Re: [RFC PATCH] mm: use precise current RSS for high-water accounting

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-07-23 09:41, Nirmoy Das wrote:
> get_mm_rss() reads approximate per-CPU RSS counters. On large systems,
> this can under-report the current RSS used for high-water accounting.

Did you compare this with my hierarchical percpu counters series ?

https://lore.kernel.org/lkml/20260707131544.75906-1-mathieu.desnoyers@efficios.com/

I've sent it to Andrew two weeks ago but have not heard back from him
though. My series attack the same issue you are trying to solve without
increasing overhead when reading those counters.

Thanks,

Mathieu

> 
> On a 352-CPU, 2-node system with 64 KiB pages, reproduce with:
> 
>    sudo -E env LTPROOT=/opt/ltp \
>        PATH=/opt/ltp/testcases/bin:$PATH LTP_TIMEOUT_MUL=5 \
>        bash -c "cd /opt/ltp/testcases/bin && ./getrusage03"
> 
> The test failed 10/10 times with child.children=270336 instead of
> 307200.
> 
> Use get_mm_rss_sum() when reading and resetting high-water RSS. Keep
> update_hiwater_rss() on the approximate path because it runs before
> RSS-decreasing operations.
> 
> The patched kernel passed getrusage03 20/20 times. getrusage01,
> getrusage02 and getrusage04 also passed.
> 
> Fixes: f1a7941243c1 ("mm: convert mm's rss stats into percpu_counter")
> Cc: stable@vger.kernel.org
> Assisted-by: Cursor:claude-opus-4-8-thinking-max
> Signed-off-by: Nirmoy Das <nirmoyd@nvidia.com>
> ---
>   include/linux/mm.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 485df9c2dbddb..1b613c6356baa 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3405,7 +3405,7 @@ static inline unsigned long get_mm_rss_sum(struct mm_struct *mm)
>   
>   static inline unsigned long get_mm_hiwater_rss(struct mm_struct *mm)
>   {
> -	return max(mm->hiwater_rss, get_mm_rss(mm));
> +	return max(mm->hiwater_rss, get_mm_rss_sum(mm));
>   }
>   
>   static inline unsigned long get_mm_hiwater_vm(struct mm_struct *mm)
> @@ -3429,7 +3429,7 @@ static inline void update_hiwater_vm(struct mm_struct *mm)
>   
>   static inline void reset_mm_hiwater_rss(struct mm_struct *mm)
>   {
> -	mm->hiwater_rss = get_mm_rss(mm);
> +	mm->hiwater_rss = get_mm_rss_sum(mm);
>   }
>   
>   static inline void setmax_mm_hiwater_rss(unsigned long *maxrss,
> 
> base-commit: 4539944e515183668109bdf4d0c3d7d228383d88


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com