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

[Jens Axboe]

On 11/21/2017 01:19 PM, Christian Borntraeger wrote:
> 
> 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?

I did, thanks for doing that. Now I wonder how to mark this patch,
as we should revert it from kernels that have the bad commit. 4.12
is fine, 4.12.later-stable is not.

-- 
Jens Axboe