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

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

On 2025/5/23 22:11, Shakeel Butt wrote:
> CC Mathieu
> 
> On Fri, May 23, 2025 at 11:16:13AM +0800, Baolin Wang wrote:
>> On some large machines with a high number of CPUs running a 64K 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.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> Hi Baolin, this seems reasonale. For long term Mathieu is planning to
> fix this with newer hierarchical percpu counter until then this looks
> good.

OK. Good.

> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

Thanks.