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

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Message-ID: <aEaOzpQElnG2I3Tz@tiehlicka>
Date: Mon, 9 Jun 2025 09:35:42 +0200
From: Michal Hocko <mhocko@...e.com>
To: Ritesh Harjani <ritesh.list@...il.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
	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 v2] mm: fix the inaccurate memory statistics issue for
 users

On Mon 09-06-25 10:57:41, Ritesh Harjani wrote:
> Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
> 
> > 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.
> >
> > In addition, the 'mm->rss_stat' is updated by using add_mm_counter() and
> > dec/inc_mm_counter(), which are all wrappers around percpu_counter_add_batch().
> > In percpu_counter_add_batch(), there is percpu batch caching to avoid 'fbc->lock'
> > contention. This patch changes task_mem() and task_statm() to get the accurate
> > mm counters under the 'fbc->lock', but this should not exacerbate kernel
> > 'mm->rss_stat' lock contention due to the percpu batch caching of the mm
> > counters. The following test also confirm the theoretical analysis.
> >
> > I run the stress-ng that stresses anon page faults in 32 threads on my 32 cores
> > machine, while simultaneously running a script that starts 32 threads to
> > busy-loop pread each stress-ng thread's /proc/pid/status interface. From the
> > following data, I did not observe any obvious impact of this patch on the
> > stress-ng tests.
> >
> > w/o patch:
> > stress-ng: info:  [6848]          4,399,219,085,152 CPU Cycles          67.327 B/sec
> > stress-ng: info:  [6848]          1,616,524,844,832 Instructions          24.740 B/sec (0.367 instr. per cycle)
> > stress-ng: info:  [6848]          39,529,792 Page Faults Total           0.605 M/sec
> > stress-ng: info:  [6848]          39,529,792 Page Faults Minor           0.605 M/sec
> >
> > w/patch:
> > stress-ng: info:  [2485]          4,462,440,381,856 CPU Cycles          68.382 B/sec
> > stress-ng: info:  [2485]          1,615,101,503,296 Instructions          24.750 B/sec (0.362 instr. per cycle)
> > stress-ng: info:  [2485]          39,439,232 Page Faults Total           0.604 M/sec
> > stress-ng: info:  [2485]          39,439,232 Page Faults Minor           0.604 M/sec
> >
> > 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>
> > Acked-by: Michal Hocko <mhocko@...e.com>
> > Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> > ---
> > Changes from v1:
> >  - Update the commit message to add some measurements.
> >  - Add acked tag from Michal. Thanks.
> >  - Drop the Fixes tag.
> 
> 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.
-- 
Michal Hocko
SUSE Labs