Re: bisected: btrfs dedupe regression in v5.11-rc1

[Thorsten Leemhuis <regressions@leemhuis.info>]

[Adding Chris and David to the list of recipients]

Hi, this is your Linux kernel regression tracker again.

@Btrfs-Maintainers: is anyone addressing this regression that got
bisected many weeks ago? It from here looks like things are stuck: Libor
asked for a status update 24 days ago, I did the same 15 days ago, but
neither of us got a answer. Is there some good reason for this? Or did
the discussion continue somewhere else?

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I'm getting a lot of
reports on my table. I can only look briefly into most of them and lack
knowledge about most of the areas they concern. I thus unfortunately
will sometimes get things wrong or miss something important. I hope
that's not the case here; if you think it is, don't hesitate to tell me
in a public reply, it's in everyone's interest to set the public record
straight.

#regzbot poke

On 18.02.22 15:46, Thorsten Leemhuis wrote:
> Hi, this is your Linux kernel regression tracker speaking. Top-posting
> for once, to make this easy accessible to everyone.
> 
> What's up here? Can anyone (Zygo? Josef?) please provide a status
> update? Yes, it took quite a while till this regression got found, but
> nevertheless this looks to take awfully long to get resolved for a
> regression was bisected weeks ago.
> 
> Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
> 
> P.S.: As the Linux kernel's regression tracker I'm getting a lot of
> reports on my table. I can only look briefly into most of them and lack
> knowledge about most of the areas they concern. I thus unfortunately
> will sometimes get things wrong or miss something important. I hope
> that's not the case here; if you think it is, don't hesitate to tell me
> in a public reply, it's in everyone's interest to set the public record
> straight.
> 
> 
> On 21.01.22 01:27, Zygo Blaxell wrote:
>> On Thu, Jan 20, 2022 at 03:04:19PM +0100, Thorsten Leemhuis wrote:
>>> Hi, this is your Linux kernel regression tracker speaking.
>>>
>>> On 07.01.22 19:31, Zygo Blaxell wrote:
>>>> On Fri, Dec 17, 2021 at 03:50:44PM -0500, Zygo Blaxell wrote:
>>>> I left my VM running tests for a few weeks and got some more information.
>>>> Or at least more data, I'm not feeling particularly informed by it.  :-P
>>>>
>>>> 1.  It's not a regression.  5.10 has the same symptoms, but about 100x
>>>> less often (once a week under these test conditions, compared to once
>>>> every 90 minutes or so on 5.11-rc1).
>>>
>>> Well, I'd still call it a regression, as it's now happening way more
>>> often and thus will likely hit more users. It's thus a bit like a
>>> problem that leads to higher energy consumption: things still work, but
>>> worse than before -- nevertheless it's considered a regression. Anway:
>>>
>>> What's the status here? Are you still investigating the issue? Are any
>>> developers looking out for the root cause?
>>
>> I think Josef's plan (start inside the logical_ino ioctl with bpftrace
>> and work upwards to see where the looping is getting stuck) is a good plan,
>> but due to conflicting priorities I haven't found the time to act on it.
>>
>> I can take experimental patches and throw them at my repro setup if
>> anyone would like to supply some.
>>
>>> Ciao, Thorsten
>>>
>>> P.S.: As a Linux kernel regression tracker I'm getting a lot of reports
>>> on my table. I can only look briefly into most of them. Unfortunately
>>> therefore I sometimes will get things wrong or miss something important.
>>> I hope that's not the case here; if you think it is, don't hesitate to
>>> tell me about it in a public reply, that's in everyone's interest.
>>>
>>> BTW, I have no personal interest in this issue, which is tracked using
