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Message-ID: <6d6dcad5-169f-4bfc-91be-c620fef811e4@linux.alibaba.com>
Date: Sat, 24 May 2025 09:24:39 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Aboorva Devarajan <aboorvad@...ux.ibm.com>, akpm@...ux-foundation.org,
david@...hat.com, shakeel.butt@...ux.dev
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 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@...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);
>> + 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@...ux.ibm.com>
> Tested-by: Aboorva Devarajan <aboorvad@...ux.ibm.com>
>
Thanks for reviewing and testing.
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