Re: md_raid: mdX_raid6 looping after sync_action "check" to "idle" transition

[Donald Buczek <buczek@molgen.mpg.de>]

On 9/25/23 03:11, Yu Kuai wrote:
> Hi,
> 
> 在 2023/09/24 22:35, Donald Buczek 写道:
>> On 9/17/23 10:55, Donald Buczek wrote:
>>> On 9/14/23 08:03, Donald Buczek wrote:
>>>> On 9/13/23 16:16, Dragan Stancevic wrote:
>>>>> Hi Donald-
>>>>> [...]
>>>>> Here is a list of changes for 6.1:
>>>>>
>>>>> e5e9b9cb71a0 md: factor out a helper to wake up md_thread directly
>>>>> f71209b1f21c md: enhance checking in md_check_recovery()
>>>>> 753260ed0b46 md: wake up 'resync_wait' at last in md_reap_sync_thread()
>>>>> 130443d60b1b md: refactor idle/frozen_sync_thread() to fix deadlock
>>>>> 6f56f0c4f124 md: add a mutex to synchronize idle and frozen in action_store()
>>>>> 64e5e09afc14 md: refactor action_store() for 'idle' and 'frozen'
>>>>> a865b96c513b Revert "md: unlock mddev before reap sync_thread in action_store"
>>>>
>>>> Thanks!
>>>>
>>>> I've put these patches on v6.1.52. I've started a script which transitions the three md-devices of a very active backup server through idle->check->idle every 6 minutes a few ours ago.  It went through ~400 iterations till now. No lock-ups so far.
>>>
>>> Oh dear, looks like the deadlock problem is _not_fixed with these patches.
>>
>> Some more info after another incident:
>>
>> - We've hit the deadlock with 5.15.131 (so it is NOT introduced by any of the above patches)
>> - The symptoms are not exactly the same as with the original year-old problem. Differences:
>> - - mdX_raid6 is NOT busy looping
>> - - /sys/devices/virtual/block/mdX/md/array_state says "active" not "write pending"
>> - - `echo active > /sys/devices/virtual/block/mdX/md/array_state` does not resolve the deadlock
>> - - After hours in the deadlock state the system resumed operation when a script of mine read(!) lots of sysfs files.
>> - But in both cases, `echo idle > /sys/devices/virtual/block/mdX/md/sync_action` hangs as does all I/O operation on the raid.
>>
>> The fact that we didn't hit the problem for many month on 5.15.94 might hint that it was introduced between 5.15.94 and 5.15.131
>>
>> We'll try to reproduce the problem on a test machine for analysis, but this make take time (vacation imminent for one...).
>>
>> But its not like these patches caused the problem. Any maybe they _did_ fix the original problem, as we didn't hit that one.
> 
> Sorry for the late reply, yes, this looks like a different problem. I'm
> pretty confident that the orignal problem is fixed since that echo
> idle/frozen doesn't hold the lock 'reconfig_mutex' to wait for
> sync_thread to be done.
> 
> I'll check patches between 5.15.94 and 5.15.131.

We've got another event today. Some more information to save you work. I'm sorry, this comes dripping in, but as I said, currently we can't reproduce it and hit it on production machines only, where we have limited time to analyze:

* In the last two events, "echo idle > sys/devices/virtual/block/mdX/md/sync_action" was not even executing. This is not a trigger, but was a random victim when it happened the first time. This deceived me to believe this is some variation of the old problem.

* It's not filesystem related, yesterday `blkid -o value -s LABEL /dev/md1` was hanging, too, and today, for example, `df`.

* /sys/devices/virtual/block/md0/inflight today was (frozen at) "2      579"

* iotop showed no disk activity (on the raid) at all. Only a single member device had activity from time to time (usually after ~30 seconds, but sometimes after a few seconds) with usually 1-4 tps, but sometimes more, max 136 tps.

* As I said, I use a script to take a snapshot of various /sys and /proc information and running this script resolved the deadlock twice.

* The recorded stack traces of mdX_raid6 of the hanging raid recorded in the two events were

     [<0>] md_bitmap_unplug.part.0+0xce/0x100
     [<0>] raid5d+0xe4/0x5a0
     [<0>] md_thread+0xab/0x160
     [<0>] kthread+0x127/0x150
     [<0>] ret_from_fork+0x22/0x30

and

     [<0>] md_super_wait+0x72/0xa0
     [<0>] md_bitmap_unplug.part.0+0xce/0x100
     [<0>] raid5d+0xe4/0x5a0
     [<0>] md_thread+0xab/0x160
     [<0>] kthread+0x127/0x150
     [<0>] ret_from_fork+0x22/0x30

But note, that these probably were taken after the previous commands in the script already unfroze the system. Today I've manually looked at the stack while the system was still frozen, and it was just

     [<0>] md_thread+0x122/0x160
     [<0>] kthread+0x127/0x150
     [<0>] ret_from_fork+0x22/0x30

* Because I knew that my script seems to unblock the system, I've run it slowly line by line to see what actually unfreezes the system. There is one loop which takes "comm" "cmdline" and "stack" of all threads:

     for task in /proc/*/task/*; do
         echo  "# # $task: $(cat $task/comm) : $(cat $task/cmdline | xargs -0 echo)"
         cmd cat $task/stack
     done

I've added a few "read" to single-step it. Unfortunately, when it came to the 64 nfsd threads, I've got a bit impatient and hit "return" faster then I should have and when the unfreeze happened, I couldn't say exactly were it was triggered. But it must have been somewhere in this tail:

# # /proc/1299/task/1299: nfsd

[<0>] svc_recv+0x7a7/0x8c0 [sunrpc]
[<0>] nfsd+0xd6/0x140 [nfsd]
[<0>] kthread+0x127/0x150
[<0>] ret_from_fork+0x22/0x30

# # /proc/13/task/13: ksoftirqd/0

[<0>] smpboot_thread_fn+0xf3/0x140
[<0>] kthread+0x127/0x150
[<0>] ret_from_fork+0x22/0x30

# # /proc/130/task/130: cpuhp/22

[<0>] smpboot_thread_fn+0xf3/0x140
[<0>] kthread+0x127/0x150
[<0>] ret_from_fork+0x22/0x30

# # /proc/1300/task/1300: nfsd

[<0>] svc_recv+0x7a7/0x8c0 [sunrpc]
[<0>] nfsd+0xd6/0x140 [nfsd]
[<0>] kthread+0x127/0x150
[<0>] ret_from_fork+0x22/0x30

## (3 more repetitions of other nfsd threads which exactly the same stack skipped here ##

So it appears, that possibly a cat /proc/PID/stack of a "ksoftirqd" or a (maybe) a "cpuhp" thread unblocks the system. "nfsd" seems unlikely, as there shouldn't and wasn't anything nfs-mounted from this system.

Conclusion: This is probably not related to mdraid at all and might be a problem of the block or some infrastructure subsystem. Do you agree?

Best

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