[PATCH 2/2] ext4: mballoc - limit scanning of uninitialized groups

[Alex Zhuravlev <azhuravlev@whamcloud.com>]

cr=0 is supposed to be an optimization to save CPU cycles, but if
buddy data (in memory) is not initialized then all this makes no
sense as we have to do sync IO taking a lot of cycles.
also, at cr=0 mballoc doesn't store any available chunk. cr=1 also
skips groups using heuristic based on avg. fragment size. it's more
useful to skip such groups and switch to cr=2 where groups will be
scanned for available chunks.

The goal group is not skipped to prevent allocations in foreign groups,
which can happen after mount while buddy data is still being populated.

using sparse image and dm-slow virtual device of 120TB was simulated.
then the image was formatted and filled using debugfs to mark ~85% of
available space as busy. the very first allocation w/o the patch could
not complete in half an hour (according to vmstat it would take ~10-1
hours). with the patch applied the allocation took ~20 seconds.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>

 fs/ext4/mballoc.c | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 30d5d97548c4..f719714862b5 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1877,6 +1877,21 @@ int ext4_mb_find_by_goal(struct ext4_allocation_context *ac,
 	return 0;
 }
 
+static inline int ext4_mb_uninit_on_disk(struct super_block *sb,
+				    ext4_group_t group)
+{
+	struct ext4_group_desc *desc;
+
+	if (!ext4_has_group_desc_csum(sb))
+		return 0;
+
+	desc = ext4_get_group_desc(sb, group, NULL);
+	if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT))
+		return 1;
+
+	return 0;
+}
+
 /*
  * The routine scans buddy structures (not bitmap!) from given order
  * to max order and tries to find big enough chunk to satisfy the req
@@ -2060,7 +2075,20 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
 
 	/* We only do this if the grp has never been initialized */
 	if (unlikely(EXT4_MB_GRP_NEED_INIT(grp))) {
-		int ret = ext4_mb_init_group(ac->ac_sb, group, GFP_NOFS);
+		int ret;
+
+		/* cr=0/1 is a very optimistic search to find large
+		 * good chunks almost for free. if buddy data is
+		 * not ready, then this optimization makes no sense.
+		 * instead it leads to loading (synchronously) lots
+		 * of groups and very slow allocations.
+		 * but don't skip the goal group to keep blocks in
+		 * the inode's group. */
+
+		if (cr < 2 && !ext4_mb_uninit_on_disk(ac->ac_sb, group) &&
+		    ac->ac_g_ex.fe_group != group)
+			return 0;
+		ret = ext4_mb_init_group(ac->ac_sb, group, GFP_NOFS);
 		if (ret)
 			return ret;
 	}
-- 
2.21.3