Re: General protection fault with use_blk_mq=1.

[Zephaniah E. Loss-Cutler-Hull]

On 03/28/2018 10:13 PM, Paolo Valente wrote:
> 
> 
>> Il giorno 29 mar 2018, alle ore 05:22, Jens Axboe <axboe@xxxxxxxxx> ha scritto:
>>
>> On 3/28/18 9:13 PM, Zephaniah E. Loss-Cutler-Hull wrote:
>>> On 03/28/2018 06:02 PM, Jens Axboe wrote:
>>>> On 3/28/18 5:03 PM, Zephaniah E. Loss-Cutler-Hull wrote:
>>>>> I am not subscribed to any of the lists on the To list here, please CC
>>>>> me on any replies.
>>>>>
>>>>> I am encountering a fairly consistent crash anywhere from 15 minutes to
>>>>> 12 hours after boot with scsi_mod.use_blk_mq=1 dm_mod.use_blk_mq=1> 
>>>>> The crash looks like:
>>>>>
>>>
>>>>>
>>>>> Looking through the code, I'd guess that this is dying inside
>>>>> blkg_rwstat_add, which calls percpu_counter_add_batch, which is what RIP
>>>>> is pointing at.
>>>>
>>>> Leaving the whole thing here for Paolo - it's crashing off insertion of
>>>> a request coming out of SG_IO. Don't think we've seen this BFQ failure
>>>> case before.
>>>>
>>>> You can mitigate this by switching the scsi-mq devices to mq-deadline
>>>> instead.
>>>>
>>>
>>> I'm thinking that I should also be able to mitigate it by disabling
>>> CONFIG_DEBUG_BLK_CGROUP.
>>>
>>> That should remove that entire chunk of code.
>>>
>>> Of course, that won't help if this is actually a symptom of a bigger
>>> problem.
>>
>> Yes, it's not a given that it will fully mask the issue at hand. But
>> turning off BFQ has a much higher chance of working for you.
>>
>> This time actually CC'ing Paolo.
>>
> 
> Hi Zephaniah,
> if you are actually interested in the benefits of BFQ (low latency,
> high responsiveness, fairness, ...) then it may be worth to try what
> you yourself suggest: disabling CONFIG_DEBUG_BLK_CGROUP.  Also because
> this option activates the heavy computation of debug cgroup statistics,
> which probably you don't use.

I definitely am.
> 
> In addition, the outcome of your attempt without
> CONFIG_DEBUG_BLK_CGROUP would give us useful bisection information:
> - if no failure occurs, then the issue is likely to be confined in
> that debugging code (which, on the bright side, is likely to be of
> occasional interest, for only a handful of developers)
> - if the issue still shows up, then we may have new hints on this odd
> failure
> 
> Finally, consider that this issue has been reported to disappear from
> 4.16 [1], and, as a plus, that the service quality of BFQ had a
> further boost exactly from 4.16.

I look forward to that either way then.
> 
> Looking forward to your feedback, in case you try BFQ without
> CONFIG_DEBUG_BLK_CGROUP,

I'm running that now, judging from the past if it survives until
tomorrow evening then we're good, so I should hopefully know in the next
day.

Thank you,
Zephaniah E. Loss-Cutler-Hull.

> Paolo
> 
> [1] https://www.spinics.net/lists/linux-block/msg21422.html
> 
>>
>> -- 
>> Jens Axboe
>

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