Re: [PATCH] xfs: refactor dir2 leaf readahead shadow buffer cleverness

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, May 01, 2017 at 02:32:43PM -0400, Brian Foster wrote:
> On Fri, Apr 28, 2017 at 12:46:52PM -0700, Darrick J. Wong wrote:
> > Currently, the dir2 leaf block getdents function uses a complex state
> > tracking mechanism to create a shadow copy of the block mappings and
> > then uses the shadow copy to schedule readahead.  Since the read and
> > readahead functions are perfectly capable of reading the mappings
> > themselves, we can tear all that out in favor of a simpler function that
> > simply keeps pushing the readahead window further out.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> > v3: use sliding window to constrain the amount of readahead
> > v2: fix readahead of more than ra_want
> > ---
> 
> Thanks for the updates. This looks much more simple and seems true to
> the current readahead behavior. The code also looks fine to me (one bit
> of whitespace damage noted below).
> 
> That aside, have you happened to test this against a huge/ugly directory
> to verify it works as expected? Note that I don't think in depth
> performance analysis is required. Verification of any kind of dir that
> is known to benefit from readahead is probably sufficient IMO. Perhaps
> dm-delay with a small enough latency to allow us to measure the effect
> of readahead could help us here.

Yeah, I used xfs/349 to generate a filesystem containing a directory
with a freeindex block, then ls'd the entire directory to see how long
the getdents calls took.  The readhead calls were nearly identical with
similar runtimes.

> >  fs/xfs/xfs_dir2_readdir.c |  316 ++++++++++++---------------------------------
> >  1 file changed, 82 insertions(+), 234 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_dir2_readdir.c b/fs/xfs/xfs_dir2_readdir.c
> > index 20b7a5c..d05c1ec 100644
> > --- a/fs/xfs/xfs_dir2_readdir.c
> > +++ b/fs/xfs/xfs_dir2_readdir.c
> > @@ -243,214 +243,98 @@ xfs_dir2_block_getdents(
> >  	return 0;
> >  }
> >  
> ...
> >  	/*
> > -	 * Do we need more readahead?
> > -	 * Each loop tries to process 1 full dir blk; last may be partial.
> > +	 * Start readahead for the next bufsize's worth of dir data blocks.
> > +	 * We may have already issued readahead for some of that range;
> > +	 * ra_blk tracks the last block we tried to read(ahead).
> >  	 */
> > +	ra_want = howmany(bufsize + geo->blksize, (1 << geo->fsblog));
> > +	if (*ra_blk >= last_da)
> > +		goto out;
> > +	else if (*ra_blk == 0)
> > +		*ra_blk = map.br_startoff;
> > +	next_ra = map.br_startoff + geo->fsbcount;
> > +	if (next_ra >= last_da)
> > +		goto out_no_ra;
> > +	found = xfs_iext_lookup_extent(dp, ifp, next_ra, &idx, &map);
> > +	if (!found || map.br_startoff >= last_da)
> > +		goto out_no_ra;
> > +	xfs_trim_extent(&map, next_ra, last_da - next_ra);
> > + 
> 
> ^ trailing space.

(I don't see it...?)

