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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/5/30 21:39, Michal Hocko wrote:
> On Thu 29-05-25 20:53:13, Andrew Morton wrote:
>> On Sat, 24 May 2025 09:59:53 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
>>
>>> 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.
>>>
>>> Fixes: f1a7941243c1 ("mm: convert mm's rss stats into percpu_counter")
>>
>> Three years ago.
>>
>>> 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>
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>
>> Thanks, I added cc:stable to this.
> 
> I have only noticed this new posting now. I do not think this is a
> stable material. I am also not convinced that the impact of the pcp lock
> exposure to the userspace has been properly analyzed and documented in
> the changelog. I am not nacking the patch (yet) but I would like to see
> a serious analyses that this has been properly thought through.

Good point. I did a quick measurement on my 32 cores Arm machine. I ran 
two workloads, one is the 'top' command: top -d 1 (updating every 
second). Another workload is kernel building (time make -j32).

 From the following data, I did not see any significant impact of the 
patch changes on the execution of the kernel building workload.

w/o patch:
real	4m33.887s
user	118m24.153s
sys	9m51.402s

w/ patch:
real	4m34.495s
user	118m21.739s
sys	9m39.232s