linux-kernel - Re: [PATCH 1/2] mm: Keep int fields in struct shrink

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Date:   Thu, 19 Jul 2018 13:10:31 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     akpm@...ux-foundation.org, vdavydov.dev@...il.com,
        penguin-kernel@...ove.SAKURA.ne.jp, chris@...is-wilson.co.uk,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: Keep int fields in struct shrink_control together

On Thu 19-07-18 13:51:19, Kirill Tkhai wrote:
> gfp_t is of unsigned type, so let's move nid to keep
> them together.

I guess you meant unsigned int so you can fill up a hole in
shrink_control and as such reduce the stack consumption a bit (at least
on 64b).

An output from pahole is usually a great tool to visualize this.

die__process_function: tag not supported (INVALID)!
struct shrink_control {
	gfp_t                      gfp_mask;             /*     0     4 */

	/* XXX 4 bytes hole, try to pack */

	long unsigned int          nr_to_scan;           /*     8     8 */
	long unsigned int          nr_scanned;           /*    16     8 */
	int                        nid;                  /*    24     4 */

	/* XXX 4 bytes hole, try to pack */

	struct mem_cgroup *        memcg;                /*    32     8 */

	/* size: 40, cachelines: 1, members: 5 */
	/* sum members: 32, holes: 2, sum holes: 8 */
	/* last cacheline: 40 bytes */
};

> 
> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  include/linux/shrinker.h |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index b154fd2b084c..d58aaaed34a4 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -12,6 +12,9 @@
>  struct shrink_control {
>  	gfp_t gfp_mask;
>  
> +	/* current node being shrunk (for NUMA aware shrinkers) */
> +	int nid;
> +
>  	/*
>  	 * How many objects scan_objects should scan and try to reclaim.
>  	 * This is reset before every call, so it is safe for callees
> @@ -26,9 +29,6 @@ struct shrink_control {
>  	 */
>  	unsigned long nr_scanned;
>  
> -	/* current node being shrunk (for NUMA aware shrinkers) */
> -	int nid;
> -
>  	/* current memcg being shrunk (for memcg aware shrinkers) */
>  	struct mem_cgroup *memcg;
>  };

-- 
Michal Hocko
SUSE Labs