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[Peter Waller <p@xxxxxxxxxxx>]

(otherwise unloaded test machines)

On a dual core machine, 100% system CPU usage with zero writes is seen
on one core for 5-10 minutes, spending time in BTRFS threads.

On a single thread machine 100% system CPU is used and I haven't yet
been able to cause it to hang entirely. I do observe almost completely
100% system CPU usage and very low IO rates, 0 IOPS for up to minutes at
a time before it then goes to >1000 IOPs.

The system goes to IOWAIT when there is significant IO traffic, and
otherwise kernel threads are consuming CPU.

However the latter is on a non-prewarmed EBS. So I am not sure if the
behaviour we're seeing is entirely due to cold block storage where we
have no latency/rate guarantees.

Generally, I can't tell if we have hit a bug in the BTRFS free extent
code, or if it is all due to bad EBS performance and a spinlock
somewhere.

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https://bugs.launchpad.net/bugs/1349711

Title:
  Machine lockup in btrfs-transaction

Status in “linux” package in Ubuntu:
  Confirmed

Bug description:
  This has happened twice now.

  I'm on an AWS EC2 m3.large instance with the official Ubuntu AMI ami-
  776d9700.

  # cat /proc/version_signature
  Ubuntu 3.13.0-32.57-generic 3.13.11.4

  After running for many days, the machine locked up with the below
  messages appearing on the console. The machine would respond to ping
  but not SSH or HTTP requests. The machine has one BTRFS volume which
  is 87% full and lives on an Logical Volume Manager (LVM) block device
  on top of one Amazon Elastic Block Store (EBS) device.

  Error messages after first reboot:

  [   77.609490] BTRFS error (device dm-0): block group 10766778368 has wrong amount of free space
  [   77.613678] BTRFS error (device dm-0): failed to load free space cache for block group 10766778368
  [   77.643801] BTRFS error (device dm-0): block group 19356712960 has wrong amount of free space
  [   77.648952] BTRFS error (device dm-0): failed to load free space cache for block group 19356712960
  [   77.926325] BTRFS error (device dm-0): block group 20430454784 has wrong amount of free space
  [   77.931078] BTRFS error (device dm-0): failed to load free space cache for block group 20430454784
  [   78.111437] BTRFS error (device dm-0): block group 21504196608 has wrong amount of free space
  [   78.116165] BTRFS error (device dm-0): failed to load free space cache for block group 21504196608

  Error messages after second reboot:

  [   45.390221] BTRFS error (device dm-0): free space inode generation (0) did not match free space cache generation (70012)
  [   45.413472] BTRFS error (device dm-0): free space inode generation (0) did not match free space cache generation (70012)
  [  467.423961] BTRFS error (device dm-0): block group 518646661120 has wrong amount of free space
  [  467.429251] BTRFS error (device dm-0): failed to load free space cache for block group 518646661120

  Error messages on the console after second lock-up follow:

  [246736.752053] INFO: rcu_sched self-detected stall on CPU { 0}  (t=2220246 jiffies g=35399662 c=35399661 q=0)
  [246736.756059] INFO: rcu_sched detected stalls on CPUs/tasks: { 0} (detected by 1, t=2220247 jiffies, g=35399662, c=35399661, q=0)
  [246764.192014] BUG: soft lockup - CPU#0 stuck for 23s! [kworker/u30:2:1828]
  [246764.212058] BUG: soft lockup - CPU#1 stuck for 23s! [btrfs-transacti:492]
  [246792.192022] BUG: soft lockup - CPU#0 stuck for 23s! [kworker/u30:2:1828]
  [246792.212057] BUG: soft lockup - CPU#1 stuck for 23s! [btrfs-transacti:492]
  [246820.192052] BUG: soft lockup - CPU#0 stuck for 23s! [kworker/u30:2:1828]
  [246820.212018] BUG: soft lockup - CPU#1 stuck for 23s! [btrfs-transacti:492]
  [246848.192052] BUG: soft lockup - CPU#0 stuck for 23s! [kworker/u30:2:1828]
  [246848.212058] BUG: soft lockup - CPU#1 stuck for 23s! [btrfs-transacti:492]
  [246876.192053] BUG: soft lockup - CPU#0 stuck for 22s! [kworker/u30:2:1828]
  [246876.212057] BUG: soft lockup - CPU#1 stuck for 22s! [btrfs-transacti:492]
  [246904.192053] BUG: soft lockup - CPU#0 stuck for 22s! [kworker/u30:2:1828]
  [246904.212058] BUG: soft lockup - CPU#1 stuck for 22s! [btrfs-transacti:492]
  [246916.772052] INFO: rcu_sched self-detected stall on CPU[246916.776058] INFO: rcu_sched detected stalls on CPUs/tasks:
  [246944.192053] BUG: soft lockup - CPU#0 stuck for 22s! [kworker/u30:2:1828]
  [246944.212058] BUG: soft lockup - CPU#1 stuck for 22s! [btrfs-transacti:492]
  [246972.192053] BUG: soft lockup - CPU#0 stuck for 22s! [kworker/u30:2:1828]
  [246972.212018] BUG: soft lockup - CPU#1 stuck for 22s! [btrfs-transacti:492]
  [247000.192053] BUG: soft lockup - CPU#0 stuck for 22s! [kworker/u30:2:1828]
  [247000.212058] BUG: soft lockup - CPU#1 stuck for 22s! [btrfs-transacti:492]
  [247028.192054] BUG: soft lockup - CPU#0 stuck for 22s! [kworker/u30:2:1828]
  [247028.212058] BUG: soft lockup - CPU#1 stuck for 22s! [btrfs-transacti:492]
  [247056.192053] BUG: soft lockup - CPU#0 stuck for 23s! [kworker/u30:2:1828]
  [247056.212061] BUG: soft lockup - CPU#1 stuck for 23s! [btrfs-transacti:492]

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