Re: [PATCH 2/4] btrfs: reorder members in btrfs_delayed_root for better packing

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2026/1/10 03:47, David Sterba 写道:
> There are two unnecessary 4B holes in btrfs_delayed_root;
> 
> struct btrfs_delayed_root {
>          spinlock_t                 lock;                 /*     0     4 */
> 
>          /* XXX 4 bytes hole, try to pack */
> 
>          struct list_head           node_list;            /*     8    16 */
>          struct list_head           prepare_list;         /*    24    16 */
>          atomic_t                   items;                /*    40     4 */
>          atomic_t                   items_seq;            /*    44     4 */
>          int                        nodes;                /*    48     4 */
> 
>          /* XXX 4 bytes hole, try to pack */
> 
>          wait_queue_head_t          wait;                 /*    56    24 */
> 
>          /* size: 80, cachelines: 2, members: 7 */
>          /* sum members: 72, holes: 2, sum holes: 8 */
>          /* last cacheline: 16 bytes */
> };
> 
> Reordering 'nodes' after 'lock' reduces size by 8B, to 72 on release
> config.

Not a huge thing, but can we put members in descend order of their sizes 
if they are properly aligned.

For this particular case, wait_queue_heat_t itself isn't properly 
power-of-2 sized, but with 2 32bits member, it can be shrink even futher:

struct btrfs_delayed_root {
         wait_queue_head_t          wait;                 /*     0    24 */
         int                        nodes;                /*    24     4 */
         spinlock_t                 lock;                 /*    28     4 */
         struct list_head           node_list;            /*    32    16 */
         struct list_head           prepare_list;         /*    48    16 */
         /* --- cacheline 1 boundary (64 bytes) --- */
         atomic_t                   items;                /*    64     4 */
         atomic_t                   items_seq;            /*    68     4 */

         /* size: 72, cachelines: 2, members: 7 */
         /* last cacheline: 8 bytes */
};

Thanks,
Qu

> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>   fs/btrfs/fs.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
> index dd4944f9a98553..4d721fbd390c55 100644
> --- a/fs/btrfs/fs.h
> +++ b/fs/btrfs/fs.h
> @@ -456,6 +456,7 @@ struct btrfs_commit_stats {
>   
>   struct btrfs_delayed_root {
>   	spinlock_t lock;
> +	int nodes;		/* for delayed nodes */
>   	struct list_head node_list;
>   	/*
>   	 * Used for delayed nodes which is waiting to be dealt with by the
> @@ -465,7 +466,6 @@ struct btrfs_delayed_root {
>   	struct list_head prepare_list;
>   	atomic_t items;		/* for delayed items */
>   	atomic_t items_seq;	/* for delayed items */
> -	int nodes;		/* for delayed nodes */
>   	wait_queue_head_t wait;
>   };
>