Re: Tasks stuck jbd2 for a long time

[Jan Kara <jack@suse.cz>]

On Wed 16-08-23 11:32:47, Bhatnagar, Rishabh wrote:
> On 8/16/23 7:53 AM, Jan Kara wrote:
> > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> > On Tue 15-08-23 20:57:14, Bhatnagar, Rishabh wrote:
> > > On 8/15/23 7:28 PM, Theodore Ts'o wrote:
> > > > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> > > > 
> > > > 
> > > > 
> > > > 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.

Thanks!

> 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?

Well, even that would be a bug but low memory conditions are certainly some
of less tested paths so it is possible there's a bug lurking there.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR