Hi,
We've run into an odd bug and I'm looking for some advice on how to
proceed. Please excuse the cross-posting, I wasn't sure where this
belongs. Any guidance would be greatly appreciated.
The problem we're seeing is that by copying a large (10GB) file to an
nfs-mounted directory we can cause all the nfsd threads to hang in D
state on the server.
Background:
We've got a couple of systems running 3.4.97, with the "rt120" patch for
CONFIG_PREEMPT_RT. On the server we've got an nfs-exported ext4
filesystem, on top of drbd (used for mirroring over the network via
protocol C), on top of LVM (simple vg on top of pv, nothing fancy), on
top of a SATA SSD.
Stuff we've ruled out:
With the above system within 1-3 iterations of copying the file we can
cause a hang. If we switch to spinning SATA disks we were unable to
trigger the problem. However, if we run within virtualbox with virtual
PIIX4 IDE disks then we can reproduce the problem. This seems to point
to a timing issue as opposed to a hardware issue.
We've confirmed that we can reproduce the problem with drbd out of the
picture (NFS-exported ext4 on LVM on virtual disks in virtualbox) but it
takes longer than with the drbd disks.
We've confirmed that we can reproduce the problem with both "deadline"
and "noop" I/O schedulers.
Stuff still to try:
We're working on setting up a minimal test-case with both the RT and
standard kernels. As it stands there are a lot of other components
running that make it harder to rebuild with different kernel options.
Details:
These are the processes we saw stuck in disk sleep:
$ ps aux | grep D
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1797 4.2 0.0 0 0 ? D 19:46 9:51 [nfsd]
root 1798 4.2 0.0 0 0 ? D 19:46 9:54 [nfsd]
root 1799 4.2 0.0 0 0 ? D 19:46 9:54 [nfsd]
root 1800 4.2 0.0 0 0 ? D 19:46 9:52 [nfsd]
root 1801 4.2 0.0 0 0 ? D 19:46 9:53 [nfsd]
root 1802 4.2 0.0 0 0 ? D 19:46 9:52 [nfsd]
root 1803 4.2 0.0 0 0 ? D 19:46 9:52 [nfsd]
root 1804 4.2 0.0 0 0 ? D 19:46 9:52 [nfsd]
root 5305 0.0 0.0 0 0 ? D 19:49 0:03
[jbd2/drbd3-8]
root 5634 0.0 0.0 0 0 ? D 19:49 0:05
[flush-147:3]
Most of the nfsd threads are stuck waiting for the mutex on an inode in
order to do a write to the disk. For example, the traceback for pid 1797
[nfsd]:
[<ffffffff8110fd46>] generic_file_aio_write+0x56/0xe0
[<ffffffff8120591f>] ext4_file_write+0xbf/0x250
[<ffffffff811689ab>] do_sync_readv_writev+0xdb/0x120
[<ffffffff81168c94>] do_readv_writev+0xd4/0x1d0
[<ffffffff81168dcc>] vfs_writev+0x3c/0x50
[<ffffffffa01701b8>] nfsd_vfs_write.isra.12+0xf8/0x2e0 [nfsd]
[<ffffffffa0172888>] nfsd_write+0xf8/0x110 [nfsd]
[<ffffffffa0178b5f>] nfsd3_proc_write+0x9f/0xe0 [nfsd]
[<ffffffffa016cade>] nfsd_dispatch+0xbe/0x1c0 [nfsd]
[<ffffffff81871679>] svc_process+0x499/0x790
[<ffffffffa016c125>] nfsd+0xc5/0x1a0 [nfsd]
[<ffffffff810609bb>] kthread+0xdb/0xe0
[<ffffffff8193b904>] kernel_thread_helper+0x4/0x10
Where generic_file_aio_write() is calling mutex_lock(&inode->i_mutex).