>>> regzbot, my Linux kernel regression tracking bot
>>> (https://linux-regtracking.leemhuis.info/regzbot/). I'm only posting
>>> this mail to get things rolling again and hence don't need to be CC on
>>> all further activities wrt to this regression.
>>>
>>> #regzbot poke
>>>
>>>> 2.  Bisection doesn't work, because there are patches that are repeatably
>>>> good and bad mixed together, so the bisect algorithm (even with stochastic
>>>> enhancement) repeatably picks the wrong commits and converges with
>>>> high confidence on nonsense.  Instead of bisecting, I picked commits
>>>> semi-randomly from 5.11-rc1's patch set, and got these results:
>>>>
>>>>    124  3a160a933111 btrfs: drop never met disk total bytes check in verify_one_dev_extent
>>>> 	1x hang, 2x slower
>>>>    125  bacce86ae8a7 btrfs: drop unused argument step from btrfs_free_extra_devids
>>>> 	1x pass (fast)
>>>>    126  2766ff61762c btrfs: update the number of bytes used by an inode atomically
>>>> 	1x hang (<20 minutes)
>>>>    127  7f458a3873ae btrfs: fix race when defragmenting leads to unnecessary IO
>>>> 	1x hang, runs 3x slower
>>>>    128  5893dfb98f25 btrfs: refactor btrfs_drop_extents() to make it easier to extend
>>>> 	2x hang (<20 minutes)
>>>>    129  e114c545bb69 btrfs: set the lockdep class for extent buffers on creation
>>>> 	2x pass (but runs 2x slower, both times)
>>>>    130  3fbaf25817f7 btrfs: pass the owner_root and level to alloc_extent_buffer
>>>> 	1x pass
>>>>    131  5d81230baa90 btrfs: pass the root owner and level around for readahead
>>>> 	1x pass
>>>>    132  1b7ec85ef490 btrfs: pass root owner to read_tree_block
>>>> 	1x pass
>>>>    133  182c79fcb857 btrfs: use btrfs_read_node_slot in btrfs_qgroup_trace_subtree
>>>>    134  3acfbd6a990c btrfs: use btrfs_read_node_slot in qgroup_trace_new_subtree_blocks
>>>> 	1x hang
>>>>    135  6b2cb7cb959a btrfs: use btrfs_read_node_slot in qgroup_trace_extent_swap
>>>>    136  c990ada2a0bb btrfs: use btrfs_read_node_slot in walk_down_tree
>>>> 	1x hang
>>>>    137  6b3426be27de btrfs: use btrfs_read_node_slot in replace_path
>>>> 	1x hang, 1x pass
>>>>    138  c975253682e0 btrfs: use btrfs_read_node_slot in do_relocation
>>>> 	1x hang
>>>>    139  8ef385bbf099 btrfs: use btrfs_read_node_slot in walk_down_reloc_tree
>>>> 	1x hang, 1x pass
>>>>    140  206983b72a36 btrfs: use btrfs_read_node_slot in btrfs_realloc_node
>>>> 	1x pass
>>>>    141  bfb484d922a3 btrfs: cleanup extent buffer readahead
>>>> 	1x pass
>>>>    142  416e3445ef80 btrfs: remove lockdep classes for the fs tree
>>>>    143  3e48d8d2540d btrfs: discard: reschedule work after sysfs param update
>>>>    144  df903e5d294f btrfs: don't miss async discards after scheduled work override
>>>>    145  6e88f116bd4c btrfs: discard: store async discard delay as ns not as jiffies
>>>> 	2x hang
>>>>    146  e50404a8a699 btrfs: discard: speed up async discard up to iops_limit
>>>>
>>>>    [snip]
>>>>
>>>>    155  0d01e247a06b btrfs: assert page mapping lock in attach_extent_buffer_page
>>>> 	1x hang, 1x pass
>>>>    156  bbb86a371791 btrfs: protect fs_info->caching_block_groups by block_group_cache_lock
>>>> 	1x hang
>>>>    157  e747853cae3a btrfs: load free space cache asynchronously
>>>> 	1x pass
>>>>    158  4d7240f0abda btrfs: load the free space cache inode extents from commit root
>>>> 	1x hang
>>>>    159  cd79909bc7cd btrfs: load free space cache into a temporary ctl
>>>> 	2x pass
>>>>    160  66b53bae46c8 btrfs: cleanup btrfs_discard_update_discardable usage
>>>> 	2x hang, 1x pass
>>>>    161  2ca08c56e813 btrfs: explicitly protect ->last_byte_to_unpin in unpin_extent_range
>>>> 	2x pass
>>>>    162  27d56e62e474 btrfs: update last_byte_to_unpin in switch_commit_roots
>>>> 	2x pass
>>>>    163  9076dbd5ee83 btrfs: do not shorten unpin len for caching block groups
