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

[Vlastimil Babka (SUSE)]

On 3/6/26 05:55, Harry Yoo wrote:
> 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".

Indeed, thanks! Ming, could you retry with that fixed up please?

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