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

[Stefan Haberland]

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 11b2025c3326f7096ceb588c3117c7883850c068ÂÂÂ -> bad
> > > > > >  ÂÂÂÂ blk-mq: create a blk_mq_ctx for each possible CPU
> > > > > > does not boot on DASD and
> > > > > > commit 9c6ae239e01ae9a9f8657f05c55c4372e9fc8bccÂÂÂ -> good
> > > > > >  ÂÂÂ 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