[PATCH] xfs: speed up directory bestfree block scanning

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

When running a "create millions inodes in a directory" test
recently, I noticed we were spending a huge amount of time
converting freespace block headers from disk format to in-memory
format:

 31.47%  [kernel]  [k] xfs_dir2_node_addname
 17.86%  [kernel]  [k] xfs_dir3_free_hdr_from_disk
  3.55%  [kernel]  [k] xfs_dir3_free_bests_p

We shouldn't be hitting the best free block scanning code so hard
when doing sequential directory creates, and it turns out there's
a highly suboptimal loop searching the the best free array in
the freespace block - it decodes the block header before checking
each entry inside a loop, instead of decoding the header once before
running the entry search loop.

This makes a massive difference to create rates. Profile now looks
like this:

  13.15%  [kernel]  [k] xfs_dir2_node_addname
   3.52%  [kernel]  [k] xfs_dir3_leaf_check_int
   3.11%  [kernel]  [k] xfs_log_commit_cil

And the wall time/average file create rate differences are
just as stark:

		create time(sec) / rate (files/s)
File count	     vanilla		    patched
  10k		   0.54 / 18.5k		   0.53 / 18.9k
  20k		   1.10	/ 18.1k		   1.05 / 19.0k
 100k		   4.21	/ 23.8k		   3.91 / 25.6k
 200k		   9.66	/ 20,7k		   7.37 / 27.1k
   1M		  86.61	/ 11.5k		  48.26 / 20.7k
   2M		 206.13	/  9.7k		 129.71 / 15.4k

The larger the directory, the bigger the performance improvement.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/libxfs/xfs_dir2_node.c | 30 +++++++++++++++---------------
 1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_dir2_node.c b/fs/xfs/libxfs/xfs_dir2_node.c
index 682e2bf370c7..bcf0d43cd6a8 100644
--- a/fs/xfs/libxfs/xfs_dir2_node.c
+++ b/fs/xfs/libxfs/xfs_dir2_node.c
@@ -1829,24 +1829,24 @@ xfs_dir2_node_addname_int(
 		 */
 		bests = dp->d_ops->free_bests_p(free);
 		dp->d_ops->free_hdr_from_disk(&freehdr, free);
-		if (be16_to_cpu(bests[findex]) != NULLDATAOFF &&
-		    be16_to_cpu(bests[findex]) >= length)
-			dbno = freehdr.firstdb + findex;
-		else {
-			/*
-			 * Are we done with the freeblock?
-			 */
-			if (++findex == freehdr.nvalid) {
-				/*
-				 * Drop the block.
-				 */
-				xfs_trans_brelse(tp, fbp);
-				fbp = NULL;
-				if (fblk && fblk->bp)
-					fblk->bp = NULL;
+		do {
+
+			if (be16_to_cpu(bests[findex]) != NULLDATAOFF &&
+			    be16_to_cpu(bests[findex]) >= length) {
+				dbno = freehdr.firstdb + findex;
+				break;
 			}
+		} while (++findex < freehdr.nvalid);
+
+		/* Drop the block if we done with the freeblock */
+		if (findex == freehdr.nvalid) {
+			xfs_trans_brelse(tp, fbp);
+			fbp = NULL;
+			if (fblk)
+				fblk->bp = NULL;
 		}
 	}
+
 	/*
 	 * If we don't have a data block, we need to allocate one and make
 	 * the freespace entries refer to it.
-- 
2.15.0