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

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

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.

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>