Comment 18 for bug 1891773

Looks like the same issue kcompactd and btrfs-transaction locking up. Can not immediately reproduce, seemingly takes ~2weeks to appear. More problematic systems generally also have higher disk i/o, all btrfs.

%Cpu0  :  3.8 us,  1.3 sy,  0.0 ni,  0.0 id, 94.9 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu1  :  1.3 us,  2.6 sy,  0.0 ni, 96.1 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st

%CPU    PID    TID     TIME STAT   RSS    SZ F COMMAND
 0.0     27     27 00:00:03 D        0     0 1 [kcompactd0]
 0.0    618    618 00:03:04 D        0     0 1 [btrfs-transacti]

cat /proc/27/stack
[<0>] __lock_page+0xff/0x140
[<0>] migrate_pages+0x91f/0xb80
[<0>] compact_zone+0x681/0x950
[<0>] kcompactd_do_work+0xfe/0x2a0
[<0>] kcompactd+0x86/0x1c0
[<0>] kthread+0x121/0x140
[<0>] ret_from_fork+0x35/0x40
[<0>] 0xffffffffffffffff

cat /proc/618/stack
[<0>] lock_extent_buffer_for_io+0x100/0x2a0 [btrfs]
[<0>] btree_write_cache_pages+0x1b8/0x420 [btrfs]
[<0>] btree_writepages+0x5d/0x70 [btrfs]
[<0>] do_writepages+0x4b/0xe0
[<0>] __filemap_fdatawrite_range+0xcf/0x100
[<0>] filemap_fdatawrite_range+0x13/0x20
[<0>] btrfs_write_marked_extents+0x68/0x140 [btrfs]
[<0>] btrfs_write_and_wait_marked_extents.constprop.20+0x4f/0x90 [btrfs]
[<0>] btrfs_commit_transaction+0x696/0x910 [btrfs]
[<0>] transaction_kthread+0x18d/0x1b0 [btrfs]
[<0>] kthread+0x121/0x140
[<0>] ret_from_fork+0x35/0x40
[<0>] 0xffffffffffffffff

INFO: task kcompactd0:27 blocked for more than 120 seconds.
      Tainted: P E 4.15.0-122-generic #124-Ubuntu
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kcompactd0 D 0 27 2 0x90000000
Call Trace:
 __schedule+0x24e/0x880
 schedule+0x2c/0x80
 io_schedule+0x16/0x40
 __lock_page+0xff/0x140
 ? page_cache_tree_insert+0xe0/0xe0
 migrate_pages+0x91f/0xb80
 ? __ClearPageMovable+0x10/0x10
 ? isolate_freepages_block+0x3b0/0x3b0
 compact_zone+0x681/0x950
 kcompactd_do_work+0xfe/0x2a0
 ? __switch_to_asm+0x35/0x70
 ? __switch_to_asm+0x41/0x70
 kcompactd+0x86/0x1c0
 ? kcompactd+0x86/0x1c0
 ? wait_woken+0x80/0x80
 kthread+0x121/0x140
 ? kcompactd_do_work+0x2a0/0x2a0
 ? kthread_create_worker_on_cpu+0x70/0x70
 ret_from_fork+0x35/0x40
~