Re: 4.14: WARNING: CPU: 4 PID: 2895 at block/blk-mq.c:1144 with virtio-blk

[Christian Borntraeger]

On 11/20/2017 09:52 PM, Jens Axboe wrote:
> On 11/20/2017 01:49 PM, Christian Borntraeger wrote:
>>
>>
>> On 11/20/2017 08:42 PM, Jens Axboe wrote:
>>> On 11/20/2017 12:29 PM, Christian Borntraeger wrote:
>>>>
>>>>
>>>> On 11/20/2017 08:20 PM, Bart Van Assche wrote:
>>>>> On Fri, 2017-11-17 at 15:42 +0100, Christian Borntraeger wrote:
>>>>>> This is 
>>>>>>
>>>>>> b7a71e66d (Jens Axboe                2017-08-01 09:28:24 -0600 1141)     * are mapped to it.
>>>>>> b7a71e66d (Jens Axboe                2017-08-01 09:28:24 -0600 1142)     */
>>>>>> 6a83e74d2 (Bart Van Assche           2016-11-02 10:09:51 -0600 1143)    WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
>>>>>> 6a83e74d2 (Bart Van Assche           2016-11-02 10:09:51 -0600 1144)            cpu_online(hctx->next_cpu));
>>>>>> 6a83e74d2 (Bart Van Assche           2016-11-02 10:09:51 -0600 1145) 
>>>>>> b7a71e66d (Jens Axboe                2017-08-01 09:28:24 -0600 1146)    /*
>>>>>
>>>>> Did you really try to figure out when the code that reported the warning
>>>>> was introduced? I think that warning was introduced through the following
>>>>> commit:
>>>>
>>>> This was more a cut'n'paste to show which warning triggered since line numbers are somewhat volatile.
>>>>
>>>>>
>>>>> commit fd1270d5df6a005e1248e87042159a799cc4b2c9
>>>>> Date:   Wed Apr 16 09:23:48 2014 -0600
>>>>>
>>>>>     blk-mq: don't use preempt_count() to check for right CPU
>>>>>      
>>>>>     UP or CONFIG_PREEMPT_NONE will return 0, and what we really
>>>>>     want to check is whether or not we are on the right CPU.
>>>>>     So don't make PREEMPT part of this, just test the CPU in
>>>>>     the mask directly.
>>>>>
>>>>> Anyway, I think that warning is appropriate and useful. So the next step
>>>>> is to figure out what work item was involved and why that work item got
>>>>> executed on the wrong CPU.
>>>>
>>>> It seems to be related to virtio-blk (is triggered by fio on such disks). Your comment basically
>>>> says: "no this is not a known issue" then :-)
>>>> I will try to take a dump to find out the work item
>>>
>>> blk-mq does not attempt to freeze/sync existing work if a CPU goes away,
>>> and we reconfigure the mappings. So I don't think the above is unexpected,
>>> if you are doing CPU hot unplug while running a fio job.
>>
>> I did a cpu hot plug (adding a CPU) and I started fio AFTER that.
> 
> OK, that's different, we should not be triggering a warning for that.
> What does your machine/virtblk topology look like in terms of CPUS,
> nr of queues for virtblk, etc?

FWIW, 4.11 does work, 4.12 and later is broken.

> 
> You can probably get this info the easiest by just doing a:
> 
> # find /sys/kernel/debug/block/virtX
> 
> replace virtX with your virtblk device name. Generate this info both
> before and after the hotplug event.

It happens in all variants (1 cpu to 2 or 16 to 17 and independent of the
number of disks).

What I can see is that the block layer does not yet sees the new CPU:

[root@zhyp137 ~]# find /sys/kernel/debug/block/vd* 
/sys/kernel/debug/block/vda
/sys/kernel/debug/block/vda/hctx0
/sys/kernel/debug/block/vda/hctx0/cpu0
/sys/kernel/debug/block/vda/hctx0/cpu0/completed
/sys/kernel/debug/block/vda/hctx0/cpu0/merged
/sys/kernel/debug/block/vda/hctx0/cpu0/dispatched
/sys/kernel/debug/block/vda/hctx0/cpu0/rq_list
/sys/kernel/debug/block/vda/hctx0/active
/sys/kernel/debug/block/vda/hctx0/run
/sys/kernel/debug/block/vda/hctx0/queued
/sys/kernel/debug/block/vda/hctx0/dispatched
/sys/kernel/debug/block/vda/hctx0/io_poll
/sys/kernel/debug/block/vda/hctx0/sched_tags_bitmap
/sys/kernel/debug/block/vda/hctx0/sched_tags
/sys/kernel/debug/block/vda/hctx0/tags_bitmap
/sys/kernel/debug/block/vda/hctx0/tags
/sys/kernel/debug/block/vda/hctx0/ctx_map
/sys/kernel/debug/block/vda/hctx0/busy
/sys/kernel/debug/block/vda/hctx0/dispatch
/sys/kernel/debug/block/vda/hctx0/flags
/sys/kernel/debug/block/vda/hctx0/state
/sys/kernel/debug/block/vda/sched
/sys/kernel/debug/block/vda/sched/dispatch
/sys/kernel/debug/block/vda/sched/starved
/sys/kernel/debug/block/vda/sched/batching
/sys/kernel/debug/block/vda/sched/write_next_rq
/sys/kernel/debug/block/vda/sched/write_fifo_list
/sys/kernel/debug/block/vda/sched/read_next_rq
/sys/kernel/debug/block/vda/sched/read_fifo_list
/sys/kernel/debug/block/vda/write_hints
/sys/kernel/debug/block/vda/state
/sys/kernel/debug/block/vda/requeue_list
/sys/kernel/debug/block/vda/poll_stat

