linux-kernel - Re: Weird delay introduced in v6.16-rc only, possible regression

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Message-ID: <90fa0bd9-71b7-44f7-9175-641d43e9fe1b@kernel.dk>
Date: Tue, 1 Jul 2025 16:11:37 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Qu Wenruo <quwenruo.btrfs@....com>,
 "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>
Subject: Re: Weird delay introduced in v6.16-rc only, possible regression

On 7/1/25 4:07 PM, Qu Wenruo wrote:
> 
> 
> ? 2025/7/2 02:00, Jens Axboe ??:
>> On 6/29/25 3:45 PM, Qu Wenruo wrote:
>>> Hi,
>>>
>>> Recently I'm hitting a very weird delay when doing development inside a x86_64 VM.
>>>
>>> The dmesg shows the delay (10+ sec) between virtio blk and device-mapper:
>>>
>>> [    3.651377] virtio_blk virtio4: 10/0/0 default/read/poll queues
>>> [    3.653075] virtio_scsi virtio2: 10/0/0 default/read/poll queues
>>> [    3.670269] virtio_blk virtio4: [vda] 83886080 512-byte logical blocks (42.9 GB/40.0 GiB)
>>> [    3.672096] scsi host6: Virtio SCSI HBA
>>> [    3.708452]  vda: vda1 vda2
>>> [    3.711073] virtio_blk virtio5: 10/0/0 default/read/poll queues
>>> [    3.729879] virtio_blk virtio5: [vdb] 167772160 512-byte logical blocks (85.9 GB/80.0 GiB)
>>> [    3.737535] virtio_blk virtio8: 10/0/0 default/read/poll queues
>>> [    3.747045] virtio_blk virtio8: [vdc] 83886080 512-byte logical blocks (42.9 GB/40.0 GiB)
>>> [   17.453833] device-mapper: uevent: version 1.0.3
>>> [   17.455689] device-mapper: ioctl: 4.50.0-ioctl (2025-04-28) initialised: dm-devel@...ts.linux.dev
>>> :: performing fsck on '/dev/os/root'
>>> /dev/mapper/os-root: clean, 240299/1048576 files, 3372218/4194304 blocks
>>> :: mounting '/dev/os/root' on real root
>>> [   17.871671] EXT4-fs (dm-0): mounted filesystem 00a85626-d289-4817-8183-ee828e221f76 r/w with ordered data mode. Quota mode: none.
>>>
>>> The VM is running kernel based on upstream commit 78f4e737a53e ("Merge tag 'for-6.16/dm-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm"), with a lot of extra btrfs patches.
>>>
>>>
>>> The v6.15 kernel from Archlinux is totally fine without any delay.
>>>
>>> The v6.16-rc kernel may have some different configs, but the config is used for a long long time, way before v6.15, so it looks like it's something in the v6.16 cycle causing problems.
>>>
>>> I can definitely do a bisection, but any clue would be appreciated.
>>
>> Probably a good idea to go ahead with a bisect to help pin it down.
>>
> 
> BTW, a little more digging shows it's the `udevadm settle` causing the long delay in the initramfs.
> 
> The rootfs is an ext4 on a LVM lv, so initramfs is required to mount the rootfs.
> 
> So it may not be the block/dm layer causing the problem.

Even the more reason to bisect it then, if we don't quite know why it's
slow.

-- 
Jens Axboe