Re: Tasks stuck jbd2 for a long time

[Jan Kara]

On Wed 16-08-23 15:53:05, Bhatnagar, Rishabh wrote:
> I collected dump and looked at some processes that were stuck in
> uninterruptible sleep.These are from upstream stable tree:
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/?h=linux-5.10.y
> (5.10.191)
> 
> One of them is the journal thread that is waiting for some other thread to
> close transaction handle.
> 
> PID: 10642  TASK: ffff9768823f4000  CPU: 37  COMMAND: "jbd2/md0-8"
>  #0 [ffffbd6c40c17c60] __schedule+617 at ffffffffbb912df9
>  #1 [ffffbd6c40c17cf8] schedule+60 at ffffffffbb91330c
>  #2 [ffffbd6c40c17d08] jbd2_journal_commit_transaction+877 at
> ffffffffc016b90d [jbd2] (/home/ec2-user/linux/fs/jbd2/commit.c:497)
>  #3 [ffffbd6c40c17ea0] kjournald2+282 at ffffffffc01723ba [jbd2]
> (/home/ec2-user/linux/fs/jbd2/journal.c:214)
>  #4 [ffffbd6c40c17f10] kthread+279 at ffffffffbb0b9167
>  #5 [ffffbd6c40c17f50] ret_from_fork+34 at ffffffffbb003802

Yes, correct. This is waiting for transaction->t_updates to drop to 0.

> One of threads that have started the handle and waiting for journal to
> commit and unlock the current transaction. This stack only shows
> ext4lazyinit but with lazyinit disabled we have seen other threads stuck in
> same place.
> 
> PID: 10644  TASK: ffff976901010000  CPU: 37  COMMAND: "ext4lazyinit"
>  #0 [ffffbd6c40c1fbe0] __schedule+617 at ffffffffbb912df9
>  #1 [ffffbd6c40c1fc78] schedule+60 at ffffffffbb91330c
>  #2 [ffffbd6c40c1fc88] wait_transaction_locked+137 at ffffffffc0168089
> [jbd2] (/home/ec2-user/linux/fs/jbd2/transaction.c:184)
>  #3 [ffffbd6c40c1fcd8] add_transaction_credits+62 at ffffffffc016813e [jbd2]
> (/home/ec2-user/linux/fs/jbd2/transaction.c:241)
>  #4 [ffffbd6c40c1fd30] start_this_handle+533 at ffffffffc0168615 [jbd2]
> (/home/ec2-user/linux/fs/jbd2/transaction.c:416)
>  #5 [ffffbd6c40c1fdc0] jbd2__journal_start+244 at ffffffffc0168dc4 [jbd2]
>  #6 [ffffbd6c40c1fe00] __ext4_journal_start_sb+250 at ffffffffc02ef65a
> [ext4]
>  #7 [ffffbd6c40c1fe40] ext4_init_inode_table+190 at ffffffffc0302ace [ext4]
>  #8 [ffffbd6c40c1feb0] ext4_lazyinit_thread+906 at ffffffffc033ec9a [ext4]
>  #9 [ffffbd6c40c1ff10] kthread+279 at ffffffffbb0b9167
> #10 [ffffbd6c40c1ff50] ret_from_fork+34 at ffffffffbb003802

This thread actually didn't start a transaction. It is *trying* to start a
transaction but it has failed and we are now waiting for transaction commit
to proceed (i.e., for jbd2/md0-8 process). So this isn't the process jbd2
is waiting for.

> To replicate the download scenario i'm just using dd to copy random data to
> disk. I launch a bunch of threads and try to stress the system. Many of
> those threads seem to be stuck in balance_dirty_pages_ratelimited as can be
> seen below.
> 
> PID: 10709  TASK: ffff9769016f8000  CPU: 25  COMMAND: "dd"
>  #0 [ffffbd6c40dafa48] __schedule+617 at ffffffffbb912df9
>  #1 [ffffbd6c40dafae0] schedule+60 at ffffffffbb91330c
>  #2 [ffffbd6c40dafaf0] schedule_timeout+570 at ffffffffbb916a7a
>  #3 [ffffbd6c40dafb68] io_schedule_timeout+25 at ffffffffbb913619 ((inlined
> by) io_schedule_finish at /home/ec2-user/linux/kernel/sched/core.c:6274)
>  #4 [ffffbd6c40dafb80] balance_dirty_pages+654 at ffffffffbb2367ce 
> (/home/ec2-user/linux/mm/page-writeback.c:1799)
>  #5 [ffffbd6c40dafcf0] balance_dirty_pages_ratelimited+763 at
> ffffffffbb23752b  (/home/ec2-user/linux/mm/page-writeback.c:1926)
>  #6 [ffffbd6c40dafd18] generic_perform_write+308 at ffffffffbb22af44
> (/home/ec2-user/linux/mm/filemap.c:3370)
>  #7 [ffffbd6c40dafd88] ext4_buffered_write_iter+161 at ffffffffc02fcba1
> [ext4] (/home/ec2-user/linux/fs/ext4/file.c:273)
>  #8 [ffffbd6c40dafdb8] ext4_file_write_iter+96 at ffffffffc02fccf0 [ext4]
>  #9 [ffffbd6c40dafe40] new_sync_write+287 at ffffffffbb2e0c0f
> #10 [ffffbd6c40dafec8] vfs_write+481 at ffffffffbb2e3161
> #11 [ffffbd6c40daff00] ksys_write+165 at ffffffffbb2e3385
> #12 [ffffbd6c40daff40] do_syscall_64+51 at ffffffffbb906213
> #13 [ffffbd6c40daff50] entry_SYSCALL_64_after_hwframe+103 at
> ffffffffbba000df

