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

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

On 2025/5/23 18:14, Aboorva Devarajan wrote:
> On Fri, 2025-05-23 at 11:16 +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>
>> ---
>>   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);
>> +	file = get_mm_counter_sum(mm, MM_FILEPAGES);
>> +	shmem = get_mm_counter_sum(mm, MM_SHMEMPAGES);
>>   
>>   	/*
>>   	 * Note: to minimize their overhead, mm maintains hiwater_vm and
>> @@ -59,7 +59,7 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
>>   	text = min(text, mm->exec_vm << PAGE_SHIFT);
>>   	lib = (mm->exec_vm << PAGE_SHIFT) - text;
>>   
>> -	swap = get_mm_counter(mm, MM_SWAPENTS);
>> +	swap = get_mm_counter_sum(mm, MM_SWAPENTS);
>>   	SEQ_PUT_DEC("VmPeak:\t", hiwater_vm);
>>   	SEQ_PUT_DEC(" kB\nVmSize:\t", total_vm);
>>   	SEQ_PUT_DEC(" kB\nVmLck:\t", mm->locked_vm);
>> @@ -92,12 +92,12 @@ unsigned long task_statm(struct mm_struct *mm,
>>   			 unsigned long *shared, unsigned long *text,
>>   			 unsigned long *data, unsigned long *resident)
>>   {
>> -	*shared = get_mm_counter(mm, MM_FILEPAGES) +
>> -			get_mm_counter(mm, MM_SHMEMPAGES);
>> +	*shared = get_mm_counter_sum(mm, MM_FILEPAGES) +
>> +			get_mm_counter_sum(mm, MM_SHMEMPAGES);
>>   	*text = (PAGE_ALIGN(mm->end_code) - (mm->start_code & PAGE_MASK))
>>   								>> PAGE_SHIFT;
>>   	*data = mm->data_vm + mm->stack_vm;
>> -	*resident = *shared + get_mm_counter(mm, MM_ANONPAGES);
>> +	*resident = *shared + get_mm_counter_sum(mm, MM_ANONPAGES);
>>   	return mm->total_vm;
>>   }
>>   
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 185424858f23..15ec5cfe9515 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2568,6 +2568,11 @@ static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
>>   	return percpu_counter_read_positive(&mm->rss_stat[member]);
>>   }
>>   
>> +static inline unsigned long get_mm_counter_sum(struct mm_struct *mm, int member)
>> +{
>> +	return percpu_counter_sum_positive(&mm->rss_stat[member]);
>> +}
>> +
>>   void mm_trace_rss_stat(struct mm_struct *mm, int member);
>>   
>>   static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
> 
> Hi Baolin,
> 
> This patch looks good to me. We observed a similar issue where the
> generic mm selftest split_huge_page_test failed due to outdated RssAnon
> values reported in /proc/[pid]/status.
> 
> ...
> 
> Without Patch:
> 
> # ./split_huge_page_test
> TAP version 13
> 1..34
> Bail out! No RssAnon is allocated before split
> # Planned tests != run tests (34 != 0)
> # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
> 
> ...
> 
> With Patch:
> 
> # ./split_huge_page_test
> # ./split_huge_page_test
> TAP version 13
> 1..34
> ...
> # Totals: pass:11 fail:0 xfail:0 xpass:0 skip:23 error:0
> 
> ...
> 
> While this change may introduce some lock contention, it only affects
> the task_mem function which is invoked only when reading
> /proc/[pid]/status. Since this is not on a performance critical path,
> it will be good to have this change in order to get accurate memory
> stats.

Agree.

> 
> This fix resolves the issue we've seen with split_huge_page_test.
> 
> Thanks!
> 
> 
> Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> Tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> 

Thanks for reviewing and testing.