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To:     linux-ext4@vger.kernel.org
From:   Chris Friesen <chris.friesen@windriver.com>
Subject: looking for assistance with jbd2 (and other processes) hung trying to
 write to disk
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Date:   Mon, 9 Nov 2020 15:11:58 -0600
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Hi,

I'm running a 3.10.0-1127.rt56.1093 CentOS kernel.  I realize you don't 
support this particular kernel but I'm hoping for some general pointers.

I've got a system with four "housekeeping" CPUs, with rcu_nocbs and 
hohz_full used to reduce system overhead on the "application" CPUs, with 
four CPUs set as "isolcpus" to try and isolate them even further.  I 
have a crashdump vmcore file from a softdog expiry when the process that 
pets the softdog hung trying to write to /dev/log after the unix socket 
backlog had been reached.

I can see a "jbd2/nvme2n1p4-" process that appears to be hung for over 9 
minutes waiting to commit a transaction.  /dev/nvme2n1p4 corresponds to 
the root filesystem.  The "ps" and "bt" output from the crashdump are 
below.  syslog-ng is also blocked waiting on filesystem access, and 
there are other tasks also blocked on disk, including a few jbd2 tasks 
that are associated with device mapper.

Can anyone give some suggestions on how to track down what's causing the 
delay here?  I suspect there's a race condition somewhere similar to 
what happened with https://access.redhat.com/solutions/3226391, although 
that one was specific to device-mapper and the root filesystem here is 
directly on the nvme device.

Any suggestions would be appreciated.

Additional info below:

crash> ps -m 930
[0 00:09:11.694] [UN]  PID: 930    TASK: ffffa14b5f9032c0  CPU: 1 
COMMAND: "jbd2/nvme2n1p4-"

crash> bt 930
PID: 930    TASK: ffffa14b5f9032c0  CPU: 1   COMMAND: "jbd2/nvme2n1p4-"
  #0 [ffffa14b5ff0ba20] __schedule at ffffffffafe1b959
  #1 [ffffa14b5ff0bab0] schedule at ffffffffafe1be80
  #2 [ffffa14b5ff0bac8] schedule_timeout at ffffffffafe19d4c
  #3 [ffffa14b5ff0bb70] io_schedule_timeout at ffffffffafe1ab6d
  #4 [ffffa14b5ff0bba0] io_schedule at ffffffffafe1ac08
  #5 [ffffa14b5ff0bbb0] bit_wait_io at ffffffffafe1a561
  #6 [ffffa14b5ff0bbc8] __wait_on_bit at ffffffffafe1a087
  #7 [ffffa14b5ff0bc08] out_of_line_wait_on_bit at ffffffffafe1a1f1
  #8 [ffffa14b5ff0bc80] __wait_on_buffer at ffffffffaf85068a
  #9 [ffffa14b5ff0bc90] jbd2_journal_commit_transaction at 
ffffffffc0e543fc [jbd2]
#10 [ffffa14b5ff0be48] kjournald2 at ffffffffc0e5a6ad [jbd2]
#11 [ffffa14b5ff0bec8] kthread at ffffffffaf6ad781
#12 [ffffa14b5ff0bf50] ret_from_fork_nospec_begin at ffffffffafe1fe5d

Possibly of interest, syslog-ng is also blocked waiting on filesystem 
access:

crash> bt 1912
PID: 1912   TASK: ffffa14b62dc2610  CPU: 1   COMMAND: "syslog-ng"
  #0 [ffffa14b635b7980] __schedule at ffffffffafe1b959
  #1 [ffffa14b635b7a10] schedule at ffffffffafe1be80
  #2 [ffffa14b635b7a28] schedule_timeout at ffffffffafe19d4c
  #3 [ffffa14b635b7ad0] io_schedule_timeout at ffffffffafe1ab6d
  #4 [ffffa14b635b7b00] io_schedule at ffffffffafe1ac08
  #5 [ffffa14b635b7b10] bit_wait_io at ffffffffafe1a561
  #6 [ffffa14b635b7b28] __wait_on_bit at ffffffffafe1a087
  #7 [ffffa14b635b7b68] out_of_line_wait_on_bit at ffffffffafe1a1f1
  #8 [ffffa14b635b7be0] do_get_write_access at ffffffffc0e51e94 [jbd2]
  #9 [ffffa14b635b7c80] jbd2_journal_get_write_access at 
ffffffffc0e521b7 [jbd2]
#10 [ffffa14b635b7ca0] __ext4_journal_get_write_access at 
ffffffffc0eb8e31 [ext4]
#11 [ffffa14b635b7cd0] ext4_reserve_inode_write at ffffffffc0e87fa0 [ext4]
#12 [ffffa14b635b7d00] ext4_mark_inode_dirty at ffffffffc0e8801e [ext4]
#13 [ffffa14b635b7d58] ext4_dirty_inode at ffffffffc0e8bc40 [ext4]
#14 [ffffa14b635b7d78] __mark_inode_dirty at ffffffffaf84855d
#15 [ffffa14b635b7da8] ext4_setattr at ffffffffc0e8b558 [ext4]
#16 [ffffa14b635b7e18] notify_change at ffffffffaf8363fc
#17 [ffffa14b635b7e60] chown_common at ffffffffaf8128ac
#18 [ffffa14b635b7f08] sys_fchown at ffffffffaf813fb7
#19 [ffffa14b635b7f50] tracesys at ffffffffafe202a8 (via system_call)

One of the hung jbd2 tasks associated with device mapper:

crash> bt 1489
PID: 1489   TASK: ffffa14b641f0000  CPU: 1   COMMAND: "jbd2/dm-0-8"
  #0 [ffffa14b5fab7a20] __schedule at ffffffffafe1b959
  #1 [ffffa14b5fab7ab0] schedule at ffffffffafe1be80
  #2 [ffffa14b5fab7ac8] schedule_timeout at ffffffffafe19d4c
  #3 [ffffa14b5fab7b70] io_schedule_timeout at ffffffffafe1ab6d
  #4 [ffffa14b5fab7ba0] io_schedule at ffffffffafe1ac08
  #5 [ffffa14b5fab7bb0] bit_wait_io at ffffffffafe1a561
  #6 [ffffa14b5fab7bc8] __wait_on_bit at ffffffffafe1a087
  #7 [ffffa14b5fab7c08] out_of_line_wait_on_bit at ffffffffafe1a1f1
  #8 [ffffa14b5fab7c80] __wait_on_buffer at ffffffffaf85068a
  #9 [ffffa14b5fab7c90] jbd2_journal_commit_transaction at 
ffffffffc0e543fc [jbd2]
#10 [ffffa14b5fab7e48] kjournald2 at ffffffffc0e5a6ad [jbd2]
#11 [ffffa14b5fab7ec8] kthread at ffffffffaf6ad781
#12 [ffffa14b5fab7f50] ret_from_fork_nospec_begin at ffffffffafe1fe5d

Thanks,

Chris