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Date: Thu, 3 Apr 2008 13:15:00 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Jan Kara <jack@...e.cz>, David Chinner <dgc@....com>,
lkml <linux-kernel@...r.kernel.org>, marcelo <marcelo@...ck.org>
Subject: Re: BUG: ext3 hang in transaction commit
On Thu, Apr 03, 2008 at 05:08:55PM +0200, Peter Zijlstra wrote:
>
> Marcelo has seen something similar, kjournald got stuck and the rest of
> userspace piled on top of it.
>
> The softlockup watchdog triggered because kjournald got stuck in D state
> for over 2 minutes. However Marcelo told me that it did get going
> eventually...
>
> He had a rather easy to reproduce case IIRC, I'll let him tell you more
> about it :-)
Run iozone (-t 4) with a sufficiently large file size under a KVM guest
with a disk file residing in an ext3 filesystem with ordered data mode.
There's nothing special in this testcase, boils down to the QEMU process
writing data to a file.
Issue is not seen under "writeback" mode. While fsync latency in ordered
data mode is known to be bad, _2 minutes_ feels more like a DoS rather
than latency issue to me.
vmstat 2 output on the host shows that the first write pass (initial
writers and rewriters) blasts at full disk speed:
0 2 0 6552980 748972 662876 0 0 58 69838 322 9912 5 6 78 10 0
1 1 0 6493620 779452 693596 0 0 10 54884 417 7032 1 3 78 17 0
1 2 0 6322968 870556 772156 0 0 58 43042 379 12651 4 7 77 12 0
1 1 0 6248416 915004 806428 0 0 20 37600 367 7800 2 4 71 23 0
1 0 0 5981148 1070300 906120 0 0 88 64 243 19734 8 10 80 1 0
0 4 0 5875992 1165220 906700 0 0 80 62620 332 13192 5 7 76 13 0
0 4 0 5867244 1177440 906724 0 0 10 66728 385 2753 1 2 66 31 0
0 2 0 5792912 1252668 907900 0 0 86 26288 401 12130 4 6 72 18 0
2 0 0 5760280 1290860 908092 0 0 18 34704 487 7130 2 3 80 15 0
But in the "mixed workload" stage write throughtput slows down drastically:
0 2 0 5666016 468036 1823052 0 0 0 1248 433 8236 5 5 83 7 0
0 2 0 5666016 468036 1823052 0 0 0 3162 899 679 0 0 78 22 0
0 2 0 5666016 468036 1823052 0 0 0 3290 864 829 0 0 76 24 0
0 2 0 5666024 468036 1823052 0 0 0 3414 925 2170 1 1 79 19 0
1 1 0 5665528 468036 1823052 0 0 0 3148 854 7935 6 6 83 4 0
0 2 0 5665024 468036 1823052 0 0 0 2570 799 4065 3 4 80 13 0
0 2 0 5665024 468036 1823052 0 0 0 2462 779 464 0 0 88 12 0
0 2 0 5664908 468036 1823052 0 0 0 2458 794 1028 0 0 88 12 0
1 1 0 5664908 468036 1823052 0 0 0 2548 799 4153 2 2 87 9 0
Which can be seen in the iozone performance inside the guest:
Children see throughput for 4 mixed workload = 106073.50 KB/sec
Parent sees throughput for 4 mixed workload = 2560.13 KB/sec
Min throughput per process = 3700.33 KB/sec
Max throughput per process = 66393.62 KB/sec
Avg throughput per process = 26518.38 KB/sec
Min xfer = 44424.00 KB
And this is when fsync becomes nasty.
blktrace output shows that the maximum latency for a single write
request io complete is 1.5 seconds, which is similar to what is seen
under "writeback" mode.
I reduced hung_task_timeout_secs to 30 for this report, but vim and
rsyslogd have been seen hung up to 120 seconds.
INFO: task vim:4476 blocked for more than 30 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
vim D 00000001000e3c44 0 4476 3591
ffff8101da80dd28 0000000000000046 0000000000000000 00000000001cc549
ffff8101fe1e8a00 ffff81022f20e740 ffff8101fe1e8c30 0000000600000246
00000000ffffffff 0000000000000246 0000000000000000 0000000000000000
Call Trace:
[<ffffffff8030985b>] log_wait_commit+0xa9/0xfe
[<ffffffff80242347>] ? autoremove_wake_function+0x0/0x38
[<ffffffff80304c19>] journal_stop+0x1a4/0x1ed
[<ffffffff803056c6>] journal_force_commit+0x23/0x25
[<ffffffff802fe337>] ext3_force_commit+0x26/0x28
[<ffffffff802f8756>] ext3_write_inode+0x39/0x3f
[<ffffffff802a58e9>] __writeback_single_inode+0x18c/0x2ab
[<ffffffff802a5a2c>] sync_inode+0x24/0x39
[<ffffffff802f4baf>] ext3_sync_file+0x8f/0xa0
[<ffffffff802a892d>] do_fsync+0x56/0xac
[<ffffffff802a89b1>] __do_fsync+0x2e/0x44
[<ffffffff802a89e2>] sys_fsync+0xb/0xd
[<ffffffff8020b21b>] system_call_after_swapgs+0x7b/0x80
2 locks held by vim/4476:
#0: (&sb->s_type->i_mutex_key#9){--..}, at: [<ffffffff802a891c>] do_fsync+0x45/0xac
