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

[Christian Borntraeger]

On 11/21/2017 09:14 PM, Jens Axboe wrote:
> On 11/21/2017 01:12 PM, Christian Borntraeger wrote:
>>
>>
>> On 11/21/2017 08:30 PM, Jens Axboe wrote:
>>> On 11/21/2017 12:15 PM, Christian Borntraeger wrote:
>>>>
>>>>
>>>> On 11/21/2017 07:39 PM, Jens Axboe wrote:
>>>>> On 11/21/2017 11:27 AM, Jens Axboe wrote:
>>>>>> On 11/21/2017 11:12 AM, Christian Borntraeger wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 11/21/2017 07:09 PM, Jens Axboe wrote:
>>>>>>>> On 11/21/2017 10:27 AM, Jens Axboe wrote:
>>>>>>>>> On 11/21/2017 03:14 AM, Christian Borntraeger wrote:
>>>>>>>>>> Bisect points to
>>>>>>>>>>
>>>>>>>>>> 1b5a7455d345b223d3a4658a9e5fce985b7998c1 is the first bad commit
>>>>>>>>>> commit 1b5a7455d345b223d3a4658a9e5fce985b7998c1
>>>>>>>>>> Author: Christoph Hellwig <hch@xxxxxx>
>>>>>>>>>> Date:   Mon Jun 26 12:20:57 2017 +0200
>>>>>>>>>>
>>>>>>>>>>     blk-mq: Create hctx for each present CPU
>>>>>>>>>>     
>>>>>>>>>>     commit 4b855ad37194f7bdbb200ce7a1c7051fecb56a08 upstream.
>>>>>>>>>>     
>>>>>>>>>>     Currently we only create hctx for online CPUs, which can lead to a lot
>>>>>>>>>>     of churn due to frequent soft offline / online operations.  Instead
>>>>>>>>>>     allocate one for each present CPU to avoid this and dramatically simplify
>>>>>>>>>>     the code.
>>>>>>>>>>     
>>>>>>>>>>     Signed-off-by: Christoph Hellwig <hch@xxxxxx>
>>>>>>>>>>     Reviewed-by: Jens Axboe <axboe@xxxxxxxxx>
>>>>>>>>>>     Cc: Keith Busch <keith.busch@xxxxxxxxx>
>>>>>>>>>>     Cc: linux-block@xxxxxxxxxxxxxxx
>>>>>>>>>>     Cc: linux-nvme@xxxxxxxxxxxxxxxxxxx
>>>>>>>>>>     Link: http://lkml.kernel.org/r/20170626102058.10200-3-hch@xxxxxx
>>>>>>>>>>     Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>>>>>>>>>     Cc: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx>
>>>>>>>>>>     Cc: Mike Galbraith <efault@xxxxxx>
>>>>>>>>>>     Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>>>>>>>>>
>>>>>>>>> I wonder if we're simply not getting the masks updated correctly. I'll
>>>>>>>>> take a look.
>>>>>>>>
>>>>>>>> Can't make it trigger here. We do init for each present CPU, which means
>>>>>>>> that if I offline a few CPUs here and register a queue, those still show
>>>>>>>> up as present (just offline) and get mapped accordingly.
>>>>>>>>
>>>>>>>> From the looks of it, your setup is different. If the CPU doesn't show
>>>>>>>> up as present and it gets hotplugged, then I can see how this condition
>>>>>>>> would trigger. What environment are you running this in? We might have
>>>>>>>> to re-introduce the cpu hotplug notifier, right now we just monitor
>>>>>>>> for a dead cpu and handle that.
>>>>>>>
>>>>>>> I am not doing a hot unplug and the replug, I use KVM and add a previously
>>>>>>> not available CPU.
>>>>>>>
>>>>>>> in libvirt/virsh speak:
>>>>>>>   <vcpu placement='static' current='1'>4</vcpu>
>>>>>>
>>>>>> So that's why we run into problems. It's not present when we load the device,
>>>>>> but becomes present and online afterwards.
>>>>>>
>>>>>> Christoph, we used to handle this just fine, your patch broke it.
>>>>>>
>>>>>> I'll see if I can come up with an appropriate fix.
>>>>>
>>>>> Can you try the below?
>>>>
>>>>
>>>> It does prevent the crash but it seems that the new CPU is not "used " after the hotplug for mq:
>>>>
>>>>
>>>> output with 2 cpus:
>>>> /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
>>>
>>> Try this, basically just a revert.
>>
>> Yes, seems to work.
>>
>> Tested-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> 
> Great, thanks for testing.
> 
>> Do you know why the original commit made it into 4.12 stable? After all
>> it has no Fixes tag and no cc stable-
> 
> I was wondering the same thing when you said it was in 4.12.stable and
> not in 4.12 release. That patch should absolutely not have gone into
> stable, it's not marked as such and it's not fixing a problem that is
> stable worthy. In fact, it's causing a regression...
> 
> Greg? Upstream commit is mentioned higher up, start of the email.
> 


Forgot to cc Greg?