<----
> From d3e8118c078996d1372a9f89285179d93971fdb2 Mon Sep 17 00:00:00 2001
> From: "Vlastimil Babka (SUSE)" <vbabka@xxxxxxxxxx>
> Date: Thu, 26 Feb 2026 18:59:56 +0100
> Subject: [PATCH] mm/slab: put barn on every online node
> 
> Including memoryless nodes.
> 
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
> ---

Just taking a quick grasp...

> @@ -6121,7 +6122,8 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>  	if (unlikely(!slab_free_hook(s, object, slab_want_init_on_free(s), false)))
>  		return;
>  
> -	if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())
> +	if (likely(!IS_ENABLED(CONFIG_NUMA) || (slab_nid(slab) == numa_mem_id())
> +			|| !node_isset(slab_nid(slab), slab_nodes))

I think you intended !node_isset(numa_mem_id(), slab_nodes)?

"Skip freeing to pcs if it's remote free, but memoryless nodes is
 an exception".

>  	    && likely(!slab_test_pfmemalloc(slab))) {
>  		if (likely(free_to_pcs(s, object, true)))
>  			return;

-- 
Cheers,
Harry / Hyeonggon