linux-kernel - Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup

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Message-ID: <14b2584b-3ac7-4358-9d81-40ca32683e4f@huaweicloud.com>
Date: Mon, 8 Dec 2025 10:35:48 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: zhongjinji <zhongjinji@...or.com>, hannes@...xchg.org
Cc: Liam.Howlett@...cle.com, akpm@...ux-foundation.org,
 axelrasmussen@...gle.com, cgroups@...r.kernel.org, chenridong@...wei.com,
 corbet@....net, david@...nel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 lorenzo.stoakes@...cle.com, lujialin4@...wei.com, mhocko@...e.com,
 muchun.song@...ux.dev, roman.gushchin@...ux.dev, rppt@...nel.org,
 shakeel.butt@...ux.dev, surenb@...gle.com, vbabka@...e.cz,
 weixugc@...gle.com, yuanchu@...gle.com, yuzhao@...gle.com,
 zhengqi.arch@...edance.com
Subject: Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global
 reclaim



On 2025/12/5 10:57, zhongjinji wrote:
>> From: Chen Ridong <chenridong@...wei.com>
>>
>> The memcg LRU was originally introduced for global reclaim to enhance
>> scalability. However, its implementation complexity has led to performance
>> regressions when dealing with a large number of memory cgroups [1].
>>
>> As suggested by Johannes [1], this patch adopts mem_cgroup_iter with
>> cookie-based iteration for global reclaim, aligning with the approach
>> already used in shrink_node_memcgs. This simplification removes the
>> dedicated memcg LRU tracking while maintaining the core functionality.
>>
>> It performed a stress test based on Zhao Yu's methodology [2] on a
>> 1 TB, 4-node NUMA system. The results are summarized below:
>>
>> 					memcg LRU    memcg iter
>> stddev(pgsteal) / mean(pgsteal)            91.2%         75.7%
>> sum(pgsteal) / sum(requested)             216.4%        230.5%
> 
> Are there more data available? For example, the load of kswapd or the refault values.
> 
> I am concerned about these two data points because Yu Zhao's implementation controls
> the fairness of aging through memcg gen (get_memcg_gen). This helps reduce excessive
> aging for certain cgroups, which is beneficial for kswapd's power consumption.
> 
> At the same time, pages that age earlier can be considered colder pages (in the entire system),
> so reclaiming them should also help with the refault values.
> 

I re-ran the test and observed a 3.2% increase in refaults. Is this enough for what you were
concerned about?

The complete data set is offered in my earlier email:

https://lore.kernel.org/all/e657d5ac-6f92-4dbb-bf32-76084988d024@huaweicloud.com/

>> The new implementation demonstrates a significant improvement in
>> fairness, reducing the standard deviation relative to the mean by
>> 15.5 percentage points. While the reclaim accuracy shows a slight
>> increase in overscan (from 85086871 to 90633890, 6.5%).
>>
>> The primary benefits of this change are:
>> 1. Simplified codebase by removing custom memcg LRU infrastructure
>> 2. Improved fairness in memory reclaim across multiple cgroups
>> 3. Better performance when creating many memory cgroups
>>
>> [1] https://lore.kernel.org/r/20251126171513.GC135004@cmpxchg.org
>> [2] https://lore.kernel.org/r/20221222041905.2431096-7-yuzhao@google.com
>> Signed-off-by: Chen Ridong <chenridong@...wei.com>

-- 
Best regards,
Ridong