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

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Message-ID: <20251204182850.GA481418@cmpxchg.org>
Date: Thu, 4 Dec 2025 13:28:50 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Chen Ridong <chenridong@...weicloud.com>
Cc: akpm@...ux-foundation.org, axelrasmussen@...gle.com, yuanchu@...gle.com,
	weixugc@...gle.com, david@...nel.org, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
	surenb@...gle.com, mhocko@...e.com, corbet@....net,
	roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
	muchun.song@...ux.dev, yuzhao@...gle.com,
	zhengqi.arch@...edance.com, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, lujialin4@...wei.com,
	chenridong@...wei.com
Subject: Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global
 reclaim

On Thu, Dec 04, 2025 at 12:31:23PM +0000, Chen Ridong 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%
> 
> 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>

Acked-by: Johannes Weiner <hannes@...pchg.org>

The diff and the test results look good to me. Comparing the resulting
shrink_many() with shrink_node_memcgs(), this also looks like a great
step towards maintainability and unification.

Thanks!