--D

> 
> Brian
> 
> > +	/* Start ra for each dir (not fs) block that has a mapping. */
> >  	blk_start_plug(&plug);
> > -	for (mip->ra_index = mip->ra_offset = i = 0;
> > -	     mip->ra_want > mip->ra_current && i < mip->map_blocks;
> > -	     i += geo->fsbcount) {
> > -		ASSERT(mip->ra_index < mip->map_valid);
> > -		/*
> > -		 * Read-ahead a contiguous directory block.
> > -		 */
> > -		if (i > mip->ra_current &&
> > -		    (map[mip->ra_index].br_blockcount - mip->ra_offset) >=
> > -		    geo->fsbcount) {
> > -			xfs_dir3_data_readahead(dp,
> > -				map[mip->ra_index].br_startoff + mip->ra_offset,
> > -				XFS_FSB_TO_DADDR(dp->i_mount,
> > -					map[mip->ra_index].br_startblock +
> > -							mip->ra_offset));
> > -			mip->ra_current = i;
> > -		}
> > -
> > -		/*
> > -		 * Read-ahead a non-contiguous directory block.  This doesn't
> > -		 * use our mapping, but this is a very rare case.
> > -		 */
> > -		else if (i > mip->ra_current) {
> > -			xfs_dir3_data_readahead(dp,
> > -					map[mip->ra_index].br_startoff +
> > -							mip->ra_offset, -1);
> > -			mip->ra_current = i;
> > -		}
> > -
> > -		/*
> > -		 * Advance offset through the mapping table, processing a full
> > -		 * dir block even if it is fragmented into several extents.
> > -		 * But stop if we have consumed all valid mappings, even if
> > -		 * it's not yet a full directory block.
> > -		 */
> > -		for (j = 0;
> > -		     j < geo->fsbcount && mip->ra_index < mip->map_valid;
> > -		     j += length ) {
> > -			/*
> > -			 * The rest of this extent but not more than a dir
> > -			 * block.
> > -			 */
> > -			length = min_t(int, geo->fsbcount - j,
> > -					map[mip->ra_index].br_blockcount -
> > -							mip->ra_offset);
> > -			mip->ra_offset += length;
> > -
> > -			/*
> > -			 * Advance to the next mapping if this one is used up.
> > -			 */
> > -			if (mip->ra_offset == map[mip->ra_index].br_blockcount) {
> > -				mip->ra_offset = 0;
> > -				mip->ra_index++;
> > +	while (ra_want > 0) {
> > +		next_ra = roundup((xfs_dablk_t)map.br_startoff, geo->fsbcount);
> > +		while (ra_want > 0 &&
> > +		       next_ra < map.br_startoff + map.br_blockcount) {
> > +			if (next_ra >= last_da) {
> > +				*ra_blk = last_da;
> > +				break;
> > +			} else if (next_ra > *ra_blk) {
> > +				xfs_dir3_data_readahead(dp, next_ra, -2);
> > +				*ra_blk = next_ra;
> >  			}
> > +			ra_want -= geo->fsbcount;
> > +			next_ra += geo->fsbcount;
> > +		}
> > +		found = xfs_iext_get_extent(ifp, ++idx, &map);
> > +		if (!found) {
> > +			*ra_blk = last_da;
> > +			break;
> >  		}
> >  	}
> >  	blk_finish_plug(&plug);
> > @@ -458,6 +342,9 @@ xfs_dir2_leaf_readbuf(
> >  out:
> >  	*bpp = bp;
> >  	return error;
> > +out_no_ra:
> > +	*ra_blk = last_da;
> > +	goto out;
> >  }
> >  
> >  /*
> > @@ -475,14 +362,14 @@ xfs_dir2_leaf_getdents(
> >  	xfs_dir2_data_hdr_t	*hdr;		/* data block header */
> >  	xfs_dir2_data_entry_t	*dep;		/* data entry */
> >  	xfs_dir2_data_unused_t	*dup;		/* unused entry */
> > -	int			error = 0;	/* error return value */
> > -	int			length;		/* temporary length value */
> > -	int			byteoff;	/* offset in current block */
> > -	xfs_dir2_off_t		curoff;		/* current overall offset */
> > -	xfs_dir2_off_t		newoff;		/* new curoff after new blk */
