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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2025/7/2 07:41, Jens Axboe 写道:
> 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@lists.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.
> 

It looks like my memory is blurry, I tried as old as v6.14 (v6.13 failed 
to compile using the latest gcc 15.1.1), all the same delay.

Thus I believe it may be some missing kernel config causing the problem.

Let me retry with the base Archlinux kernel config and see what's going 
wrong.

Thanks,
Qu