linux-kernel - Re: [PATCH 3/3] mm: optimize free_area_empty() check using per-migratetype counts

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Message-ID: <CAGsJ_4wCDW=3YdPboinTNUp5qvcSR8LM+6vEhFM5q8FXHVNzHg@mail.gmail.com>
Date: Sat, 29 Nov 2025 08:04:40 +0800
From: Barry Song <21cnbao@...il.com>
To: Hongru Zhang <zhanghongru06@...il.com>
Cc: akpm@...ux-foundation.org, vbabka@...e.cz, david@...nel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, surenb@...gle.com, 
	mhocko@...e.com, jackmanb@...gle.com, hannes@...xchg.org, ziy@...dia.com, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, rppt@...nel.org, 
	axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com, 
	Hongru Zhang <zhanghongru@...omi.com>
Subject: Re: [PATCH 3/3] mm: optimize free_area_empty() check using
 per-migratetype counts

On Fri, Nov 28, 2025 at 11:13 AM Hongru Zhang <zhanghongru06@...il.com> wrote:
>
> From: Hongru Zhang <zhanghongru@...omi.com>
>
> Use per-migratetype counts instead of list_empty() helps reduce a
> few cpu instructions.
>
> Signed-off-by: Hongru Zhang <zhanghongru@...omi.com>
> ---
>  mm/internal.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 1561fc2ff5b8..7759f8fdf445 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -954,7 +954,7 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
>
>  static inline bool free_area_empty(struct free_area *area, int migratetype)
>  {
> -       return list_empty(&area->free_list[migratetype]);
> +       return !READ_ONCE(area->mt_nr_free[migratetype]);

I'm not quite sure about this. Since the counter is written and read more
frequently, cache coherence traffic may actually be higher than for the list
head.

I'd prefer to drop this unless there is real data showing it performs better.

Thanks
Barry