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

[David Ahern <dsahern@gmail.com>]

On 4/1/14, 9:09 AM, Stefan Hajnoczi wrote:
> 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?

virtio-net.

David