Re: Tasks stuck jbd2 for a long time

[Bhatnagar, Rishabh]

On 8/16/23 7:53 AM, Jan Kara wrote:
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> On Tue 15-08-23 20:57:14, Bhatnagar, Rishabh wrote:
> > On 8/15/23 7:28 PM, Theodore Ts'o wrote:
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> > >
> > >
> > > It would be helpful if you can translate address in the stack trace to
> > > line numbers.  See [1] and the script in
> > > ./scripts/decode_stacktrace.sh in the kernel sources.  (It is
> > > referenced in the web page at [1].)
> > >
> > > [1] https://docs.kernel.org/admin-guide/bug-hunting.html
> > >
> > > Of course, in order to interpret the line numbers, we'll need a
> > > pointer to the git repo of your kernel sources and the git commit ID
> > > you were using that presumably corresponds to 5.10.184-175.731.amzn2.x86_64.
> > >
> > > The stack trace for which I am particularly interested is the one for
> > > the jbd2/md0-8 task, e.g.:
> > Thanks for checking Ted.
> >
> > We don't have fast_commit feature enabled. So it should correspond to this
> > line:
> > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/fs/jbd2/commit.c?h=linux-5.10.y#n496
> >
> > > >         Not tainted 5.10.184-175.731.amzn2.x86_64 #1
> > > > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > > > task:jbd2/md0-8      state:D stack:    0 pid: 8068 ppid:     2
> > > > flags:0x00004080
> > > > Call Trace:
> > > > __schedule+0x1f9/0x660
> > > >    schedule+0x46/0xb0
> > > >    jbd2_journal_commit_transaction+0x35d/0x1880 [jbd2]  <--------- line #?
> > > >    ? update_load_avg+0x7a/0x5d0
> > > >    ? add_wait_queue_exclusive+0x70/0x70
> > > >    ? lock_timer_base+0x61/0x80
> > > >    ? kjournald2+0xcf/0x360 [jbd2]
> > > >    kjournald2+0xcf/0x360 [jbd2]
> > > Most of the other stack traces you refenced are tasks that are waiting
> > > for the transaction commit to complete so they can proceed with some
> > > file system operation.  The stack traces which have
> > > start_this_handle() in them are examples of this going on.  Stack
> > > traces of tasks that do *not* have start_this_handle() would be
> > > specially interesting.
> > I see all other stacks apart from kjournald have "start_this_handle".
> That would be strange. Can you post full output of "echo w
> > /proc/sysrq-trigger" to dmesg, ideally passed through scripts/faddr2line as
> Ted suggests. Thanks!

Sure i'll try to collect that. The system freezes when such a situation 
happens and i'm not able
to collect much information. I'll try to crash the kernel and collect 
kdump and see if i can get that info.

Can low available memory be a reason for a thread to not be able to 
close the transaction handle for a long time?
Maybe some writeback thread starts the handle but is not able to 
complete writeback?

>                                                                  Honza
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR