linux-kernel - Re: I/O scheduler deadlocks on Xen virtual block devices

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Message-ID: <AANLkTim76cMyb8VC7TUkNGRagWmjFzbwf7-fMAMLzCd5@mail.gmail.com>
Date:	Thu, 2 Sep 2010 16:43:28 +0100
From:	Doug Neal <dneallkml@...il.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: I/O scheduler deadlocks on Xen virtual block devices

On Thu, Sep 2, 2010 at 4:25 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Doug Neal <dneallkml@...il.com> writes:
>
>>>>>
>>>>> Did you try these different I/O schedulers in the domU or on the dom0?
>>>>> Does switching I/O schedulers in either place make the problem go away
>>>>> when it happens?
>>>>>
>>>>
>>>> In the domU, and the bug was present in all cases. The dom0 was using
>>>> cfq. I'll run the tests again using each scheduler in the dom0 with
>>>> domU set to noop and report back.
>>>
>>> While I think this is an interesting test, you need only test one I/O
>>> scheduler in the dom0.  Also, I think you misunderstood the second
>>> question.  I'd like to know if switching I/O schedulers while the system
>>> is in this bad state helps get I/O going again.
>>>
>> Jeff,
>>
>> Issue confirmed in the following cases:
>>
>> dom0:domU
>> noop:noop
>> noop:deadline
>> noop:anticipatory
>> cfq:noop
>> cfq:deadline
>> cfq:anticipatory
>> cfq:cfq
>>
>> I try switching schedulers in the domU during the lockup, the echo x >
>> /sys/block/xvda/queue/scheduler command also hangs. In the dom0, no
>> effect.
>
> OK.  So, this means that the I/O is not hung up in the I/O scheduler.  I
> don't have a whole lot of experience with the Xen I/O stack, but I think
> that's where I'd start digging (especially if there's nothing in the
> logs about command timed out).
>
> I hope this was a somewhat helpful exercise.  Sorry I can't be of more
> help.
>
> Cheers!
> Jeff
>

No problem, thanks for your assistance.


Doug
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