linux-ext4 - [PATCH 2/4] ext4: skip non-loaded groups at cr=0/1 when scanning for good groups

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Date:   Fri, 17 Jul 2020 11:53:50 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     Alex Zhuravlev <bzzz@...mcloud.com>,
        Alex Zhuravlev <azhuravlev@...mcloud.com>,
        Andreas Dilger <adilger@...mcloud.com>
Subject: [PATCH 2/4] ext4: skip non-loaded groups at cr=0/1 when scanning for good groups

From: Alex Zhuravlev <azhuravlev@...mcloud.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 avaibale 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.

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.  mount process w/o the patch
couldn't complete in half an hour (according to vmstat it would take
~10-11 hours).  With the patch applied mount took ~20 seconds.

Lustre-bug-id: https://jira.whamcloud.com/browse/LU-12988
Signed-off-by: Alex Zhuravlev <bzzz@...mcloud.com>
Reviewed-by: Andreas Dilger <adilger@...mcloud.com>
---
 fs/ext4/mballoc.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 8a1e6e03c088..172994349bf6 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2195,7 +2195,18 @@ static int ext4_mb_good_group_nolock(struct ext4_allocation_context *ac,

 	/* We only do this if the grp has never been initialized */
 	if (unlikely(EXT4_MB_GRP_NEED_INIT(grp))) {
-		ret = ext4_mb_init_group(ac->ac_sb, group, GFP_NOFS);
+		struct ext4_group_desc *gdp = ext4_get_group_desc(sb, group,
+								  NULL);
+		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 */
+		if (cr < 2 &&
+		    !(ext4_has_group_desc_csum(sb) &&
+		      (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT))))
+			return 0;
+		ret = ext4_mb_init_group(sb, group, GFP_NOFS);
 		if (ret)
 			return ret;
 	}
-- 
2.24.1