[Qemu-devel] Re: Network shutdown under load

[Anthony Liguori]

On 02/08/2010 10:10 AM, Tom Lendacky wrote:
> Fix a race condition where qemu finds that there are not enough virtio
> ring buffers available and the guest make more buffers available before
> qemu can enable notifications.
>
> Signed-off-by: Tom Lendacky<address@hidden>
> Signed-off-by: Anthony Liguori<address@hidden>
>    

I've walked through the changes in this series and I'm pretty certain 
that this is the only problem.  I'd appreciate if others could review 
though.

Regards,

Anthony Liguori

>   hw/virtio-net.c |   10 +++++++++-
>   1 files changed, 9 insertions(+), 1 deletions(-)
>
> diff --git a/hw/virtio-net.c b/hw/virtio-net.c
> index 6e48997..5c0093e 100644
> --- a/hw/virtio-net.c
> +++ b/hw/virtio-net.c
> @@ -379,7 +379,15 @@ static int virtio_net_has_buffers(VirtIONet *n, int 
> bufsize)
>           (n->mergeable_rx_bufs&&
>            !virtqueue_avail_bytes(n->rx_vq, bufsize, 0))) {
>           virtio_queue_set_notification(n->rx_vq, 1);
> -        return 0;
> +
> +        /* To avoid a race condition where the guest has made some buffers
> +         * available after the above check but before notification was
> +         * enabled, check for available buffers again.
> +         */
> +        if (virtio_queue_empty(n->rx_vq) ||
> +            (n->mergeable_rx_bufs&&
> +             !virtqueue_avail_bytes(n->rx_vq, bufsize, 0)))
> +            return 0;
>       }
>
>       virtio_queue_set_notification(n->rx_vq, 0);
>
> On Friday 29 January 2010 02:06:41 pm Tom Lendacky wrote:
>    
> > There's been some discussion of this already in the kvm list, but I want to
> > summarize what I've found and also include the qemu-devel list in an effort
> >   to find a solution to this problem.
> >
> > Running a netperf test between two kvm guests results in the guest's
> >   network interface shutting down. I originally found this using kvm guests
> >   on two different machines that were connected via a 10GbE link.  However,
> >   I found this problem can be easily reproduced using two guests on the same
> >   machine.
> >
> > I am running the 2.6.32 level of the kvm.git tree and the 0.12.1.2 level of
> > the qemu-kvm.git tree.
> >
> > The setup includes two bridges, br0 and br1.
> >
> > The commands used to start the guests are as follows:
> > usr/local/bin/qemu-system-x86_64 -name cape-vm001 -m 1024 -drive
> > file=/autobench/var/tmp/cape-vm001-
> > raw.img,if=virtio,index=0,media=disk,boot=on -net
> > nic,model=virtio,vlan=0,macaddr=00:16:3E:00:62:51,netdev=cape-vm001-eth0 -
> > netdev tap,id=cape-vm001-eth0,script=/autobench/var/tmp/ifup-kvm-
> > br0,downscript=/autobench/var/tmp/ifdown-kvm-br0 -net
> > nic,model=virtio,vlan=1,macaddr=00:16:3E:00:62:D1,netdev=cape-vm001-eth1 -
> > netdev tap,id=cape-vm001-eth1,script=/autobench/var/tmp/ifup-kvm-
> > br1,downscript=/autobench/var/tmp/ifdown-kvm-br1 -vnc :1 -monitor
> > telnet::5701,server,nowait -snapshot -daemonize
> >
> > usr/local/bin/qemu-system-x86_64 -name cape-vm002 -m 1024 -drive
> > file=/autobench/var/tmp/cape-vm002-
> > raw.img,if=virtio,index=0,media=disk,boot=on -net
> > nic,model=virtio,vlan=0,macaddr=00:16:3E:00:62:61,netdev=cape-vm002-eth0 -
> > netdev tap,id=cape-vm002-eth0,script=/autobench/var/tmp/ifup-kvm-
> > br0,downscript=/autobench/var/tmp/ifdown-kvm-br0 -net
> > nic,model=virtio,vlan=1,macaddr=00:16:3E:00:62:E1,netdev=cape-vm002-eth1 -
> > netdev tap,id=cape-vm002-eth1,script=/autobench/var/tmp/ifup-kvm-
> > br1,downscript=/autobench/var/tmp/ifdown-kvm-br1 -vnc :2 -monitor
> > telnet::5702,server,nowait -snapshot -daemonize
> >
> > The ifup-kvm-br0 script takes the (first) qemu created tap device and
> >   brings it up and adds it to bridge br0.  The ifup-kvm-br1 script take the
> >   (second) qemu created tap device and brings it up and adds it to bridge
> >   br1.
> >
> > Each ethernet device within a guest is on it's own subnet.  For example:
> >    guest 1 eth0 has addr 192.168.100.32 and eth1 has addr 192.168.101.32
> >    guest 2 eth0 has addr 192.168.100.64 and eth1 has addr 192.168.101.64
> >
> > On one of the guests run netserver:
> >    netserver -L 192.168.101.32 -p 12000
> >
> > On the other guest run netperf:
> >    netperf -L 192.168.101.64 -H 192.168.101.32 -p 12000 -t TCP_STREAM -l 60
> >   -c -C -- -m 16K -M 16K
> >
> > It may take more than one netperf run (I find that my second run almost
> >   always causes the shutdown) but the network on the eth1 links will stop
> >   working.
> >
> > I did some debugging and found that in qemu on the guest running netserver:
> >   - the receive_disabled variable is set and never gets reset
> >   - the read_poll event handler for the eth1 tap device is disabled and
> >   never re-enabled
> > These conditions result in no packets being read from the tap device and
> >   sent to the guest - effectively shutting down the network.  Network
> >   connectivity can be restored by shutting down the guest interfaces,
> >   unloading the virtio_net module, re-loading the virtio_net module and
> >   re-starting the guest interfaces.
> >
> > I'm continuing to work on debugging this, but would appreciate if some
> >   folks with more qemu network experience could try to recreate and debug
> >   this.
> >
> > If my kernel config matters, I can provide that.
> >
> > Thanks,
> > Tom
> > --
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> >
> >