linux-kernel - Re: [PATCH] slub: avoid scanning all partial slabs in get

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Date: Fri, 23 Feb 2024 10:51:15 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Chengming Zhou <chengming.zhou@...ux.dev>,
 "Christoph Lameter (Ampere)" <cl@...ux.com>
Cc: David Rientjes <rientjes@...gle.com>,
 Jianfeng Wang <jianfeng.w.wang@...cle.com>, penberg@...nel.org,
 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: [PATCH] slub: avoid scanning all partial slabs in get_slabinfo()

On 2/23/24 10:37, Chengming Zhou wrote:
> On 2024/2/23 17:24, Vlastimil Babka wrote:
>> 
>>>>
>>>
>>> I think this is a better direction! We can use RCU list if slab can be freed by RCU.
>> 
>> Often we remove slab from the partial list for other purposes than freeing -
>> i.e. to become a cpu (partial) slab, and that can't be handled by a rcu
>> callback nor can we wait a grace period in such situations.
> 
> IMHO, only free_slab() need to use call_rcu() to delay free the slab,
> other paths like taking partial slabs from node partial list don't need
> to wait for RCU grace period.
> 
> All we want is safely lockless iterate over the node partial list, right?

Yes, and for that there's the "list_head slab_list", which is in union with
"struct slab *next" and "int slabs" for the cpu partial list. So if we
remove a slab from the partial list and rewrite the list_head for the
partial list purposes, it will break the lockless iterators, right? We would
have to wait a grace period between unlinking the slab from partial list (so
no new iterators can reach it), and reusing the list_head (so we are sure
the existing iterators stopped looking at our slab).

Maybe there's more advanced rcu tricks but this is my basic understanding
how this works.

> Thanks.