On 8/17/23 3:49 AM, Jan Kara wrote:
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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...
Hi Jan
I think I found the thread that is holding the transaction handle. It
seems to be in runnable state though.
It has the journal_info set to the journal handle that has the matching
transaction as the journal's running transaction.
Here is the associated stack trace. It is converting unwritten extents
to extents.
PID: 287 TASK: ffff976801890000 CPU: 20 COMMAND: "kworker/u96:35"
#0 [ffffbd6c40b3f498] __schedule+617 at ffffffffbb912df9
#1 [ffffbd6c40b3f530] _cond_resched+38 at ffffffffbb9133e6
#2 [ffffbd6c40b3f538] shrink_lruvec+670 at ffffffffbb244d1e
#3 [ffffbd6c40b3f640] _cond_resched+21 at ffffffffbb9133d5
#4 [ffffbd6c40b3f648] shrink_node+552 at ffffffffbb2452a8
#5 [ffffbd6c40b3f6c8] do_try_to_free_pages+201 at ffffffffbb245829
#6 [ffffbd6c40b3f718] try_to_free_pages+239 at ffffffffbb246c0f
#7 [ffffbd6c40b3f7b0] __alloc_pages_slowpath.constprop.114+913 at
ffffffffbb28d741
#8 [ffffbd6c40b3f890] __alloc_pages_nodemask+679 at ffffffffbb28e2e7
#9 [ffffbd6c40b3f900] allocate_slab+726 at ffffffffbb2b0886
#10 [ffffbd6c40b3f958] ___slab_alloc+1173 at ffffffffbb2b3ff5
#11 [ffffbd6c40b3f988] insert_revoke_hash+37 at ffffffffc016f435 [jbd2]
(/home/ec2-user/linux/fs/jbd2/revoke.c:146)
#12 [ffffbd6c40b3f9b8] kmem_cache_free+924 at ffffffffbb2b712c )(inlined
by) slab_alloc at /home/ec2-user/linux/mm/slub.c:2904)
#13 [ffffbd6c40b3fa18] insert_revoke_hash+37 at ffffffffc016f435 [jbd2]
(/home/ec2-user/linux/fs/jbd2/revoke.c:146)
#14 [ffffbd6c40b3fa40] kmem_cache_alloc+928 at ffffffffbb2b4590
(/home/ec2-user/linux/mm/slub.c:290)
#15 [ffffbd6c40b3fa78] insert_revoke_hash+37 at ffffffffc016f435 [jbd2]
(/home/ec2-user/linux/fs/jbd2/revoke.c:146)
#16 [ffffbd6c40b3faa0] __ext4_forget+338 at ffffffffc02efb32 [ext4]
(/home/ec2-user/linux/fs/ext4/ext4_jbd2.c:298)
#17 [ffffbd6c40b3fae0] ext4_free_blocks+2437 at ffffffffc031fd55 [ext4]
(/home/ec2-user/linux/fs/ext4/mballoc.c:5709 (discriminator 2))
#18 [ffffbd6c40b3fbb0] ext4_ext_handle_unwritten_extents+596 at
ffffffffc02f56a4 [ext4] ((inlined by) ext4_ext_handle_unwritten_extents
at /home/ec2-user/linux/fs/ext4/extents.c:3892)
#19 [ffffbd6c40b3fc98] ext4_ext_map_blocks+1325 at ffffffffc02f710d
[ext4] (/home/ec2-user/linux/fs/ext4/extents.c:4165)
#20 [ffffbd6c40b3fd60] ext4_map_blocks+813 at ffffffffc030bd0d [ext4]
(/home/ec2-user/linux/fs/ext4/inode.c:659)
#21 [ffffbd6c40b3fdd0] ext4_convert_unwritten_extents+303 at
ffffffffc02f8adf [ext4] (/home/ec2-user/linux/fs/ext4/extents.c:4810)
#22 [ffffbd6c40b3fe28] ext4_convert_unwritten_io_end_vec+95 at
ffffffffc02f8c5f [ext4] (/home/ec2-user/linux/fs/ext4/extents.c:4850)
#23 [ffffbd6c40b3fe58] ext4_end_io_rsv_work+269 at ffffffffc032c3fd
[ext4] ((inlined by) ext4_do_flush_completed_IO at
/home/ec2-user/linux/fs/ext4/page-io.c:262)
#24 [ffffbd6c40b3fe98] process_one_work+405 at ffffffffbb0b2725
#25 [ffffbd6c40b3fed8] worker_thread+48 at ffffffffbb0b2920
#26 [ffffbd6c40b3ff10] kthread+279 at ffffffffbb0b9167
#27 [ffffbd6c40b3ff50] ret_from_fork+34 at ffffffffbb003802
Thanks
Rishabh
