linux-kernel - Re: [PATCH mm-new v2] mm/memcontrol: Flush stats when write stat file

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Message-ID: <37aa86c5-2659-4626-a80b-b3d07c2512c9@gmail.com>
Date: Thu, 6 Nov 2025 09:02:41 -0800
From: JP Kobryn <inwardvessel@...il.com>
To: Leon Huang Fu <leon.huangfu@...pee.com>, linux-mm@...ck.org
Cc: hannes@...xchg.org, mhocko@...nel.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 v2] mm/memcontrol: Flush stats when write stat file

On 11/4/25 11:49 PM, Leon Huang Fu wrote:
> On high-core count systems, memory cgroup statistics can become stale
> due to per-CPU caching and deferred aggregation. Monitoring tools and
> management applications sometimes need guaranteed up-to-date statistics
> at specific points in time to make accurate decisions.
> 
> This patch adds write handlers to both memory.stat and memory.numa_stat
> files to allow userspace to explicitly force an immediate flush of
> memory statistics. When "1" is written to either file, it triggers
> __mem_cgroup_flush_stats(memcg, true), which unconditionally flushes
> all pending statistics for the cgroup and its descendants.
> 
> The write operation validates the input and only accepts the value "1",
> returning -EINVAL for any other input.
> 
> Usage example:
>    # Force immediate flush before reading critical statistics
>    echo 1 > /sys/fs/cgroup/mygroup/memory.stat
>    cat /sys/fs/cgroup/mygroup/memory.stat
> 
> This provides several benefits:
> 
> 1. On-demand accuracy: Tools can flush only when needed, avoiding
>     continuous overhead
> 
> 2. Targeted flushing: Allows flushing specific cgroups when precision
>     is required for particular workloads

I'm curious about your use case. Since you mention required precision,
are you planning on manually flushing before every read?

> 
> 3. Integration flexibility: Monitoring scripts can decide when to pay
>     the flush cost based on their specific accuracy requirements

[...]
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c34029e92bab..d6a5d872fbcb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4531,6 +4531,17 @@ int memory_stat_show(struct seq_file *m, void *v)
>   	return 0;
>   }
> 
> +int memory_stat_write(struct cgroup_subsys_state *css, struct cftype *cft, u64 val)
> +{
> +	if (val != 1)
> +		return -EINVAL;
> +
> +	if (css)
> +		css_rstat_flush(css);

This is a kfunc. You can do this right now from a bpf program without
any kernel changes.