Re: [RFC 2/2] xfs: implement get_inode_wb_ctx_idx() for per-AG parallel writeback

[Christoph Hellwig <hch@lst.de>]

On Sun, Sep 14, 2025 at 08:11:09PM +0800, wangyufei wrote:
> The number of writeback contexts is set to the number of CPUs by
> default. This allows XFS to decide how to assign inodes to writeback
> contexts based on its allocation groups.
> 
> Implement get_inode_wb_ctx_idx() in xfs_super_operations as follows:
> - Limit the number of active writeback contexts to the number of AGs.
> - Assign inodes from the same AG to a unique writeback context.

I'm not sure this actually works.  Data is spread over AGs, just with
a default to the parent inode AG if there is space, and even that isn't
true for the inode32 option or when using the RT subvolume.

> +
> +	if (mp->m_sb.sb_agcount <= nr_wb_ctx)
> +		return XFS_INO_TO_AGNO(mp, xfs_inode->i_ino);
> +	return xfs_inode->i_ino % nr_wb_ctx;
> +}
> +
>  static const struct super_operations xfs_super_operations = {
>  	.alloc_inode		= xfs_fs_alloc_inode,
>  	.destroy_inode		= xfs_fs_destroy_inode,
> @@ -1295,6 +1308,7 @@ static const struct super_operations xfs_super_operations = {
>  	.free_cached_objects	= xfs_fs_free_cached_objects,
>  	.shutdown		= xfs_fs_shutdown,
>  	.show_stats		= xfs_fs_show_stats,
> +	.get_inode_wb_ctx_idx   = xfs_fs_get_inode_wb_ctx_idx,
>  };
>  
>  static int
> -- 
> 2.34.1
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