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

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Message-Id: <20250529205313.a1285b431bbec2c54d80266d@linux-foundation.org>
Date: Thu, 29 May 2025 20:53:13 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: david@...hat.com, shakeel.butt@...ux.dev, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
 surenb@...gle.com, mhocko@...e.com, donettom@...ux.ibm.com,
 aboorvad@...ux.ibm.com, sj@...nel.org, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix the inaccurate memory statistics issue for
 users

On Sat, 24 May 2025 09:59:53 +0800 Baolin Wang <baolin.wang@...ux.alibaba.com> wrote:

> On some large machines with a high number of CPUs running a 64K pagesize
> 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.
> 
> Fixes: f1a7941243c1 ("mm: convert mm's rss stats into percpu_counter")

Three years ago.

> Tested-by Donet Tom <donettom@...ux.ibm.com>
> Reviewed-by: Aboorva Devarajan <aboorvad@...ux.ibm.com>
> Tested-by: Aboorva Devarajan <aboorvad@...ux.ibm.com>
> Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
> Acked-by: SeongJae Park <sj@...nel.org>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>

Thanks, I added cc:stable to this.