linux-kernel - Re: [PATCH 4/4] vmalloc: Align nr_vmalloc_pages and vmap_lazy

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Message-ID: <aAJTAUwhP1w8v9rD@MiWiFi-R3L-srv>
Date: Fri, 18 Apr 2025 21:26:25 +0800
From: Baoquan He <bhe@...hat.com>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
	Mateusz Guzik <mjguzik@...il.com>
Subject: Re: [PATCH 4/4] vmalloc: Align nr_vmalloc_pages and vmap_lazy_nr

On 04/17/25 at 06:12pm, Uladzislau Rezki (Sony) wrote:
> Currently both atomics share one cache-line:
> 
> <snip>
> ...
> ffffffff83eab400 b vmap_lazy_nr
> ffffffff83eab408 b nr_vmalloc_pages
> ...
> <snip>
> 
> those are global variables and they are only 8 bytes apart.
> Since they are modified by different threads this causes a
> false sharing. This can lead to a performance drop due to
> unnecessary cache invalidations.
> 
> After this patch it is aligned to a cache line boundary:
> 
> <snip>
> ...
> ffffffff8260a600 d vmap_lazy_nr
> ffffffff8260a640 d nr_vmalloc_pages

A great catch and improvement.

Reviewed-by: Baoquan He <bhe@...hat.com>

> ...
> <snip>
> 
> Cc: Mateusz Guzik <mjguzik@...il.com>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  mm/vmalloc.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 77da4613f07ff..54f60d62051da 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1008,7 +1008,8 @@ static BLOCKING_NOTIFIER_HEAD(vmap_notify_list);
>  static void drain_vmap_area_work(struct work_struct *work);
>  static DECLARE_WORK(drain_vmap_work, drain_vmap_area_work);
>  
> -static atomic_long_t nr_vmalloc_pages;
> +static __cacheline_aligned_in_smp atomic_long_t nr_vmalloc_pages;
> +static __cacheline_aligned_in_smp atomic_long_t vmap_lazy_nr;
>  
>  unsigned long vmalloc_nr_pages(void)
>  {
> @@ -2117,8 +2118,6 @@ static unsigned long lazy_max_pages(void)
>  	return log * (32UL * 1024 * 1024 / PAGE_SIZE);
>  }
>  
> -static atomic_long_t vmap_lazy_nr = ATOMIC_LONG_INIT(0);
> -
>  /*
>   * Serialize vmap purging.  There is no actual critical section protected
>   * by this lock, but we want to avoid concurrent calls for performance
> -- 
> 2.39.5
>