--> in host virsh setvcpu zhyp137 2

[root@zhyp137 ~]# chcpu -e 1
CPU 1 enabled
[root@zhyp137 ~]# find /sys/kernel/debug/block/vd* 
/sys/kernel/debug/block/vda
/sys/kernel/debug/block/vda/hctx0
/sys/kernel/debug/block/vda/hctx0/cpu0
/sys/kernel/debug/block/vda/hctx0/cpu0/completed
/sys/kernel/debug/block/vda/hctx0/cpu0/merged
/sys/kernel/debug/block/vda/hctx0/cpu0/dispatched
/sys/kernel/debug/block/vda/hctx0/cpu0/rq_list
/sys/kernel/debug/block/vda/hctx0/active
/sys/kernel/debug/block/vda/hctx0/run
/sys/kernel/debug/block/vda/hctx0/queued
/sys/kernel/debug/block/vda/hctx0/dispatched
/sys/kernel/debug/block/vda/hctx0/io_poll
/sys/kernel/debug/block/vda/hctx0/sched_tags_bitmap
/sys/kernel/debug/block/vda/hctx0/sched_tags
/sys/kernel/debug/block/vda/hctx0/tags_bitmap
/sys/kernel/debug/block/vda/hctx0/tags
/sys/kernel/debug/block/vda/hctx0/ctx_map
/sys/kernel/debug/block/vda/hctx0/busy
/sys/kernel/debug/block/vda/hctx0/dispatch
/sys/kernel/debug/block/vda/hctx0/flags
/sys/kernel/debug/block/vda/hctx0/state
/sys/kernel/debug/block/vda/sched
/sys/kernel/debug/block/vda/sched/dispatch
/sys/kernel/debug/block/vda/sched/starved
/sys/kernel/debug/block/vda/sched/batching
/sys/kernel/debug/block/vda/sched/write_next_rq
/sys/kernel/debug/block/vda/sched/write_fifo_list
/sys/kernel/debug/block/vda/sched/read_next_rq
/sys/kernel/debug/block/vda/sched/read_fifo_list
/sys/kernel/debug/block/vda/write_hints
/sys/kernel/debug/block/vda/state
/sys/kernel/debug/block/vda/requeue_list
/sys/kernel/debug/block/vda/poll_stat



If I already start with 2 cpus it looks like the following (all cpu1 entries are new)

[root@zhyp137 ~]# find /sys/kernel/debug/block/vd* 
/sys/kernel/debug/block/vda
/sys/kernel/debug/block/vda/hctx0
/sys/kernel/debug/block/vda/hctx0/cpu1
/sys/kernel/debug/block/vda/hctx0/cpu1/completed
/sys/kernel/debug/block/vda/hctx0/cpu1/merged
/sys/kernel/debug/block/vda/hctx0/cpu1/dispatched
/sys/kernel/debug/block/vda/hctx0/cpu1/rq_list
/sys/kernel/debug/block/vda/hctx0/cpu0
/sys/kernel/debug/block/vda/hctx0/cpu0/completed
/sys/kernel/debug/block/vda/hctx0/cpu0/merged
/sys/kernel/debug/block/vda/hctx0/cpu0/dispatched
/sys/kernel/debug/block/vda/hctx0/cpu0/rq_list
/sys/kernel/debug/block/vda/hctx0/active
/sys/kernel/debug/block/vda/hctx0/run
/sys/kernel/debug/block/vda/hctx0/queued
/sys/kernel/debug/block/vda/hctx0/dispatched
/sys/kernel/debug/block/vda/hctx0/io_poll
/sys/kernel/debug/block/vda/hctx0/sched_tags_bitmap
/sys/kernel/debug/block/vda/hctx0/sched_tags
/sys/kernel/debug/block/vda/hctx0/tags_bitmap
/sys/kernel/debug/block/vda/hctx0/tags
/sys/kernel/debug/block/vda/hctx0/ctx_map
/sys/kernel/debug/block/vda/hctx0/busy
/sys/kernel/debug/block/vda/hctx0/dispatch
/sys/kernel/debug/block/vda/hctx0/flags
/sys/kernel/debug/block/vda/hctx0/state
/sys/kernel/debug/block/vda/sched
/sys/kernel/debug/block/vda/sched/dispatch
/sys/kernel/debug/block/vda/sched/starved
/sys/kernel/debug/block/vda/sched/batching
/sys/kernel/debug/block/vda/sched/write_next_rq
/sys/kernel/debug/block/vda/sched/write_fifo_list
/sys/kernel/debug/block/vda/sched/read_next_rq
/sys/kernel/debug/block/vda/sched/read_fifo_list
/sys/kernel/debug/block/vda/write_hints
/sys/kernel/debug/block/vda/state
/sys/kernel/debug/block/vda/requeue_list
/sys/kernel/debug/block/vda/poll_stat