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Message-Id: <20251008150705.4090434-1-matt@readmodwrite.com>
Date: Wed, 8 Oct 2025 16:07:05 +0100
From: Matt Fleming <matt@...dmodwrite.com>
To: Matt Fleming <matt@...dmodwrite.com>
Cc: adilger.kernel@...ger.ca,
kernel-team@...udflare.com,
linux-ext4@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org,
tytso@....edu,
willy@...radead.org,
Baokun Li <libaokun1@...wei.com>,
Jan Kara <jack@...e.cz>
Subject: Re: ext4 writeback performance issue in 6.12
(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
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