Re: v5.10.1 xfs deadlock
From: Donald Buczek
Date: Fri Dec 18 2020 - 13:36:40 EST
On 18.12.20 16:35, Brian Foster wrote:
On Thu, Dec 17, 2020 at 10:30:37PM +0100, Donald Buczek wrote:
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
Do you have any indication of whether these workloads actually hung or
just became incredibly slow?
There is zero progress. iostat doesn't show any I/O on any of the block devices (md or member)
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);
This looks like the transaction commit throttling code. That was
introduced earlier this year in v5.7 via commit 0e7ab7efe7745 ("xfs:
Throttle commits on delayed background CIL push"). The purpose of that
change was to prevent the CIL from growing too large. FWIW, I don't
recall that being a functional problem so it should be possible to
simply remove that blocking point and see if that avoids the problem or
if we simply stall out somewhere else, if you wanted to give that a
test.
Will do. Before trying with this commit reverted, I will repeat the test without any change to see if the problem is reproducible at all.
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
Is writeback still blocked as such or was this just a transient stack?
This is frozen. I did another collection of the stack dump today and all the stack trace (related to these two filesystems) are unchanged.
However, I discovered, that my original report was not correct. The above stack trace is from PID 348 ("kworker/u81:8+flush-9:0") while the other thread for the other device (PID 20, "kworker/u82:0+flush-9:1" doesn't have an identical thread, it has three more frames and is blocking at xfs_log_commit_cil+0x6cc as most other processes.
[<0>] xfs_log_commit_cil+0x6cc/0x7c0
[<0>] __xfs_trans_commit+0xab/0x320
[<0>] xfs_bmapi_convert_delalloc+0x437/0x4b0
[<0>] xfs_map_blocks+0x1e3/0x400
[<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@deadbird:~# for f in /sys/fs/xfs/*/log/*; do echo $f : $(cat $f);done
/sys/fs/xfs/md0/log/log_head_lsn : 5:714808
/sys/fs/xfs/md0/log/log_tail_lsn : 5:581592
/sys/fs/xfs/md0/log/reserve_grant_head : 5:365981696
/sys/fs/xfs/md0/log/write_grant_head : 5:365981696
Hm, so it looks like the log is populated but not necessarily full. What
looks more interesting is that the grant heads (365981696 bytes) line up
with the physical log head (714808 512b sectors). That suggests there is
no outstanding transaction reservation and thus perhaps all workload
tasks are sitting at that throttling point just after the current
transaction commits and releases unused reservation. That certainly
shouldn't be such a longstanding blocking point as it only waits for the
CIL push to start.
Out of curiosity, have any of the above values changed since the sample
provided here was collected? As above, I'm curious if the filesystem
happens to be moving along slowly or not at all, whether the AIL has
been drained in the background, etc.
Zero I/O on the block devices.
All related thread have exact same stack frame after >2days
Could you post the xfs_info for the affected filesystems?
buczek@deadbird:~/linux_problems/mdX_raid6_looping/tests_with_deadbird/2020-12-16-01$ xfs_info /dev/md0
meta-data=/dev/md0 isize=512 agcount=102, agsize=268435328 blks
= sectsz=4096 attr=2, projid32bit=1
= crc=1 finobt=1, sparse=1, rmapbt=0
= reflink=1
data = bsize=4096 blocks=27348629504, imaxpct=1
= sunit=128 swidth=1792 blks
naming =version 2 bsize=4096 ascii-ci=0, ftype=1
log =internal log bsize=4096 blocks=521728, version=2
= sectsz=4096 sunit=1 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
buczek@deadbird:~/linux_problems/mdX_raid6_looping/tests_with_deadbird/2020-12-16-01$ xfs_info /dev/md1
meta-data=/dev/md1 isize=512 agcount=102, agsize=268435328 blks
= sectsz=4096 attr=2, projid32bit=1
= crc=1 finobt=1, sparse=1, rmapbt=0
= reflink=1
data = bsize=4096 blocks=27348629504, imaxpct=1
= sunit=128 swidth=1792 blks
naming =version 2 bsize=4096 ascii-ci=0, ftype=1
log =internal log bsize=4096 blocks=521728, version=2
= sectsz=4096 sunit=1 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
Also since it seems like you should have plenty of available log
reservation, are you able to perform any writable operations on the fs
(i.e., touch <file>)? If so, I wonder if you were able to start a new
copy workload on of the fs' capable of triggering the blocking threshold
again, if that might eventually unstick the currently blocked tasks when
the next CIL push occurs...
When I do something like
touch /jbod/M8067/scratch/x.001
This doesn't block and shortly thereafter I see momentary write activity on the md and member devices. But the blocked processes don't resume.
root@deadbird:~# iostat|grep md0 ; sleep 30 ; iostat|grep md0 ; touch /jbod/M8067/scratch/x.008 ; sleep 30 ; iostat|grep md0 ; sleep 30 ; iostat|grep md0
md0 25.81 52.45 2385.56 0.00 12806780 582460904 0
md0 25.81 52.45 2385.27 0.00 12806780 582460904 0
md0 25.81 52.44 2384.98 0.00 12806780 582460908 0
md0 25.80 52.43 2384.68 0.00 12806780 582460908 0
The touched files are visible with `ls`.
So then I tried something longer:
cp -a /jbod/M8067/scratch/linux /jbod/M8067/scratch/linux.001
which also triggerd a few I/Os in the beginning but then came to a perceived halt (no I/O in several minutes).
Interestingly, this new "cp" is not blocking on xfs_log_commit_cil+0x6cc as the four "cp" commands from the stress test but has this stack:
[<0>] balance_dirty_pages+0x31c/0xd80
[<0>] balance_dirty_pages_ratelimited+0x2f9/0x3c0
[<0>] iomap_write_actor+0x11d/0x190
[<0>] iomap_apply+0x117/0x2e0
[<0>] iomap_file_buffered_write+0x62/0x90
[<0>] xfs_file_buffered_aio_write+0xd3/0x320
[<0>] new_sync_write+0x11f/0x1b0
[<0>] vfs_write+0x1ea/0x250
[<0>] ksys_write+0xa1/0xe0
[<0>] do_syscall_64+0x33/0x40
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
Maybe this would change if I waited a few hours more.
So I will reboot the system now and retry test test exactly the same configuration as then again with 0e7ab7efe7745 ("xfs: Throttle commits on delayed background CIL push") reverted.
Donald
Brian
/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@xxxxxxxxxxxxx
Tel: +49 30 8413 1433
--
Donald Buczek
buczek@xxxxxxxxxxxxx
Tel: +49 30 8413 1433