Re: [Qemu-devel] networking stalls in the guest -- backlog in the host

[Stefan Hajnoczi]

On Thu, Mar 27, 2014 at 04:13:15PM -0600, David Ahern wrote:
> 
> We are hitting a networking problem and hoping someone has an idea
> -- perhaps a known bug.
> 
> After a couple of hours of runtime with low level traffic (e.g., 1
> sec pings) the VM stops receiving packets. In the host running tc on
> the tap device shows a full backlog and packets getting dropped:
> 
> tc -s qdisc show dev vnet0
> qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1
> 1 1 1 1 1 1 1
> Sent 5806496634 bytes 4163358 pkt (dropped 116079, overlimits 0 requeues 4)
> backlog 33834b 500p requeues 4
> 
> 
> The tap device is passed to qemu as fd=24. Running strace on the IO
> thread does not show the fd in the list passed to select. e.g.,
> 
> select(55, [7 8 11 18 52 53 54], [], [], {1, 0}) = 1 (in [8], left
> {0, 872402})
> 
> That would explain why the packets are not pulled from the tap
> device into the VM. When networking is functioning properly, you do
> see fd=24 in the list followed by read(24, ...).
> 
> Why would qemu stop adding the fd to the list passed to select?
> 
> 
> This is qemu-kvm-1.0 (upgrading is not an option), started by
> libvirt (libvirt 1.0.2). The command line is rather long. Snippets:
> 
> /usr/bin/kvm -M pc-1.0 -cpu host -enable-kvm -m 4096 -smp 4
> ...
> -netdev tap,fd=24,id=hostnet0 -device 
> virtio-net-pci,netdev=hostnet0,id=net0,mac=00:16:3e:ba:55:60,bus=pci.0,addr=0xa
> ...
> 
> Host kernel: 3.2.0-60-generic
> Guest kernel:  3.8.0-29-generic

Are you using vhost_net or the userspace virtio-net emulation?

Stefan