>>>>    164  dc5161648693 btrfs: reorder extent buffer members for better packing
>>>> 	2x pass
>>>>    165  b9729ce014f6 btrfs: locking: rip out path->leave_spinning
>>>>    166  ac5887c8e013 btrfs: locking: remove all the blocking helpers
>>>>    167  2ae0c2d80d25 btrfs: scrub: remove local copy of csum_size from context
>>>>    168  419b791ce760 btrfs: check integrity: remove local copy of csum_size
>>>> 	1x hang, 1x pass
>>>>    169  713cebfb9891 btrfs: remove unnecessary local variables for checksum size
>>>>    170  223486c27b36 btrfs: switch cached fs_info::csum_size from u16 to u32
>>>>    171  55fc29bed8dd btrfs: use cached value of fs_info::csum_size everywhere
>>>>    172  fe5ecbe818de btrfs: precalculate checksums per leaf once
>>>>    173  22b6331d9617 btrfs: store precalculated csum_size in fs_info
>>>>    174  265fdfa6ce0a btrfs: replace s_blocksize_bits with fs_info::sectorsize_bits
>>>>    175  098e63082b9b btrfs: replace div_u64 by shift in free_space_bitmap_size
>>>> 	2x pass
>>>>    176  ab108d992b12 btrfs: use precalculated sectorsize_bits from fs_info
>>>>
>>>>    [snip]
>>>>
>>>>    200  5e8b9ef30392 btrfs: move pos increment and pagecache extension to btrfs_buffered_write
>>>> 	1x pass
>>>>    201  4e4cabece9f9 btrfs: split btrfs_direct_IO to read and write
>>>>
>>>>    [snip]
>>>>
>>>>    215  d70bf7484f72 btrfs: unify the ro checking for mount options
>>>> 	1x pass
>>>>    216  a6889caf6ec6 btrfs: do not start readahead for csum tree when scrubbing non-data block groups
>>>>    217  a57ad681f12e btrfs: assert we are holding the reada_lock when releasing a readahead zone
>>>>    218  aa8c1a41a1e6 btrfs: set EXTENT_NORESERVE bits side btrfs_dirty_pages()
>>>>    219  13f0dd8f7861 btrfs: use round_down while calculating start position in btrfs_dirty_pages()
>>>>    220  949b32732eab btrfs: use iosize while reading compressed pages
>>>>    221  eefa45f59379 btrfs: calculate num_pages, reserve_bytes once in btrfs_buffered_write
>>>>    222  fb8a7e941b1b btrfs: calculate more accurate remaining time to sleep in transaction_kthread
>>>> 	1x pass
>>>>
>>>> There is some repeatability in these results--some commits have a much
>>>> lower failure rate than others--but I don't see a reason why the bad
>>>> commits are bad or the good commits are good.  There are some commits with
>>>> locking and concurrency implications, but they're as likely to produce
>>>> good as bad results in test.  Sometimes there's a consistent change in
>>>> test result after a commit that only rearranges function arguments on
>>>> the stack.
>>>>
>>>> Maybe what we're looking at is a subtle race that is popping up due
>>>> to unrelated changes in the kernel, and disappearing just as often,
>>>> and 5.11-rc1 in particular did something innocent that aggravates
>>>> it somehow, so all later kernels hit the problem more often than
>>>> 5.10 did.
>>>>
>>>> 3.  Somewhere around "7f458a3873ae btrfs: fix race when defragmenting
>>>> leads to unnecessary IO" bees starts running about 3x slower than on
>>>> earlier kernels.  bees is a nightmare of nondeterministically racing
>>>> worker threads, so I'm not sure how important this observation is,
>>>> but it keeps showing up in the data.
>>>>
>>>> 4.  I had one machine on 5.10.84 (not a test VM) with a shell process
>>>> that got stuck spinning 100% CPU in the kernel on sys_write.  bees was
>>>> also running, but its threads were all stuck waiting for the shell to
>>>> release the transaction.  Other crashes on 5.10.8x kernels look more
>>>> like the one in this thread, with a logical_ino spinning.
>>>>
>>>>>> If it's not looping there, it may be looping higher up, but I don't see where it
>>>>>> would be doing that.  Lets start here and work our way up if we need to.
>>>>>> Thanks,
>>>
>