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RE: [Xen-devel] new netfront and occasional receive path lockup

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Pasi Kärkkäinen <pasik@xxxxxx>
Subject: RE: [Xen-devel] new netfront and occasional receive path lockup
From: "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>
Date: Fri, 10 Sep 2010 09:45:32 +0800
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Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Christophe Saout <christophe@xxxxxxxx>
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Thread-topic: [Xen-devel] new netfront and occasional receive path lockup
Hi Jeremy and Pasi,

I was frustrated that I couldn't reproduce this bug in my site. 

However I investigated the code, indeed there is one race condition that
probably cause the bug. See the attached patch.

Could anybody who can see this bug help to try it? Appreciate much!


Jeremy Fitzhardinge wrote:
>  On 09/10/2010 04:50 AM, Pasi Kärkkäinen wrote:
>> On Wed, Aug 25, 2010 at 08:51:09AM +0800, Xu, Dongxiao wrote:
>>> Hi Christophe,
>>> Thanks for finding and checking the problem.
>>> I will try to reproduce the issue and check what caused the problem.
>> Hello,
>> Was this issue resolved? Some users have been complaining "network
>> freezing up" issues recently on ##xen on irc..
> Yeah, I'll add a command-line parameter to disable smartpoll (and
> leave it off by default). 
>     J
>> -- Pasi
>>> Thanks,
>>> Dongxiao
>>> Jeremy Fitzhardinge wrote:
>>>>  On 08/22/2010 09:43 AM, Christophe Saout wrote:
>>>>> Hi,
>>>>> I've been playing with some of the new pvops code, namely DomU
>>>>> guest code.  What I've been observing on one of the virtual
>>>>> machines is that the network (vif) is dying after about ten to
>>>>> sixty minutes of uptime. The unfortunate thing here is that I can
>>>>> only repoduce it on a production VM and have been unlucky so far
>>>>> to trigger the bug on a test machine.  While this has not been
>>>>> tragic - rebooting fixed the issue, unfortunately I can't spend
>>>>> very much time on debugging after the issue pops up.
>>>> Ah, OK.  I've seen this a couple of times as well.  And it just
>>>> happened to me then... 
>>>>> Now, what is happening is that the receive path goes dead.  The
>>>>> DomU can send packets to Dom0 and those are visible using tcpdump
>>>>> on the Dom0 on the virtual interface, but not the other way
>>>>> around. 
>>>> I hadn't got to that level of diagnosis, but I can confirm that
>>>> that's what seems to be happening here too.
>>>>> Now, I have done more than one change at a time (I'd like to avoid
>>>>> going into pinning it down since I can only reproduce it on a
>>>>> production machine, as I said, so suggestions are welcome), but my
>>>>> suspicion is that it might have to do with the new "smart polling"
>>>>> feature in xen/netfront.  Note that I have also updated Dom0 to
>>>>> pull in the latest dom0/backend and netback changes, just to make
>>>>> sure it's not due to an issue that has been fixed there, but I'm
>>>>> still seeing the same.
>>>> I agree.  I think I started seeing this once I merged smartpoll
>>>> into netfront. 
>>>>     J
>>>>> The production machine is a machine that doesn't have much network
>>>>> load, but deals with a lot of small network requests (DNS and smtp
>>>>> mostly).  A workload which is hard to reproduce on the test
>>>>> machine. Heavy network load (NFS, FTP and so on) for days hasn't
>>>>> triggered the problem.  Also, segmentation offloading and similar
>>>>> settings don't have any effect. 
>>>>> The machine has 2 physical and the VM 2 virtual CPUs, DomU has
>>>>> PREEMPT enabled. 
>>>>> I've been looking at the code, if there might be a race condition
>>>>> somewhere, something like where one could run into a situation
>>>>> where the hrtimer doesn't run and Dom0 believes the DomU should be
>>>>> polling and doesn't emit an interrupt or something, but I'm afraid
>>>>> I don't know enough to judge this (I mean, there are spinlocks
>>>>> which look safe to me). 
>>>>> Do you have any suggestions what to try?  I can trigger the issue
>>>>> on the production VM again, but debugging should not take more
>>>>> than a few minutes if it happens.  Access is only possible via
>>>>> the console. Neither Dom0 nor the guest show anything unusual in
>>>>> the kernel message and continue to behave normally after the
>>>>> network goes dead (also able to shut down the guest normally).
>>>>> Thanks,
>>>>>   Christophe
>>>>> _______________________________________________
>>>>> Xen-devel mailing list
>>>>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>>>>> http://lists.xensource.com/xen-devel
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>>> http://lists.xensource.com/xen-devel

Attachment: 0001-Fix-one-race-condition-for-netfront-smartpoll-logic.patch
Description: 0001-Fix-one-race-condition-for-netfront-smartpoll-logic.patch

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