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

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

On 2025/5/23 13:25, Donet Tom wrote:
> 
> On 5/23/25 8:46 AM, 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>
>> ---
>>   fs/proc/task_mmu.c | 14 +++++++-------
>>   include/linux/mm.h |  5 +++++
>>   2 files changed, 12 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index b9e4fbbdf6e6..f629e6526935 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -36,9 +36,9 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
>>       unsigned long text, lib, swap, anon, file, shmem;
>>       unsigned long hiwater_vm, total_vm, hiwater_rss, total_rss;
>> -    anon = get_mm_counter(mm, MM_ANONPAGES);
>> -    file = get_mm_counter(mm, MM_FILEPAGES);
>> -    shmem = get_mm_counter(mm, MM_SHMEMPAGES);
>> +    anon = get_mm_counter_sum(mm, MM_ANONPAGES);
> 
> 
> Hi Baolin Wang,
> 
> We also observed the same issue where the RSS value in /proc/PID/status
> was 0 on machines with a high number of CPUs. With this patch, the issue
> got fixedl

Yes, we also observed this issue.

> Rss value without this patch
> ----------------------------
>   # cat /proc/87406/status
> .....
> VmRSS:           0 kB
> RssAnon:           0 kB
> RssFile:           0 k
> 
> 
> Rss values with this patch
> --------------------------
>   # cat /proc/3055/status
> VmRSS:        2176 kB
> RssAnon:         512 kB
> RssFile:        1664 kB
> RssShmem:           0 kB
> 
> Tested-by Donet Tom <donettom@linux.ibm.com>

Thanks for testing.