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

[Chen Ridong <chenridong@huaweicloud.com>]

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?

> Use mem_cgroup_iter() with a reclaim cookie in shrink_many(), like we
> do in shrink_node_memcgs().
> 
> The memcg LRU is complicated, and it only works for global reclaim; if
> you have a subtree with a memory.max at the top, it'll go through
> shrink_node_memcgs() already anyway.

-- 
Best regards,
Ridong