[Qemu-devel] Re: Network shutdown under load

[Tom Lendacky <tahm@linux.vnet.ibm.com>]

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 <toml@us.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

 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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