Subject: Re: 4.14: WARNING: CPU: 4 PID: 2895 at block/blk-mq.c:1144 with
 virtio-blk (also 4.12 stable)
To: Stefan Haberland <sth@linux.vnet.ibm.com>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>
Cc: Bart Van Assche <Bart.VanAssche@wdc.com>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
References: <cdca47a9-05cc-2676-57fe-904ce8ad5fbc@de.ibm.com>
 <20171123182542.GA2680@lst.de>
 <899f1638-cca4-28e6-3225-51505a053d45@de.ibm.com>
 <20171123183232.GA2845@lst.de>
 <92ef1aae-90b5-f14f-390e-bfab97899431@de.ibm.com>
 <419d8565-9cbe-16ac-3d5d-5945098694bc@de.ibm.com>
 <20171127155409.GA6937@lst.de>
 <d0f39408-b697-8d1a-2cce-b833cb8fa118@de.ibm.com>
 <20171204162108.GA12482@lst.de>
 <5ab91c56-b117-f4fa-3049-a4f8a5493155@de.ibm.com>
 <20171206232924.GA16584@lst.de>
 <0520e469-563b-486c-9ab8-00d8944ffa9d@linux.vnet.ibm.com>
From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Wed, 20 Dec 2017 16:47:21 +0100
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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).

Christian