linux-kernel - Re: [RFC PATCH] mm: fix the inaccurate memory statistics issue for users

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Message-ID: <67n3snrowiyxjw6grddyer7np5rpnpg4x5f6bsyonmgcc5k5eq@s5v4ux27i4fw>
Date: Fri, 23 May 2025 07:11:37 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, Liam.Howlett@...cle.com, 
	vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, 
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: fix the inaccurate memory statistics issue for
 users

CC Mathieu

On Fri, May 23, 2025 at 11:16:13AM +0800, Baolin Wang wrote:
> On some large machines with a high number of CPUs running a 64K kernel,
> we found that the 'RES' field is always 0 displayed by the top command
> for some processes, which will cause a lot of confusion for users.
> 
>     PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
>  875525 root      20   0   12480      0      0 R   0.3   0.0   0:00.08 top
>       1 root      20   0  172800      0      0 S   0.0   0.0   0:04.52 systemd
> 
> The main reason is that the batch size of the percpu counter is quite large
> on these machines, caching a significant percpu value, since converting mm's
> rss stats into percpu_counter by commit f1a7941243c1 ("mm: convert mm's rss
> stats into percpu_counter"). Intuitively, the batch number should be optimized,
> but on some paths, performance may take precedence over statistical accuracy.
> Therefore, introducing a new interface to add the percpu statistical count
> and display it to users, which can remove the confusion. In addition, this
> change is not expected to be on a performance-critical path, so the modification
> should be acceptable.
> 
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>

Hi Baolin, this seems reasonale. For long term Mathieu is planning to
fix this with newer hierarchical percpu counter until then this looks
good.

Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>