Yes, this is waiting for page writeback to reduce amount of dirty pages in
the pagecache. We are not holding transaction handle during this wait so
this is also not the task jbd2 is waiting for.

> There are other dd threads that are trying to read and are handling page
> fault. These are in runnable state and not uninterruptible sleep.
> 
> PID: 14581  TASK: ffff97c3cfdbc000  CPU: 29  COMMAND: "dd"
>  #0 [ffffbd6c4a1d3598] __schedule+617 at ffffffffbb912df9
>  #1 [ffffbd6c4a1d3630] _cond_resched+38 at ffffffffbb9133e6
>  #2 [ffffbd6c4a1d3638] shrink_page_list+126 at ffffffffbb2412fe
>  #3 [ffffbd6c4a1d36c8] shrink_inactive_list+478 at ffffffffbb24441e
>  #4 [ffffbd6c4a1d3768] shrink_lruvec+957 at ffffffffbb244e3d
>  #5 [ffffbd6c4a1d3870] shrink_node+552 at ffffffffbb2452a8
>  #6 [ffffbd6c4a1d38f0] do_try_to_free_pages+201 at ffffffffbb245829
>  #7 [ffffbd6c4a1d3940] try_to_free_pages+239 at ffffffffbb246c0f
>  #8 [ffffbd6c4a1d39d8] __alloc_pages_slowpath.constprop.114+913 at
> ffffffffbb28d741
>  #9 [ffffbd6c4a1d3ab8] __alloc_pages_nodemask+679 at ffffffffbb28e2e7
> #10 [ffffbd6c4a1d3b28] alloc_pages_vma+124 at ffffffffbb2a734c
> #11 [ffffbd6c4a1d3b68] handle_mm_fault+3999 at ffffffffbb26de2f
> #12 [ffffbd6c4a1d3c28] exc_page_fault+708 at ffffffffbb909c84
> #13 [ffffbd6c4a1d3c80] asm_exc_page_fault+30 at ffffffffbba00b4e
>  #14 [ffffbd6c4a1d3d30] copyout+28 at ffffffffbb5160bc
> #15 [ffffbd6c4a1d3d38] _copy_to_iter+158 at ffffffffbb5188de
> #16 [ffffbd6c4a1d3d98] get_random_bytes_user+136 at ffffffffbb644608
> #17 [ffffbd6c4a1d3e48] new_sync_read+284 at ffffffffbb2e0a5c
> #18 [ffffbd6c4a1d3ed0] vfs_read+353 at ffffffffbb2e2f51
> #19 [ffffbd6c4a1d3f00] ksys_read+165 at ffffffffbb2e3265
> #20 [ffffbd6c4a1d3f40] do_syscall_64+51 at ffffffffbb906213
> #21 [ffffbd6c4a1d3f50] entry_SYSCALL_64_after_hwframe+103 at
> ffffffffbba000df

This process is in direct reclaim trying to free more memory. It doesn't
have transaction handle started so jbd2 also isn't waiting for this
process.

> > > 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.
> Amongst the things we have tested 2 things seem to give good improvements.
> 
> One is disabling journalling. We don't see any stuck tasks. System becomes
> slow but eventually recovers. But its not something we want to disable.
> 
> Other is enabling swap memory. Adding some swap memory also avoids system
> going into low memory state and system doesn't freeze.

OK, these are just workarounds. The question really is which process holds
the transaction handle jbd2 thread is waiting for. It is none of the
processes you have shown above. Since you have the crashdump, you can also
search all the processes and find those which have non-zero
task->journal_info. And from these processes you can select those where
task->journal_info points to an object from jbd2_handle_cache and then you
can verify whether the handles indeed point (through handle->h_transaction)
to the transaction jbd2 thread is trying to commit. After you've identified
such task it is interesting to see what is it doing...

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