Looking through the other tracebacks, it appears that pid 1803 [nfsd]
probably has this mutex. Also, looking at the block_start in
/proc/1803/sched (, it appears that this was the first process to block:
se.statistics.block_start : 13948189.066634
Its traceback looks like this:
[<ffffffff8125dc55>] jbd2_log_wait_commit+0xc5/0x150
[<ffffffff8125f628>] jbd2_complete_transaction+0xb8/0xd0
[<ffffffff81205dcd>] ext4_sync_file+0x1fd/0x4b0
[<ffffffff81197ad5>] generic_write_sync+0x55/0x70
[<ffffffff8110fdb6>] generic_file_aio_write+0xc6/0xe0
[<ffffffff8120591f>] ext4_file_write+0xbf/0x250
[<ffffffff811689ab>] do_sync_readv_writev+0xdb/0x120
[<ffffffff81168c94>] do_readv_writev+0xd4/0x1d0
[<ffffffff81168dcc>] vfs_writev+0x3c/0x50
[<ffffffffa01701b8>] nfsd_vfs_write.isra.12+0xf8/0x2e0 [nfsd]
[<ffffffffa0172888>] nfsd_write+0xf8/0x110 [nfsd]
[<ffffffffa0178b5f>] nfsd3_proc_write+0x9f/0xe0 [nfsd]
[<ffffffffa016cade>] nfsd_dispatch+0xbe/0x1c0 [nfsd]
[<ffffffff81871679>] svc_process+0x499/0x790
[<ffffffffa016c125>] nfsd+0xc5/0x1a0 [nfsd]
[<ffffffff810609bb>] kthread+0xdb/0xe0
[<ffffffff8193b904>] kernel_thread_helper+0x4/0x10
The inode-i_mutex seems to be taken in ext4_sync_file() before the call
to jbd2_complete_transaction().
It looks like jbd2_log_wait_commit() has just called schedule() in
wait_event() in the code below:
while (tid_gt(tid, journal->j_commit_sequence)) {
jbd_debug(1, "JBD2: want %d, j_commit_sequence=%d\n",
tid, journal->j_commit_sequence);
wake_up(&journal->j_wait_commit);
read_unlock(&journal->j_state_lock);
wait_event(journal->j_wait_done_commit,
!tid_gt(tid, journal->j_commit_sequence));
read_lock(&journal->j_state_lock);
}
The kjournald2 thread (pid 5305) blocks significantly later:
se.statistics.block_start : 13953712.751778
Here is the traceback:
[<ffffffff8110e3ae>] sleep_on_page+0xe/0x20
[<ffffffff8110e5b8>] wait_on_page_bit+0x78/0x80
[<ffffffff8110e81c>] filemap_fdatawait_range+0x10c/0x1b0
[<ffffffff8110e8eb>] filemap_fdatawait+0x2b/0x30
[<ffffffff8125832c>] jbd2_journal_commit_transaction+0x8dc/0x1a70
This calls journal_finish_inode_data_buffers which calls
filemap_fdatawait()
[<ffffffff8125ddaf>] kjournald2+0xbf/0x280
[<ffffffff810609bb>] kthread+0xdb/0xe0
[<ffffffff8193b904>] kernel_thread_helper+0x4/0x10
It is stuck in jbd2_journal_commit_transaction, apparently waiting for
inodes to be written to disk? I'm not sure what would be preventing that
from happening.
