On Fri, Nov 23, 2012 at 10:41:21AM +0100, Peter Lieven wrote:
>
> Am 23.11.2012 um 08:02 schrieb Stefan Hajnoczi:
>
> > On Thu, Nov 22, 2012 at 03:29:52PM +0100, Peter Lieven wrote:
> >> is anyone aware of a problem with the linux network bridge that in very
> >> rare circumstances stops
> >> a bridge from sending pakets to a tap device?
> >>
> >> My problem occurs in conjunction with vanilla qemu-kvm-1.2.0 and Ubuntu
> >> Kernel 3.2.0-34.53
> >> which is based on Linux 3.2.33.
> >>
> >> I was not yet able to reproduce the issue, it happens in really rare
> >> cases. The symptom is that
> >> the tap does not have any TX packets. RX is working fine. I see the
> >> packets coming in at
> >> the physical interface on the host, but they are not forwarded to the tap
> >> interface.
> >> The bridge itself has learnt the mac address of the vServer that is
> >> connected to the tap interface.
> >> It does not help to toggle the bridge link status, the tap interface
> >> status or the interface in the vServer.
> >> It seems that problem occurs if a tap interface that has previously been
> >> used, but set to nonpersistent
> >> is set persistent again and then is by chance assigned to the same vServer
> >> (=same mac address on same
> >> bridge) again. Unfortunately it seems not to be reproducible.
> >
> > Not sure but this patch from Michael Tsirkin may help - it solves an
> > issue with persistent tap devices:
> >
> > http://patchwork.ozlabs.org/patch/198598/
>
> Hi Stefan,
>
> thanks for the pointer. I have seen this patch, but I have neglected it
> because it was dealing
> with persistent taps. But maybe the taps in the kernel are not deleted
> directly.
> Can you remember what the syptomps of the above issue have been? Sorry for
> being vague, but I currently have no clue whats going on.
>
> Can someone who has more internal knowledge of the bridging/tap code say if
> qemu can
> be responsible at all if the tap device is not receiving packets from the
> bridge.
>
> If I have the following config. Lets say packets coming in via physical
> interface eth1.123,
> and a bridge called br123.I further have a virtual machine with tap0. Both
> eth1.123
> and tap0 are member of br123.
>
> If the issue occurs the vServer has no network connectivity inbound. If I
> sent a ping
> from the vServer I see it on tap0 and leaving on eth1.123. I see further the
> arp reply coming
> in via eth1.123, but the reply can't be seen on tap0.
>
> Peter
If guest is not consuming packets, a TX queue in tap device
will with time overrun (there's space for 1000 packets there).
This is code from tun:
if (skb_queue_len(&tfile->socket.sk->sk_receive_queue)
>= dev->tx_queue_len / tun->numqueues){
if (!(tun->flags & TUN_ONE_QUEUE)) {
/* Normal queueing mode. */
/* Packet scheduler handles dropping of further
* packets. */
netif_stop_subqueue(dev, txq);
/* We won't see all dropped packets
* individually, so overrun
* error is more appropriate. */
dev->stats.tx_fifo_errors++;
So you can detect that this triggered by looking at fifo errors counter in
device.
Once this happens TX queue is stopped, then you hit this path:
if (!netif_xmit_stopped(txq)) {
__this_cpu_inc(xmit_recursion);
rc = dev_hard_start_xmit(skb, dev, txq);
__this_cpu_dec(xmit_recursion);
if (dev_xmit_complete(rc)) {
HARD_TX_UNLOCK(dev, txq);
goto out;
}
}
so packets are not passed to device anymore.
It will stay this way until guest consumes some packets and
queue is restarted.
> >
> > Stefan
