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@xxxxxxxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxx>
Tested-by: Aboorva Devarajan <aboorvad@xxxxxxxxxxxxx>
