linux-kernel - Re: [PATCH v2] mm: fix the inaccurate memory statistics issue for users

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Message-Id: <20250609171758.afc946b81451e1ad5a8ce027@linux-foundation.org>
Date: Mon, 9 Jun 2025 17:17:58 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>, Baolin Wang
 <baolin.wang@...ux.alibaba.com>, Michal Hocko <mhocko@...e.com>,
 david@...hat.com, shakeel.butt@...ux.dev, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com, 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 v2] mm: fix the inaccurate memory statistics issue for
 users

On Mon, 9 Jun 2025 10:56:46 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:

> On 6/9/25 10:52 AM, Vlastimil Babka wrote:
> > On 6/9/25 10:31 AM, Ritesh Harjani (IBM) wrote:
> >> Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
> >>
> >>> On 2025/6/9 15:35, Michal Hocko wrote:
> >>>> On Mon 09-06-25 10:57:41, Ritesh Harjani wrote:
> >>>>>
> >>>>> Any reason why we dropped the Fixes tag? I see there were a series of
> >>>>> discussion on v1 and it got concluded that the fix was correct, then why
> >>>>> drop the fixes tag?
> >>>>
> >>>> This seems more like an improvement than a bug fix.
> >>>
> >>> Yes. I don't have a strong opinion on this, but we (Alibaba) will 
> >>> backport it manually,
> >>>
> >>> because some of user-space monitoring tools depend 
> >>> on these statistics.
> >>
> >> That sounds like a regression then, isn't it?
> > 
> > Hm if counters were accurate before f1a7941243c1 and not afterwards, and
> > this is making them accurate again, and some userspace depends on it,
> > then Fixes: and stable is probably warranted then. If this was just a
> > perf improvement, then not. But AFAIU f1a7941243c1 was the perf
> > improvement...
> 
> Dang, should have re-read the commit log of f1a7941243c1 first. It seems
> like the error margin due to batching existed also before f1a7941243c1.
> 
> " This patch converts the rss_stats into percpu_counter to convert the
> error  margin from (nr_threads * 64) to approximately (nr_cpus ^ 2)."
> 
> so if on some systems this means worse margin than before, the above
> "if" chain of thought might still hold.

f1a7941243c1 seems like a good enough place to tell -stable
maintainers where to insert the patch (why does this sound rude).

The patch is simple enough.  I'll add fixes:f1a7941243c1 and cc:stable
and, as the problem has been there for years, I'll leave the patch in
mm-unstable so it will eventually get into LTS, in a well tested state.