xfs hang or slowness while removing files

[Avi Kivity]

Removing a directory with ~900 32MB files, we saw this:

[ 5645.684464] INFO: task xfsaild/md0:12247 blocked for more than 120 
seconds.
[ 5645.686488] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" 
disables this message.
[ 5645.687713] xfsaild/md0     D ffff88103f9d3680     0 12247    2 
0x00000080
[ 5645.687729]  ffff8810136f7d40 0000000000000046 ffff882026d82220 
ffff8810136f7fd8
[ 5645.687732]  ffff8810136f7fd8 ffff8810136f7fd8 ffff882026d82220 
ffff882026d82220
[ 5645.687734]  ffff88103f9d44c0 0000000000000001 0000000000000000 
ffff8820285aa928
[ 5645.687737] Call Trace:
[ 5645.687747]  [<ffffffff816098d9>] schedule+0x29/0x70
[ 5645.687768]  [<ffffffffa06cd880>] _xfs_log_force+0x230/0x290 [xfs]
[ 5645.687773]  [<ffffffff810a9510>] ? wake_up_state+0x20/0x20
[ 5645.687796]  [<ffffffffa06cd906>] xfs_log_force+0x26/0x80 [xfs]
[ 5645.687808]  [<ffffffffa06d2380>] ? 
xfs_trans_ail_cursor_first+0x90/0x90 [xfs]
[ 5645.687818]  [<ffffffffa06d24d1>] xfsaild+0x151/0x5e0 [xfs]
[ 5645.687828]  [<ffffffffa06d2380>] ? 
xfs_trans_ail_cursor_first+0x90/0x90 [xfs]
[ 5645.687831]  [<ffffffff8109727f>] kthread+0xcf/0xe0
[ 5645.687834]  [<ffffffff810971b0>] ? kthread_create_on_node+0x140/0x140
[ 5645.687837]  [<ffffffff81614358>] ret_from_fork+0x58/0x90
[ 5645.687852]  [<ffffffff810971b0>] ? kthread_create_on_node+0x140/0x140

'rm' did not complete, but was killable.  Nothing else was running on 
the system at the time.

The filesystem was mounted with the discard option set, but since that 
is discouraged, we'll retry without it.