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

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Message-ID: <ac3b8c16-1f2b-4f2f-8bcd-ef8da8544a20@linux.ibm.com>
Date: Fri, 23 May 2025 10:55:24 +0530
From: Donet Tom <donettom@...ux.ibm.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
        david@...hat.com, shakeelb@...gle.com
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
        rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: fix the inaccurate memory statistics issue for
 users


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@...ux.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

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@...ux.ibm.com>


> +	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)