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

[Ming Lei <ming.lei@redhat.com>]

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?

Thanks, 
Ming