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

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Message-ID: <aDm1GCV8yToFG1cq@tiehlicka>
Date: Fri, 30 May 2025 15:39:36 +0200
From: Michal Hocko <mhocko@...e.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, david@...hat.com,
	shakeel.butt@...ux.dev, lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
	surenb@...gle.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 Thu 29-05-25 20:53:13, Andrew Morton wrote:
> 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.

I have only noticed this new posting now. I do not think this is a
stable material. I am also not convinced that the impact of the pcp lock
exposure to the userspace has been properly analyzed and documented in
the changelog. I am not nacking the patch (yet) but I would like to see
a serious analyses that this has been properly thought through.

-- 
Michal Hocko
SUSE Labs