Re: [Xen-devel] Network drop to domU (netfront: rx->offset: 0, size: 4294967295)

["PCextreme B.V. - Wido den Hollander" <wido@xxxxxxxxxxxx>]

Hi,

My tests kept running this weekend and it seems it affects ALL the
domU's at the same moment.

For example on May 24 at 13:54 and 16:45 the traffic dropped for about
30 seconds to both domU's i have been monitoring.

My ifconfig on the dom0 shows a lot of TX drops (14663 atm) and a total
of 11047260 packets, that's about 0.1% packetloss over the uptime of the
domU (12 days atm).

The errors didn't change yet.

-  
Met vriendelijke groet,

Wido den Hollander
Hoofd Systeembeheer / CSO
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On Wed, 2009-05-20 at 06:01 -0700, Keir Fraser wrote:
> On 20/05/2009 05:50, "PCextreme B.V. - Wido den Hollander"
> <wido@xxxxxxxxxxxx> wrote:
> 
> >> If it's an issue that crops up with many guests then I suppose it's
> > more likely a netback issue, which is a pain.
> > 
> > I already assumed this would be a pain, any way to determine if it is a
> > netback issue? Adding some verbose messages to the kernel?
> 
> The visible effects of the bug start in dom0, when it presents a buffer
> reference (aka a 'grant reference') to Xen as provided to it by domU. Xen
> notes that the grant reference is bogus (the xm dmesg output shows the flag
> field of the grant is zero, which means it's currently unused). Now, does
> that mean domU forgot to initialise the buffer grant, or got out of sync
> somehow, or is the dom0 which has got out of sync? It's rather hard to tell.
> But dom0 is more likely to be affected by scaling to large numbers of
> domains than a domU is. The logic in domU netfront doesn't change, whereas
> dom0 netback has the actual multiplexing job. Hence dom0 is more likely to
> be the culprit.
> 
> If you define DEBUG at the very top of dom0's drivers/xen/netback/netback.c
> you will get more debug output from dom0 kernel when things go wrong. It may
> be not much extra help unfortunately, but extra tracing could be added I
> suppose (the pain being of course that each such change will require a dom0
> reboot or a netback module reload, which itself may require domains to be
> restarted).
> 
>  -- Keir
> 
>

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