#1: (jbd_handle){--..}, at: [<ffffffff8030567c>] journal_start+0xef/0x116
2 locks held by rsyslogd/3266:
#0: (&sb->s_type->i_mutex_key#9){--..}, at: [<ffffffff802a891c>] do_fsync+0x45/0xac
#1: (jbd_handle){--..}, at: [<ffffffff8030567c>] journal_start+0xef/0x116
INFO: task rklogd:3269 blocked for more than 30 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
rklogd D 00000001000fbb55 0 3269 1
ffff8102280b9d28 0000000000000046 0000000000000000 00000000001cc553
ffff81022d9b0b40 ffff81022f196540 ffff81022d9b0d70 0000000400000246
00000000ffffffff 0000000000000246 0000000000000000 0000000000000000
Call Trace:
[<ffffffff8030985b>] log_wait_commit+0xa9/0xfe
[<ffffffff80242347>] ? autoremove_wake_function+0x0/0x38
[<ffffffff80304c19>] journal_stop+0x1a4/0x1ed
[<ffffffff803056c6>] journal_force_commit+0x23/0x25
[<ffffffff802fe337>] ext3_force_commit+0x26/0x28
[<ffffffff802f8756>] ext3_write_inode+0x39/0x3f
[<ffffffff802a58e9>] __writeback_single_inode+0x18c/0x2ab
[<ffffffff802a5a2c>] sync_inode+0x24/0x39
[<ffffffff802f4baf>] ext3_sync_file+0x8f/0xa0
[<ffffffff802a892d>] do_fsync+0x56/0xac
[<ffffffff802a89b1>] __do_fsync+0x2e/0x44
[<ffffffff802a89e2>] sys_fsync+0xb/0xd
[<ffffffff8020b21b>] system_call_after_swapgs+0x7b/0x80
qemu-system-x D 00000001000c1809 0 4359 4356
ffff8102111fd838 0000000000000046 0000000000000000 0000000000000046
ffff81021101c080 ffff81022f196540 ffff81021101c2b0 0000000400000002
00000000ffffffff ffffffff8024bc0b 0000000000000000 0000000000000000
Call Trace:
[<ffffffff8024bc0b>] ? mark_held_locks+0x4a/0x6a
[<ffffffff805593d1>] io_schedule+0x28/0x34
[<ffffffff803596dc>] get_request_wait+0x122/0x170
[<ffffffff8055b685>] ? _spin_unlock_irqrestore+0x3f/0x47
[<ffffffff80242347>] ? autoremove_wake_function+0x0/0x38
[<ffffffff80357872>] ? elv_merge+0x1df/0x221
[<ffffffff80359d27>] __make_request+0x33e/0x3dd
[<ffffffff80245115>] ? up_read+0x26/0x2a
[<ffffffff803585ad>] generic_make_request+0x33c/0x377
[<ffffffff802626f3>] ? mempool_alloc_slab+0x11/0x13
[<ffffffff803599e0>] submit_bio+0xce/0xd7
[<ffffffff802a970e>] submit_bh+0xea/0x10e
[<ffffffff802aca88>] sync_dirty_buffer+0x6e/0xd3
[<ffffffff80305e90>] journal_dirty_data+0xeb/0x1ce
[<ffffffff802f6805>] ext3_journal_dirty_data+0x18/0x41
[<ffffffff802f5b74>] walk_page_buffers+0x67/0x8e
[<ffffffff802f67ed>] ? ext3_journal_dirty_data+0x0/0x41
[<ffffffff802f84ad>] ext3_ordered_write_end+0x73/0x110
[<ffffffff8026117d>] generic_file_buffered_write+0x1c2/0x63f
[<ffffffff8024bda1>] ? trace_hardirqs_on+0xf1/0x115
[<ffffffff80233eed>] ? current_fs_time+0x22/0x29
[<ffffffff80261953>] __generic_file_aio_write_nolock+0x359/0x3c3
[<ffffffff80249d4d>] ? debug_mutex_free_waiter+0x46/0x4a
[<ffffffff80261a23>] generic_file_aio_write+0x66/0xc2
[<ffffffff802f4a1e>] ext3_file_write+0x1e/0x9e
[<ffffffff80289912>] do_sync_write+0xe2/0x126
[<ffffffff80242347>] ? autoremove_wake_function+0x0/0x38
[<ffffffff8024c6fa>] ? __lock_acquire+0x4e0/0xc0e
[<ffffffff80246881>] ? getnstimeofday+0x31/0x88
[<ffffffff8028a09b>] vfs_write+0xae/0x137
[<ffffffff8028a6f9>] sys_pwrite64+0x55/0x76
[<ffffffff8020b21b>] system_call_after_swapgs+0x7b/0x80
kjournald D 00000001000d0612 0 1252 2
ffff81022c66fcf0 0000000000000046 0000000000000000 ffff81022c4b6280
ffff81022e90e2c0 ffff81022f196540 ffff81022e90e4f0 000000042c66fd80
00000000ffffffff ffffffff8805d272 0000000000000000 0000000000000000
Call Trace:
[<ffffffff8805d272>] ? :dm_mod:dm_unplug_all+0x2d/0x31
[<ffffffff805593d1>] io_schedule+0x28/0x34
[<ffffffff802aa7e1>] sync_buffer+0x3e/0x42
[<ffffffff8055962e>] __wait_on_bit+0x45/0x77
[<ffffffff802aa7a3>] ? sync_buffer+0x0/0x42
[<ffffffff802aa7a3>] ? sync_buffer+0x0/0x42
[<ffffffff805596ce>] out_of_line_wait_on_bit+0x6e/0x7b
[<ffffffff8024237f>] ? wake_bit_function+0x0/0x2a
[<ffffffff802aa72b>] __wait_on_buffer+0x1f/0x21
[<ffffffff80306a02>] journal_commit_transaction+0x516/0xe70
[<ffffffff8055b685>] ? _spin_unlock_irqrestore+0x3f/0x47
[<ffffffff8024bda1>] ? trace_hardirqs_on+0xf1/0x115
As Peter mentioned it eventually gets out of this state (after several minutes) and fsync
instances complete.
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