Re: [PATCH 2/2] xfs: replace ialloc space res macro with inline helper

[Dave Chinner <david@fromorbit.com>]

On Thu, Jul 16, 2020 at 08:18:49AM -0400, Brian Foster wrote:
> Rewrite the macro as a static inline helper to clean up the logic
> and have one less macro.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/xfs/libxfs/xfs_trans_space.h | 24 ++++++++++++++++--------
>  fs/xfs/xfs_inode.c              |  4 ++--
>  fs/xfs/xfs_symlink.c            |  2 +-
>  3 files changed, 19 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_trans_space.h b/fs/xfs/libxfs/xfs_trans_space.h
> index c6df01a2a158..d08dfc8795c3 100644
> --- a/fs/xfs/libxfs/xfs_trans_space.h
> +++ b/fs/xfs/libxfs/xfs_trans_space.h
> @@ -55,10 +55,18 @@
>  	 XFS_DIRENTER_MAX_SPLIT(mp,nl))
>  #define	XFS_DIRREMOVE_SPACE_RES(mp)	\
>  	XFS_DAREMOVE_SPACE_RES(mp, XFS_DATA_FORK)
> -#define	XFS_IALLOC_SPACE_RES(mp)	\
> -	(M_IGEO(mp)->ialloc_blks + \
> -	 ((xfs_sb_version_hasfinobt(&mp->m_sb) ? 2 : 1) * \
> -	  (M_IGEO(mp)->inobt_maxlevels - 1)))
> +
> +static inline int
> +xfs_ialloc_space_res(
> +	struct xfs_mount	*mp)
> +{
> +	int			res = M_IGEO(mp)->ialloc_blks;
> +
> +	res += M_IGEO(mp)->inobt_maxlevels - 1;
> +	if (xfs_sb_version_hasfinobt(&mp->m_sb))
> +		res += M_IGEO(mp)->inobt_maxlevels - 1;
> +	return res;
> +}

This misses the point I made. i.e. that the space reservation is
constant and never changes, yet we calculate it -twice- per inode
create. That means we can be calculating it hundreds of thousands of
times a second instead of just reading a variable that is likely hot
in cache.

IOWs, if we are going to improve this code, it should to be moved to
a pre-calculated, read-only, per-mount variable so the repeated
calculation goes away entirely.

Then the macro/function goes away entirely an is replaced simply
by mp->m_ialloc_space_res or M_IGEO(mp)->alloc_space_res....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com