Re: [RFC PATCH] mm: Sync percpu mm RSS counters before querying

[Shakeel Butt]

On Thu, Jun 08, 2023 at 07:12:56PM +0200, Michal Koutný wrote:
> An issue was observed with stats collected in struct rusage on ppc64le
> with 64kB pages. The percpu counters use batching with
> 	percpu_counter_batch = max(32, nr*2) # in PAGE_SIZE
> i.e. with larger pages but similar RSS consumption (bytes), there'll be
> less flushes and error more noticeable.
> 
> In this given case (getting consumption of exited child), we can request
> percpu counter's flush without worrying about contention with updaters.
> 
> Fortunately, the commit f1a7941243c1 ("mm: convert mm's rss stats into
> percpu_counter") didn't eradicate all traces of SPLIT_RSS_COUNTING and
> this mechanism already provided some synchronization points before
> reading stats.
> Therefore, use sync_mm_rss as carrier for percpu counters refreshes and
> forget SPLIT_RSS_COUNTING macro for good.
> 
> Fixes: f1a7941243c1 ("mm: convert mm's rss stats into percpu_counter")
> Reported-by: Adam Majer <amajer@xxxxxxxx>
> Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>

The patch seems reasonable to me. Are any of the callsites of
sync_mm_rss performance sensitive?

> ---
>  include/linux/mm.h | 6 ++----
>  kernel/fork.c      | 4 ----
>  2 files changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 27ce77080c79..30cfde88d5b2 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2547,13 +2547,11 @@ static inline void setmax_mm_hiwater_rss(unsigned long *maxrss,
>  		*maxrss = hiwater_rss;
>  }
>  
> -#if defined(SPLIT_RSS_COUNTING)
> -void sync_mm_rss(struct mm_struct *mm);
> -#else
>  static inline void sync_mm_rss(struct mm_struct *mm)
>  {
> +	for (int i = 0; i < NR_MM_COUNTERS; ++i)
> +		percpu_counter_sum(&mm->rss_stat[i]);
>  }
> -#endif
>  
>  #ifndef CONFIG_ARCH_HAS_PTE_SPECIAL
>  static inline int pte_special(pte_t pte)
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 81cba91f30bb..e030eb902e4b 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2412,10 +2412,6 @@ __latent_entropy struct task_struct *copy_process(
>  	p->io_uring = NULL;
>  #endif
>  
> -#if defined(SPLIT_RSS_COUNTING)
> -	memset(&p->rss_stat, 0, sizeof(p->rss_stat));
> -#endif
> -
>  	p->default_timer_slack_ns = current->timer_slack_ns;
>  
>  #ifdef CONFIG_PSI
> -- 
> 2.40.1
>