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Date: Fri, 22 May 2026 19:34:40 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>,
 Roman Gushchin <roman.gushchin@linux.dev>, Muchun Song
 <muchun.song@linux.dev>, Qi Zheng <qi.zheng@linux.dev>, Alexandre Ghiti
 <alex@ghiti.fr>, Joshua Hahn <joshua.hahnjy@gmail.com>, Harry Yoo
 <harry@kernel.org>, Meta kernel team <kernel-team@meta.com>,
 linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
 kernel test robot <oliver.sang@intel.com>
Subject: Re: [PATCH v2 0/4] memcg: shrink obj_stock_pcp and cache multiple
 objcgs
Message-Id: <20260522193440.40e20563422afcc69b8445dd@linux-foundation.org>
In-Reply-To: <20260522011908.1669332-1-shakeel.butt@linux.dev>
References: <20260522011908.1669332-1-shakeel.butt@linux.dev>
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On Thu, 21 May 2026 18:19:04 -0700 Shakeel Butt <shakeel.butt@linux.dev> wrote:

> Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg
> per-node type") split a memcg's single obj_cgroup into one per NUMA
> node so that reparenting LRU folios can take per-node lru locks. As a
> side effect, the per-CPU obj_stock_pcp -- which caches a single
> cached_objcg pointer -- thrashes on workloads where threads of the
> same memcg run on different NUMA nodes. The kernel test robot reported
> a 67.7% regression on stress-ng.switch.ops_per_sec from this pattern.
> 
> Commit d0211878ce06 ("memcg: cache obj_stock by memcg, not by objcg
> pointer") landed as a temporary fix by treating sibling per-node
> objcgs as equivalent for the cache lookup, intended to be reverted
> once per-node kmem accounting is introduced. This series takes a more
> general approach: cache multiple objcgs per CPU using the multi-slot
> pattern memcg_stock_pcp already uses, so the per-node objcg variants
> of one memcg can all coexist in the stock without ever forcing a
> drain. The temporary fix can then be reverted.
> 
> To avoid increasing the per-CPU cache footprint, the first three
> patches shrink the existing single-slot obj_stock_pcp fields.
> The final patch converts cached_objcg and nr_bytes into
> NR_OBJ_STOCK=5 slot arrays and reorders the struct so the entire
> consume/refill/account hot path fits within a single 64-byte cache
> line on non-debug 64-bit builds (verified with pahole).

Thanks, I added this to mm.git's mm-new branch, along with a couple of
possible todo notes from the review.

Sashiko asked a thing:
	https://sashiko.dev/#/patchset/20260522011908.1669332-1-shakeel.butt@linux.dev

Did you already see this?  The footers there indicate that an email was
sent out but I don't know if it works?