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

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Message-ID: <06d9981e-4a4a-4b99-9418-9dec0a3420e8@suse.cz>
Date: Mon, 9 Jun 2025 10:56:46 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Michal Hocko <mhocko@...e.com>
Cc: akpm@...ux-foundation.org, 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 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.

> 
>> -ritesh
>