Re: [PATCH v22 10/16] RFC xfs: Skip flip flags for delayed attrs

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jul 26, 2021 at 11:20:47PM -0700, Allison Henderson wrote:
> This is a clean up patch that skips the flip flag logic for delayed attr
> renames.  Since the log replay keeps the inode locked, we do not need to
> worry about race windows with attr lookups.  So we can skip over
> flipping the flag and the extra transaction roll for it
> 
> RFC: In the last review, folks asked for some performance analysis, so I
> did a few perf captures with and with out this patch.  What I found was
> that there wasnt very much difference at all between having the patch or
> not having it.  Of the time we do spend in the affected code, the
> percentage is small.  Most of the time we spend about %0.03 of the time
> in this function, with or with out the patch.  Occasionally we get a
> 0.02%, though not often.  So I think this starts to challenge needing
> this patch at all. This patch was requested some number of reviews ago,
> be perhaps in light of the findings, it may no longer be of interest.
> 
>      0.03%     0.00%  fsstress  [xfs]               [k] xfs_attr_set_iter
> 
> Keep it or drop it?
> 
> Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>

/me hates it when he notices things after review... :/

> ---
>  fs/xfs/libxfs/xfs_attr.c      | 51 +++++++++++++++++++++++++------------------
>  fs/xfs/libxfs/xfs_attr_leaf.c |  3 ++-
>  2 files changed, 32 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_attr.c b/fs/xfs/libxfs/xfs_attr.c
> index 11d8081..eee219c6 100644
> --- a/fs/xfs/libxfs/xfs_attr.c
> +++ b/fs/xfs/libxfs/xfs_attr.c
> @@ -355,6 +355,7 @@ xfs_attr_set_iter(
>  	struct xfs_inode		*dp = args->dp;
>  	struct xfs_buf			*bp = NULL;
>  	int				forkoff, error = 0;
> +	struct xfs_mount		*mp = args->dp->i_mount;
>  
>  	/* State machine switch */
>  	switch (dac->dela_state) {
> @@ -476,16 +477,21 @@ xfs_attr_set_iter(
>  		 * In a separate transaction, set the incomplete flag on the
>  		 * "old" attr and clear the incomplete flag on the "new" attr.
>  		 */
> -		error = xfs_attr3_leaf_flipflags(args);
> -		if (error)
> -			return error;
> -		/*
> -		 * Commit the flag value change and start the next trans in
> -		 * series.
> -		 */
> -		dac->dela_state = XFS_DAS_FLIP_LFLAG;
> -		trace_xfs_attr_set_iter_return(dac->dela_state, args->dp);
> -		return -EAGAIN;
> +		if (!xfs_hasdelattr(mp)) {

More nitpicking: this should be gated directly on the log incompat
feature check, not the LARP knob...

		if (!xfs_sb_version_haslogxattrs(&mp->m_sb)) {

> +			error = xfs_attr3_leaf_flipflags(args);
> +			if (error)
> +				return error;
> +			/*
> +			 * Commit the flag value change and start the next trans
> +			 * in series.
> +			 */
> +			dac->dela_state = XFS_DAS_FLIP_LFLAG;
> +			trace_xfs_attr_set_iter_return(dac->dela_state,
> +						       args->dp);
> +			return -EAGAIN;
> +		}
> +
> +		/* fallthrough */
>  	case XFS_DAS_FLIP_LFLAG:
>  		/*
>  		 * Dismantle the "old" attribute/value pair by removing a
> @@ -587,17 +593,21 @@ xfs_attr_set_iter(
>  		 * In a separate transaction, set the incomplete flag on the
>  		 * "old" attr and clear the incomplete flag on the "new" attr.
>  		 */
> -		error = xfs_attr3_leaf_flipflags(args);
> -		if (error)
> -			goto out;
> -		/*
> -		 * Commit the flag value change and start the next trans in
> -		 * series
> -		 */
> -		dac->dela_state = XFS_DAS_FLIP_NFLAG;
> -		trace_xfs_attr_set_iter_return(dac->dela_state, args->dp);
> -		return -EAGAIN;
> +		if (!xfs_hasdelattr(mp)) {

...and here...

> +			error = xfs_attr3_leaf_flipflags(args);
> +			if (error)
> +				goto out;
> +			/*
> +			 * Commit the flag value change and start the next trans
> +			 * in series
> +			 */
> +			dac->dela_state = XFS_DAS_FLIP_NFLAG;
> +			trace_xfs_attr_set_iter_return(dac->dela_state,
> +						       args->dp);
> +			return -EAGAIN;
> +		}
>  
> +		/* fallthrough */
>  	case XFS_DAS_FLIP_NFLAG:
>  		/*
>  		 * Dismantle the "old" attribute/value pair by removing a
> @@ -1241,7 +1251,6 @@ xfs_attr_node_addname_clear_incomplete(
>  	 * Re-find the "old" attribute entry after any split ops. The INCOMPLETE
>  	 * flag means that we will find the "old" attr, not the "new" one.
>  	 */
> -	args->attr_filter |= XFS_ATTR_INCOMPLETE;

Why is this removed from the query arguments?  If we're going to skip
the INCOMPLETE flag dance, I would have thought that you'd change the
XFS_DAS_CLR_FLAG case to omit xfs_attr_node_addname_clear_incomplete if
the logged xattr feature is set?

>  	state = xfs_da_state_alloc(args);
>  	state->inleaf = 0;
>  	error = xfs_da3_node_lookup_int(state, &retval);
> diff --git a/fs/xfs/libxfs/xfs_attr_leaf.c b/fs/xfs/libxfs/xfs_attr_leaf.c
> index b910bd2..a9116ee 100644
> --- a/fs/xfs/libxfs/xfs_attr_leaf.c
> +++ b/fs/xfs/libxfs/xfs_attr_leaf.c
> @@ -1482,7 +1482,8 @@ xfs_attr3_leaf_add_work(
>  	if (tmp)
>  		entry->flags |= XFS_ATTR_LOCAL;
>  	if (args->op_flags & XFS_DA_OP_RENAME) {
> -		entry->flags |= XFS_ATTR_INCOMPLETE;
> +		if (!xfs_hasdelattr(mp))

Same change here as above.

--D

> +			entry->flags |= XFS_ATTR_INCOMPLETE;
>  		if ((args->blkno2 == args->blkno) &&
>  		    (args->index2 <= args->index)) {
>  			args->index2++;
> -- 
> 2.7.4
>