[#LU-15319] Weird mballoc behaviour

A weird mballoc behavior in sudden STREAM_ALLOC allocator head jump after a target mount:

# grep -H "" /proc/fs/ldiskfs/md*/mb_last_group
/proc/fs/ldiskfs/md0/mb_last_group:0
/proc/fs/ldiskfs/md2/mb_last_group:0
# echo > /sys/kernel/debug/tracing/trace
# nobjlo=2 nobjhi=2 thrlo=1024 thrhi=1024 size=393216 rszlo=4096 rszhi=4096 tests_str="write" obdfilter-survey 2>&1 | tee /root/obdfilter-survey.log
Fri Dec  3 12:25:19 UTC 2021 Obdfilter-survey for case=disk from kjlmo1304
ost  2 sz 805306368K rsz 4096K obj    4 thr 2048 write 16552.35 [4580.64, 9382.91] 
/usr/bin/iokit-libecho: line 236: 253095 Killed                  remote_shell $host "vmstat 5 >> $host_vmstatf" &>/dev/null
done!
# grep -H "" /proc/fs/ldiskfs/md*/mb_last_group
/proc/fs/ldiskfs/md0/mb_last_group:114337
/proc/fs/ldiskfs/md2/mb_last_group:130831
#

The streaming allocator head jumped right to the first non-initialized group and now it is the last inited group (the target fs is almost empty):

[root@kjlmo1304 ~]# dumpe2fs /dev/md0 | sed '/BLOCK/q' | tail -24
....
Group 114335: (Blocks 3746529280-3746562047) csum 0x1b7a [INODE_UNINIT, ITABLE_ZEROED]
  Block bitmap at 3741319328 (bg #114176 + 160)
  Inode bitmap at 3741319584 (bg #114176 + 416)
  Inode table at 3741322225-3741322240 (bg #114176 + 3057)
  32768 free blocks, 128 free inodes, 0 directories, 128 unused inodes
  Free blocks: 3746529280-3746562047
  Free inodes: 14634881-14635008
Group 114336: (Blocks 3746562048-3746594815) csum 0x37c1 [INODE_UNINIT, ITABLE_ZEROED]
  Block bitmap at 3741319329 (bg #114176 + 161)
  Inode bitmap at 3741319585 (bg #114176 + 417)
  Inode table at 3741322241-3741322256 (bg #114176 + 3073)
  32768 free blocks, 128 free inodes, 0 directories, 128 unused inodes
  Free blocks: 3746562048-3746594815
  Free inodes: 14635009-14635136
Group 114337: (Blocks 3746594816-3746627583) csum 0xbacd [INODE_UNINIT, ITABLE_ZEROED]
  Block bitmap at 3741319330 (bg #114176 + 162)
  Inode bitmap at 3741319586 (bg #114176 + 418)
  Inode table at 3741322257-3741322272 (bg #114176 + 3089)
  32768 free blocks, 128 free inodes, 0 directories, 128 unused inodes
  Free blocks: 3746594816-3746627583
  Free inodes: 14635137-14635264
Group 114338: (Blocks 3746627584-3746660351) csum 0xca57 [INODE_UNINIT, BLOCK_UNINIT, ITABLE_ZEROED]

The above jump is not big enough to cause performance impact, but the same behavior was observed on another system with 2M block group initialized, that mb_last_group jump shifted block allocations on an empty fs over the middle of the disk device with approximately 15% write / read slowdown.

Looks like it was due to the following checks in ldiksfs_mb_good_group()

        /* We only do this if the grp has never been initialized */
        if (unlikely(LDISKFS_MB_GRP_NEED_INIT(grp))) {
                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 && !ldiskfs_mb_uninit_on_disk(ac->ac_sb, group))
                        return 0;
                ret = ldiskfs_mb_init_group(ac->ac_sb, group);
                if (ret)
                        return 0;
        }

introduced by

ecb68b8 LU-13291 ldiskfs: mballoc don't skip uninit-on-disk groups
6a7a700 LU-12988 ldiskfs: skip non-loaded groups at cr=0/1

[LU-15319] Weird mballoc behaviour Created: 06/Dec/21 Updated: 25/Sep/23 Resolved: 25/Sep/23
Status:	Resolved
Project:	Lustre
Component/s:	None
Affects Version/s:	None
Fix Version/s:	Lustre 2.16.0

[LU-15319] Weird mballoc behaviour Created: 06/Dec/21 Updated: 25/Sep/23 Resolved: 25/Sep/23