ext4: avoid unnecessary spreading of allocations among groups

mb_set_largest_free_order() updates lists containing groups with largest
chunk of free space of given order. The way it updates it leads to
always moving the group to the tail of the list. Thus allocations
looking for free space of given order effectively end up cycling through
all groups (and due to initialization in last to first order). This
spreads allocations among block groups which reduces performance for
rotating disks or low-end flash media. Change
mb_set_largest_free_order() to only update lists if the order of the
largest free chunk in the group changed.

Fixes: 196e402adf2e ("ext4: improve cr 0 / cr 1 group scanning")
CC: stable@kernel.org
Reported-and-tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/all/0d81a7c2-46b7-6010-62a4-3e6cfc1628d6@i2se.com/
Link: https://lore.kernel.org/r/20220908092136.11770-2-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 41e1cfecac3b1bf6bcd8df1893e5dd8f494b38b6..6251b4a6cc63c8a85e14582866ae0d23d4501ea6 100644 (file)
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1077,23 +1077,25 @@ mb_set_largest_free_order(struct super_block *sb, struct ext4_group_info *grp)
        struct ext4_sb_info *sbi = EXT4_SB(sb);
        int i;
 
-       if (test_opt2(sb, MB_OPTIMIZE_SCAN) && grp->bb_largest_free_order >= 0) {
+       for (i = MB_NUM_ORDERS(sb) - 1; i >= 0; i--)
+               if (grp->bb_counters[i] > 0)
+                       break;
+       /* No need to move between order lists? */
+       if (!test_opt2(sb, MB_OPTIMIZE_SCAN) ||
+           i == grp->bb_largest_free_order) {
+               grp->bb_largest_free_order = i;
+               return;
+       }
+
+       if (grp->bb_largest_free_order >= 0) {
                write_lock(&sbi->s_mb_largest_free_orders_locks[
                                              grp->bb_largest_free_order]);
                list_del_init(&grp->bb_largest_free_order_node);
                write_unlock(&sbi->s_mb_largest_free_orders_locks[
                                              grp->bb_largest_free_order]);
        }
-       grp->bb_largest_free_order = -1; /* uninit */
-
-       for (i = MB_NUM_ORDERS(sb) - 1; i >= 0; i--) {
-               if (grp->bb_counters[i] > 0) {
-                       grp->bb_largest_free_order = i;
-                       break;
-               }
-       }
-       if (test_opt2(sb, MB_OPTIMIZE_SCAN) &&
-           grp->bb_largest_free_order >= 0 && grp->bb_free) {
+       grp->bb_largest_free_order = i;
+       if (grp->bb_largest_free_order >= 0 && grp->bb_free) {
                write_lock(&sbi->s_mb_largest_free_orders_locks[
                                              grp->bb_largest_free_order]);
                list_add_tail(&grp->bb_largest_free_order_node,