linux-kernel - Re: v5.10.1 xfs deadlock

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Date:   Thu, 17 Dec 2020 22:30:37 +0100
From:   Donald Buczek <buczek@...gen.mpg.de>
To:     Brian Foster <bfoster@...hat.com>
Cc:     linux-xfs@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        it+linux-xfs@...gen.mpg.de
Subject: Re: v5.10.1 xfs deadlock

On 17.12.20 20:43, Brian Foster wrote:
> On Thu, Dec 17, 2020 at 06:44:51PM +0100, Donald Buczek wrote:
>> Dear xfs developer,
>>
>> I was doing some testing on a Linux 5.10.1 system with two 100 TB xfs filesystems on md raid6 raids.
>>
>> The stress test was essentially `cp -a`ing a Linux source repository with two threads in parallel on each filesystem.
>>
>> After about on hour, the processes to one filesystem (md1) blocked, 30 minutes later the process to the other filesystem (md0) did.
>>
>>      root      7322  2167  0 Dec16 pts/1    00:00:06 cp -a /jbod/M8068/scratch/linux /jbod/M8068/scratch/1/linux.018.TMP
>>      root      7329  2169  0 Dec16 pts/1    00:00:05 cp -a /jbod/M8068/scratch/linux /jbod/M8068/scratch/2/linux.019.TMP
>>      root     13856  2170  0 Dec16 pts/1    00:00:08 cp -a /jbod/M8067/scratch/linux /jbod/M8067/scratch/2/linux.028.TMP
>>      root     13899  2168  0 Dec16 pts/1    00:00:05 cp -a /jbod/M8067/scratch/linux /jbod/M8067/scratch/1/linux.027.TMP
>>
>> Some info from the system (all stack traces, slabinfo) is available here: https://owww.molgen.mpg.de/~buczek/2020-12-16.info.txt
>>
>> It stands out, that there are many (549 for md0, but only 10 for md1)  "xfs-conv" threads all with stacks like this
>>
>>      [<0>] xfs_log_commit_cil+0x6cc/0x7c0
>>      [<0>] __xfs_trans_commit+0xab/0x320
>>      [<0>] xfs_iomap_write_unwritten+0xcb/0x2e0
>>      [<0>] xfs_end_ioend+0xc6/0x110
>>      [<0>] xfs_end_io+0xad/0xe0
>>      [<0>] process_one_work+0x1dd/0x3e0
>>      [<0>] worker_thread+0x2d/0x3b0
>>      [<0>] kthread+0x118/0x130
>>      [<0>] ret_from_fork+0x22/0x30
>>
>> xfs_log_commit_cil+0x6cc is
>>
>>    xfs_log_commit_cil()
>>      xlog_cil_push_background(log)
>>        xlog_wait(&cil->xc_push_wait, &cil->xc_push_lock);
>>
>> Some other threads, including the four "cp" commands are also blocking at xfs_log_commit_cil+0x6cc
>>
>> There are also single "flush" process for each md device with this stack signature:
>>
>>      [<0>] xfs_map_blocks+0xbf/0x400
>>      [<0>] iomap_do_writepage+0x15e/0x880
>>      [<0>] write_cache_pages+0x175/0x3f0
>>      [<0>] iomap_writepages+0x1c/0x40
>>      [<0>] xfs_vm_writepages+0x59/0x80
>>      [<0>] do_writepages+0x4b/0xe0
>>      [<0>] __writeback_single_inode+0x42/0x300
>>      [<0>] writeback_sb_inodes+0x198/0x3f0
>>      [<0>] __writeback_inodes_wb+0x5e/0xc0
>>      [<0>] wb_writeback+0x246/0x2d0
>>      [<0>] wb_workfn+0x26e/0x490
>>      [<0>] process_one_work+0x1dd/0x3e0
>>      [<0>] worker_thread+0x2d/0x3b0
>>      [<0>] kthread+0x118/0x130
>>      [<0>] ret_from_fork+0x22/0x30
>>
>> xfs_map_blocks+0xbf is the
>>
>>      xfs_ilock(ip, XFS_ILOCK_SHARED);
>>
>> in xfs_map_blocks().
>>
>> The system is low on free memory
>>
>>      MemTotal:       197587764 kB
>>      MemFree:          2196496 kB
>>      MemAvailable:   189895408 kB
>>
>> but responsive.
>>
>> I have an out of tree driver for the HBA ( smartpqi 2.1.6-005 pulled from linux-scsi) , but it is unlikely that this blocking is related to that, because the md block devices itself are responsive (`xxd /dev/md0` )
>>
>> I can keep the system in the state for a while. Is there an idea what was going from or an idea what data I could collect from the running system to help? I have full debug info and could walk lists or retrieve data structures with gdb.
>>
> 
> It might be useful to dump the values under /sys/fs/xfs/<dev>/log/* for
> each fs to get an idea of the state of the logs as well...


root@...dbird:~# for f in /sys/fs/xfs/*/log/*; do echo $f : $(cat $f);done
/sys/fs/xfs/md0/log/log_head_lsn : 5:714808
/sys/fs/xfs/md0/log/log_tail_lsn : 5:581592
/sys/fs/xfs/md0/log/reserve_grant_head : 5:365981696
/sys/fs/xfs/md0/log/write_grant_head : 5:365981696
/sys/fs/xfs/md1/log/log_head_lsn : 3:2963880
/sys/fs/xfs/md1/log/log_tail_lsn : 3:2772656
/sys/fs/xfs/md1/log/reserve_grant_head : 3:1517506560
/sys/fs/xfs/md1/log/write_grant_head : 3:1517506560
/sys/fs/xfs/sda1/log/log_head_lsn : 233:106253
/sys/fs/xfs/sda1/log/log_tail_lsn : 233:106251
/sys/fs/xfs/sda1/log/reserve_grant_head : 233:54403812
/sys/fs/xfs/sda1/log/write_grant_head : 233:54403812
/sys/fs/xfs/sda2/log/log_head_lsn : 84:5653
/sys/fs/xfs/sda2/log/log_tail_lsn : 84:5651
/sys/fs/xfs/sda2/log/reserve_grant_head : 84:2894336
/sys/fs/xfs/sda2/log/write_grant_head : 84:2894336



> 
> Brian
> 
>> Best
>>    Donald
>>
> 

-- 
Donald Buczek
buczek@...gen.mpg.de
Tel: +49 30 8413 1433