Re: 4.14: WARNING: CPU: 4 PID: 2895 at block/blk-mq.c:1144 with virtio-blk (also 4.12 stable)

[Stefan Haberland <sth@linux.vnet.ibm.com>]

On 11.01.2018 12:44, Christian Borntraeger wrote:
>
> On 01/11/2018 10:13 AM, Ming Lei wrote:
>> On Wed, Dec 20, 2017 at 04:47:21PM +0100, Christian Borntraeger wrote:
>>> On 12/18/2017 02:56 PM, Stefan Haberland wrote:
>>>> On 07.12.2017 00:29, Christoph Hellwig wrote:
>>>>> On Wed, Dec 06, 2017 at 01:25:11PM +0100, Christian Borntraeger wrote:
>>>>> t > commit 11b2025c3326f7096ceb588c3117c7883850c068    -> bad
>>>>>>       blk-mq: create a blk_mq_ctx for each possible CPU
>>>>>> does not boot on DASD and
>>>>>> commit 9c6ae239e01ae9a9f8657f05c55c4372e9fc8bcc    -> good
>>>>>>      genirq/affinity: assign vectors to all possible CPUs
>>>>>> does boot with DASD disks.
>>>>>>
>>>>>> Also adding Stefan Haberland if he has an idea why this fails on DASD and adding Martin (for the
>>>>>> s390 irq handling code).
>>>>> That is interesting as it really isn't related to interrupts at all,
>>>>> it just ensures that possible CPUs are set in ->cpumask.
>>>>>
>>>>> I guess we'd really want:
>>>>>
>>>>> e005655c389e3d25bf3e43f71611ec12f3012de0
>>>>> "blk-mq: only select online CPUs in blk_mq_hctx_next_cpu"
>>>>>
>>>>> before this commit, but it seems like the whole stack didn't work for
>>>>> your either.
>>>>>
>>>>> I wonder if there is some weird thing about nr_cpu_ids in s390?
>>>>> -- 
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>>>>>
>>>> I tried this on my system and the blk-mq-hotplug-fix branch does not boot for me as well.
>>>> The disks get up and running and I/O works fine. At least the partition detection and EXT4-fs mount works.
>>>>
>>>> But at some point in time the disk do not get any requests.
>>>>
>>>> I currently have no clue why.
>>>> I took a dump and had a look at the disk states and they are fine. No error in the logs or in our debug entrys. Just empty DASD devices waiting to be called for I/O requests.
>>>>
>>>> Do you have anything I could have a look at?
>>> Jens, Christoph, so what do we do about this?
>>> To summarize:
>>> - commit 4b855ad37194f7 ("blk-mq: Create hctx for each present CPU") broke CPU hotplug.
>>> - Jens' quick revert did fix the issue and did not broke DASD support but has some issues
>>> with interrupt affinity.
>>> - Christoph patch set fixes the hotplug issue for virtio blk but causes I/O hangs on DASDs (even
>>> without hotplug).
>> Hello,
>>
>> This one is a valid use case for VM, I think we need to fix that.
>>
>> Looks there is issue on the fouth patch("blk-mq: only select online
>> CPUs in blk_mq_hctx_next_cpu"), I fixed it in the following tree, and
>> the other 3 patches are same with Christoph's:
>>
>> 	https://github.com/ming1/linux.git  v4.15-rc-block-for-next-cpuhot-fix
>>
>> gitweb:
>> 	https://github.com/ming1/linux/commits/v4.15-rc-block-for-next-cpuhot-fix
>>
>> Could you test it and provide the feedback?
>>
>> BTW, if it can't help this issue, could you boot from a normal disk first
>> and dump blk-mq debugfs of DASD later?
> That kernel seems to boot fine on my system with DASD disks.
>
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I did some regression testing and it works quite well. Boot works, 
attaching CPUs during runtime on z/VM and enabling them in Linux works 
as well.
I also did some DASD online/offline CPU enable/disable loops.

Regards,
Stefan