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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Baolin Wang <baolin.wang@linux.alibaba.com> writes:

> On 2025/6/9 15:35, Michal Hocko wrote:
>> On Mon 09-06-25 10:57:41, Ritesh Harjani wrote:
>>> Baolin Wang <baolin.wang@linux.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@linux.ibm.com>
>>>> Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
>>>> Tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
>>>> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
>>>> Acked-by: SeongJae Park <sj@kernel.org>
>>>> Acked-by: Michal Hocko <mhocko@suse.com>
>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.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.
>
> 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?

-ritesh