[Lguest] Virtio block problems on newer kernels

[Sakari Ailus]

Hi Rusty,

Rusty Russell wrote:
> Sakari Ailus <sakari.ailus at iki.fi> writes:
>> Hi,
>>
>> On Mon, Apr 07, 2014 at 11:33:23AM +0300, Sakari Ailus wrote:
>>> Sakari Ailus wrote:
>>>> On Mon, Mar 24, 2014 at 03:31:33AM +0200, Sakari Ailus wrote:
>>>>> I haven't had time to debug this further and bisecting will be
>>>>> painful as this isn't that easy to reproduce: I first noticed after
>>>>> running 3.13.6 for a day but first thought it was a network issue.
>>>>> The problem appeared between the two above versions (3.10.10 and
>>>>> 3.13.6).
>>>>
>>>> Oops --- after having the problem awhile and then sending the e-mail to the
>>>> list, I now realise I'm using a launcher which is quite old. I'll replace
>>>> that with a newer one and see if that fixes the issues.
>>>
>>> So I did that. No luck.
>>>
>>> It seems that virtio block devices can just get stuck. The (guest) processes
>>> that try to access them will stay in uninterruptible sleep, sometimes for
>>> minutes, sometimes apparently forever. The appears to be per-virtio block
>>> device issue when it happens. The host lguest processes are simply in sleep
>>> state.
>>>
>>> Host kernel version does not matter but that of the guest does. 3.14.0 seems
>>> to be affected, too. 3.12.0 works.
>>>
>>> I'll try to bisect and see what I can find.
>>
>> I did the bisect and found this:
>>
>> commit 1cf7e9c68fe84248174e998922b39e508375e7c1
>> Author: Jens Axboe <axboe at kernel.dk>
>> Date:   Fri Nov 1 10:52:52 2013 -0600
>>
>>     virtio_blk: blk-mq support
> 
> Interesting!
> 
>>     Switch virtio-blk from the dual support for old-style requests and bios
>>     to use the block-multiqueue.
>>     
>>     Acked-by: Asias He <asias at redhat.com>
>>     Signed-off-by: Jens Axboe <axboe at kernel.dk>
>>     Signed-off-by: Christoph Hellwig <hch at lst.de>
>>
>> Erratic block i/o behaviour begin when that patch is applied to the guest
>> kernel. I wonder if someone else has seen this. I can reproduce it
>> relatively easily by running bonnie++ in a guest:
>>
>> 	bonnie++ -n 0 -s 256M -r 10 -b
>>
>> I'm using an up-to-date lguest launcher.
> 
> I cannot reproduce this under qemu (that's how I run my 32-bit host
> these days) with Linus' latest.  Of course, if the issue is that we have
> a race and performance has increased to trigger it, it's not surprising.

The I/O performance might rather be less than on your Qemu virtual
machine in this case --- this is an Athlon XP host with virtio block
devices backed by RAID 0 + luks + lvm without hardware acceleration for
encryption.

> I'm installing on an old laptop now (I don't have any native 32 bit
> machines here).

-- 
Regards,

Sakari Ailus
sakari.ailus at iki.fi