Re: ext4 writeback performance issue in 6.12

[Matt Fleming <matt@readmodwrite.com>]

(Adding Baokun and Jan in case they have any ideas)

On Mon, Oct 06, 2025 at 12:56:15 +0100, Matt Fleming wrote:
> Hi,
> 
> We're seeing writeback take a long time and triggering blocked task
> warnings on some of our database nodes, e.g.
> 
>   INFO: task kworker/34:2:243325 blocked for more than 225 seconds.
>         Tainted: G           O       6.12.41-cloudflare-2025.8.2 #1
>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>   task:kworker/34:2    state:D stack:0     pid:243325 tgid:243325 ppid:2      task_flags:0x4208060 flags:0x00004000
>   Workqueue: cgroup_destroy css_free_rwork_fn
>   Call Trace:
>    <TASK>
>    __schedule+0x4fb/0xbf0
>    schedule+0x27/0xf0
>    wb_wait_for_completion+0x5d/0x90
>    ? __pfx_autoremove_wake_function+0x10/0x10
>    mem_cgroup_css_free+0x19/0xb0
>    css_free_rwork_fn+0x4e/0x430
>    process_one_work+0x17e/0x330
>    worker_thread+0x2ce/0x3f0
>    ? __pfx_worker_thread+0x10/0x10
>    kthread+0xd2/0x100
>    ? __pfx_kthread+0x10/0x10
>    ret_from_fork+0x34/0x50
>    ? __pfx_kthread+0x10/0x10
>    ret_from_fork_asm+0x1a/0x30
>    </TASK>
> 
> A large chunk of system time (4.43%) is being spent in the following
> code path:
> 
>    ext4_get_group_info+9
>    ext4_mb_good_group+41
>    ext4_mb_find_good_group_avg_frag_lists+136
>    ext4_mb_regular_allocator+2748
>    ext4_mb_new_blocks+2373
>    ext4_ext_map_blocks+2149
>    ext4_map_blocks+294
>    ext4_do_writepages+2031
>    ext4_writepages+173
>    do_writepages+229
>    __writeback_single_inode+65
>    writeback_sb_inodes+544
>    __writeback_inodes_wb+76
>    wb_writeback+413
>    wb_workfn+196
>    process_one_work+382
>    worker_thread+718
>    kthread+210
>    ret_from_fork+52
>    ret_from_fork_asm+26
> 
> That's the path through the CR_GOAL_LEN_FAST allocator.
> 
> The primary reason for all these cycles looks to be that we're spending
> a lot of time in ext4_mb_find_good_group_avg_frag_lists(). The fragment
> lists seem quite big and the function fails to find a suitable group
> pretty much every time it's called either because the frag list is empty
> (orders 10-13) or the average size is < 1280 (order 9). I'm assuming it
> falls back to a linear scan at that point.
> 
>   https://gist.github.com/mfleming/5b16ee4cf598e361faf54f795a98c0a8
> 
> $ sudo cat /proc/fs/ext4/md127/mb_structs_summary
> optimize_scan: 1
> max_free_order_lists:
> 	list_order_0_groups: 0
> 	list_order_1_groups: 1
> 	list_order_2_groups: 6
> 	list_order_3_groups: 42
> 	list_order_4_groups: 513
> 	list_order_5_groups: 62
> 	list_order_6_groups: 434
> 	list_order_7_groups: 2602
> 	list_order_8_groups: 10951
> 	list_order_9_groups: 44883
> 	list_order_10_groups: 152357
> 	list_order_11_groups: 24899
> 	list_order_12_groups: 30461
> 	list_order_13_groups: 18756
> avg_fragment_size_lists:
> 	list_order_0_groups: 108
> 	list_order_1_groups: 411
> 	list_order_2_groups: 1640
> 	list_order_3_groups: 5809
> 	list_order_4_groups: 14909
> 	list_order_5_groups: 31345
> 	list_order_6_groups: 54132
> 	list_order_7_groups: 90294
> 	list_order_8_groups: 77322
> 	list_order_9_groups: 10096
> 	list_order_10_groups: 0
> 	list_order_11_groups: 0
> 	list_order_12_groups: 0
> 	list_order_13_groups: 0
> 
> These machines are striped and are using noatime:
> 
> $ grep ext4 /proc/mounts
> /dev/md127 /state ext4 rw,noatime,stripe=1280 0 0
> 
> Is there some tunable or configuration option that I'm missing that
> could help here to avoid wasting time in
> ext4_mb_find_good_group_avg_frag_lists() when it's most likely going to
> fail an order 9 allocation anyway?
> 
> I'm happy to provide any more details that might help.
> 
> Thanks,
> Matt