One of the nfsd processes is stuck in a different spot:
root 1804 2 3 Oct15 ? 00:09:52 [nfsd]
This is the second process to block:
se.statistics.block_start : 13948189.759783
And the traceback:
[<ffffffff81256f85>] do_get_write_access+0x2b5/0x4e0
[<ffffffff81257300>] jbd2_journal_get_write_access+0x30/0x50
[<ffffffff8123848e>] __ext4_journal_get_write_access+0x3e/0x80
[<ffffffff8123efaf>] ext4_mb_mark_diskspace_used+0xff/0x530
[<ffffffff8124074f>] ext4_mb_new_blocks+0x1ef/0x5f0
[<ffffffff812361d5>] ext4_ext_map_blocks+0xc05/0x1e60
[<ffffffff8120ae15>] ext4_map_blocks+0x1c5/0x270
[<ffffffff8120f0dc>] mpage_da_map_and_submit+0xac/0x4c0
[<ffffffff8120fd5e>] ext4_da_writepages+0x36e/0x720
[<ffffffff8111a5f4>] do_writepages+0x24/0x40
[<ffffffff8110f5db>] __filemap_fdatawrite_range+0x5b/0x60
[<ffffffff8110f6ea>] filemap_write_and_wait_range+0x5a/0x80
[<ffffffff81205c48>] ext4_sync_file+0x78/0x4b0
[<ffffffff81197ad5>] generic_write_sync+0x55/0x70
[<ffffffff8110fdb6>] generic_file_aio_write+0xc6/0xe0
[<ffffffff8120591f>] ext4_file_write+0xbf/0x250
[<ffffffff811689ab>] do_sync_readv_writev+0xdb/0x120
[<ffffffff81168c94>] do_readv_writev+0xd4/0x1d0
[<ffffffff81168dcc>] vfs_writev+0x3c/0x50
[<ffffffffa01701b8>] nfsd_vfs_write.isra.12+0xf8/0x2e0 [nfsd]
[<ffffffffa0172888>] nfsd_write+0xf8/0x110 [nfsd]
[<ffffffffa0178b5f>] nfsd3_proc_write+0x9f/0xe0 [nfsd]
[<ffffffffa016cade>] nfsd_dispatch+0xbe/0x1c0 [nfsd]
[<ffffffff81871679>] svc_process+0x499/0x790
[<ffffffffa016c125>] nfsd+0xc5/0x1a0 [nfsd]
[<ffffffff810609bb>] kthread+0xdb/0xe0
[<ffffffff8193b904>] kernel_thread_helper+0x4/0x10
I'm pretty sure it called schedule() below in the code from
do_get_write_access():
if (jh->b_jlist == BJ_Shadow) {
DEFINE_WAIT_BIT(wait, &bh->b_state, BH_Unshadow);
wait_queue_head_t *wqh;
wqh = bit_waitqueue(&bh->b_state, BH_Unshadow);
JBUFFER_TRACE(jh, "on shadow: sleep");
jbd_unlock_bh_state(bh);
/* commit wakes up all shadow buffers after IO */
for ( ; ; ) {
prepare_to_wait(wqh, &wait.wait,
TASK_UNINTERRUPTIBLE);
if (jh->b_jlist != BJ_Shadow)
break;
schedule();
}
finish_wait(wqh, &wait.wait);
goto repeat;
}
And finally, there is a flush stuck as well:
root 5634 2 0 Oct15 ? 00:00:05 [flush-147:3]
This is the third process to block:
se.statistics.block_start : 13948189.814929
And the traceback:
[<ffffffff8109cd0c>] rt_down_read+0x2c/0x40
[<ffffffff8120ac91>] ext4_map_blocks+0x41/0x270
[<ffffffff8120f0dc>] mpage_da_map_and_submit+0xac/0x4c0
[<ffffffff8120f9c9>] write_cache_pages_da+0x3f9/0x420
[<ffffffff8120fd30>] ext4_da_writepages+0x340/0x720
[<ffffffff8111a5f4>] do_writepages+0x24/0x40
[<ffffffff81191b71>] writeback_single_inode+0x181/0x4b0
[<ffffffff811922a2>] writeback_sb_inodes+0x1b2/0x290
[<ffffffff8119241e>] __writeback_inodes_wb+0x9e/0xd0
[<ffffffff811928e3>] wb_writeback+0x223/0x3f0
[<ffffffff81192b4f>] wb_check_old_data_flush+0x9f/0xb0
[<ffffffff8119403f>] wb_do_writeback+0x12f/0x250
[<ffffffff811941f4>] bdi_writeback_thread+0x94/0x320
[<ffffffff810609bb>] kthread+0xdb/0xe0
[<ffffffff8193b904>] kernel_thread_helper+0x4/0x10
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