linux-kernel - Re: [PATCH mm-new] mm/memcontrol: Introduce sysctl vm.memcg_stats_flush

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Message-ID: <aQnFn6vPQ5D6STGw@tiehlicka>
Date: Tue, 4 Nov 2025 10:21:35 +0100
From: Michal Hocko <mhocko@...e.com>
To: Leon Huang Fu <leon.huangfu@...pee.com>
Cc: linux-mm@...ck.org, hannes@...xchg.org, roman.gushchin@...ux.dev,
	shakeel.butt@...ux.dev, muchun.song@...ux.dev,
	akpm@...ux-foundation.org, joel.granados@...nel.org, jack@...e.cz,
	laoar.shao@...il.com, mclapinski@...gle.com, kyle.meyer@....com,
	corbet@....net, lance.yang@...ux.dev, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH mm-new] mm/memcontrol: Introduce sysctl
 vm.memcg_stats_flush_threshold

On Tue 04-11-25 11:19:08, Leon Huang Fu wrote:
> The current implementation uses a flush threshold calculated as
> MEMCG_CHARGE_BATCH * num_online_cpus() for determining when to
> aggregate per-CPU memory cgroup statistics. On systems with high core
> counts, this threshold can become very large (e.g., 64 * 256 = 16,384
> on a 256-core system), leading to stale statistics when userspace reads
> memory.stat files.
> 
> This is particularly problematic for monitoring and management tools
> that rely on reasonably fresh statistics, as they may observe data that
> is thousands of updates out of date.
> 
> Introduce a new sysctl, vm.memcg_stats_flush_threshold, that allows
> administrators to override the flush threshold specifically for
> userspace reads of memory.stat. When set to 0 (default), the behavior
> remains unchanged, using the automatic calculation. When set to a
> non-zero value, userspace reads will use the custom threshold for more
> frequent flushing.

How are admins supposed to know how to tune this? Wouldn't it make more
sense to allow explicit flushing on write to the file? That would allow
admins to implement their preferred accuracy tuning by writing to the file
when the precision is required.

-- 
Michal Hocko
SUSE Labs