Re: [RFC -next] memcg: Optimize creation performance when LRU_GEN is enabled

[Chen Ridong <chenridong@huaweicloud.com>]

On 2025/11/27 17:04, Chen Ridong wrote:
> 
> 
> On 2025/11/27 1:15, Johannes Weiner wrote:
>> On Wed, Nov 19, 2025 at 08:37:22AM +0000, Chen Ridong wrote:
>>> From: Chen Ridong <chenridong@huawei.com>
>>>
>>> With LRU_GEN=y and LRU_GEN_ENABLED=n, a performance regression occurs
>>> when creating a large number of memory cgroups (memcgs):
>>>
>>> 	# time mkdir testcg_{1..10000}
>>>
>>> 	real	0m7.167s
>>> 	user	0m0.037s
>>> 	sys	0m6.773s
>>>
>>> 	# time mkdir testcg_{1..20000}
>>>
>>> 	real	0m27.158s
>>> 	user	0m0.079s
>>> 	sys	0m26.270s
>>>
>>> In contrast, with LRU_GEN=n, creation of the same number of memcgs
>>> performs better:
>>>
>>> 	# time mkdir testcg_{1..10000}
>>>
>>> 	real	0m3.386s
>>> 	user	0m0.044s
>>> 	sys	0m3.009s
>>>
>>> 	# time mkdir testcg_{1..20000}
>>>
>>> 	real	0m6.876s
>>> 	user	0m0.075s
>>> 	sys	0m6.121s
>>>
>>> The root cause is that lru_gen node onlining uses hlist_nulls_add_tail_rcu,
>>> which traverses the entire list to find the tail. This traversal scales
>>> with the number of memcgs, even when LRU_GEN is runtime-disabled.
>>
>> Can you please look into removing the memcg LRU instead?
>>
> 
> Thanks Johannes, this is indeed a promising approach.
> 
> The memcg LRU was originally designed exclusively for global reclaim scenarios. Before we move
> forward with its removal, I'd like to hear Yu's thoughts on this.
> 
> Hello Yu,
> 
> Do you have any opinions on removing the memcg LRU?
> 

Hello Johannes and Yu,

I've sent patches to remove the memcg LRU and replace it with mem_cgroup_iter.
I would appreciate it if you could take a look when you have time.

-- 
Best regards,
Ridong