[PATCH v3 3/4] xfs: randomly fall back to near mode lookup algorithm in debug mode

[Brian Foster <bfoster@redhat.com>]

The last block scan is the dominant near mode allocation algorithm
for a newer filesystem with fewer, large free extents. Add debug
mode logic to randomly fall back to lookup mode to improve
regression test coverage.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/xfs/libxfs/xfs_alloc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index 7753b61ba532..d550aa5597bf 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -1266,6 +1266,7 @@ xfs_alloc_ag_vextent_near(
 	int			i;
 	xfs_agblock_t		bno;
 	xfs_extlen_t		len;
+	bool			lastblock;
 
 	/* handle unitialized agbno range so caller doesn't have to */
 	if (!args->min_agbno && !args->max_agbno)
@@ -1291,7 +1292,12 @@ xfs_alloc_ag_vextent_near(
 	 * Otherwise run the optimized lookup search algorithm from the current
 	 * location to the end of the tree.
 	 */
-	if (xfs_btree_islastblock(acur.cnt, 0)) {
+	lastblock = xfs_btree_islastblock(acur.cnt, 0);
+#ifdef DEBUG
+	if (lastblock)
+		lastblock = prandom_u32() & 1;
+#endif
+	if (lastblock) {
 		int	j;
 
 		trace_xfs_alloc_cur_lastblock(args);
-- 
2.20.1