> >  	char			*ptr = NULL;	/* pointer to current data */
> > -	struct xfs_dir2_leaf_map_info *map_info;
> >  	struct xfs_da_geometry	*geo = args->geo;
> > +	xfs_dablk_t		rablk = 0;	/* current readahead block */
> > +	xfs_dir2_off_t		curoff;		/* current overall offset */
> > +	int			length;		/* temporary length value */
> > +	int			byteoff;	/* offset in current block */
> > +	int			lock_mode;
> > +	int			error = 0;	/* error return value */
> >  
> >  	/*
> >  	 * If the offset is at or past the largest allowed value,
> > @@ -492,30 +379,12 @@ xfs_dir2_leaf_getdents(
> >  		return 0;
> >  
> >  	/*
> > -	 * Set up to bmap a number of blocks based on the caller's
> > -	 * buffer size, the directory block size, and the filesystem
> > -	 * block size.
> > -	 */
> > -	length = howmany(bufsize + geo->blksize, (1 << geo->fsblog));
> > -	map_info = kmem_zalloc(offsetof(struct xfs_dir2_leaf_map_info, map) +
> > -				(length * sizeof(struct xfs_bmbt_irec)),
> > -			       KM_SLEEP | KM_NOFS);
> > -	map_info->map_size = length;
> > -
> > -	/*
> >  	 * Inside the loop we keep the main offset value as a byte offset
> >  	 * in the directory file.
> >  	 */
> >  	curoff = xfs_dir2_dataptr_to_byte(ctx->pos);
> >  
> >  	/*
> > -	 * Force this conversion through db so we truncate the offset
> > -	 * down to get the start of the data block.
> > -	 */
> > -	map_info->map_off = xfs_dir2_db_to_da(geo,
> > -					      xfs_dir2_byte_to_db(geo, curoff));
> > -
> > -	/*
> >  	 * Loop over directory entries until we reach the end offset.
> >  	 * Get more blocks and readahead as necessary.
> >  	 */
> > @@ -527,38 +396,18 @@ xfs_dir2_leaf_getdents(
> >  		 * current buffer, need to get another one.
> >  		 */
> >  		if (!bp || ptr >= (char *)bp->b_addr + geo->blksize) {
> > -			int	lock_mode;
> > -			bool	trim_map = false;
> > -
> >  			if (bp) {
> > -				xfs_trans_brelse(NULL, bp);
> > +				xfs_trans_brelse(args->trans, bp);
> >  				bp = NULL;
> > -				trim_map = true;
> >  			}
> >  
> >  			lock_mode = xfs_ilock_data_map_shared(dp);
> > -			error = xfs_dir2_leaf_readbuf(args, bufsize, map_info,
> > -						      &curoff, &bp, trim_map);
> > +			error = xfs_dir2_leaf_readbuf(args, bufsize, &curoff,
> > +					&rablk, &bp);
> >  			xfs_iunlock(dp, lock_mode);
> > -			if (error || !map_info->map_valid)
> > +			if (error || !bp)
> >  				break;
> >  
> > -			/*
> > -			 * Having done a read, we need to set a new offset.
> > -			 */
> > -			newoff = xfs_dir2_db_off_to_byte(geo,
> > -							 map_info->curdb, 0);
> > -			/*
> > -			 * Start of the current block.
> > -			 */
> > -			if (curoff < newoff)
> > -				curoff = newoff;
> > -			/*
> > -			 * Make sure we're in the right block.
> > -			 */
> > -			else if (curoff > newoff)
> > -				ASSERT(xfs_dir2_byte_to_db(geo, curoff) ==
> > -				       map_info->curdb);
> >  			hdr = bp->b_addr;
> >  			xfs_dir3_data_check(dp, bp);
> >  			/*
> > @@ -643,7 +492,6 @@ xfs_dir2_leaf_getdents(
> >  		ctx->pos = XFS_DIR2_MAX_DATAPTR & 0x7fffffff;
> >  	else
> >  		ctx->pos = xfs_dir2_byte_to_dataptr(curoff) & 0x7fffffff;
> > -	kmem_free(map_info);
> >  	if (bp)
> >  		xfs_trans_brelse(NULL, bp);
> >  	return error;
> > --
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