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

[Baolin Wang]

On 2025/6/9 15:35, Michal Hocko wrote:
> On Mon 09-06-25 10:57:41, Ritesh Harjani wrote:
> > Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > Reviewed-by: Aboorva Devarajan <aboorvad@xxxxxxxxxxxxx>
> > > Tested-by: Aboorva Devarajan <aboorvad@xxxxxxxxxxxxx>
> > > Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > > Acked-by: SeongJae Park <sj@xxxxxxxxxx>
> > > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > ---
> > > 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.

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.