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Date: Tue, 12 Feb 2013 10:10:24 +0100
From: Peter Lieven <pl@...net.de>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Stefan Hajnoczi <stefanha@...il.com>, qemu-devel@...gnu.org,
netdev@...r.kernel.org
Subject: Re: [Qemu-devel] tap devices not receiving packets from a bridge
Am 12.02.2013 um 10:08 schrieb "Michael S. Tsirkin" <mst@...hat.com>:
> On Tue, Feb 12, 2013 at 08:06:04AM +0100, Peter Lieven wrote:
>> On 23.01.2013 11:03, Michael S. Tsirkin wrote:
>>> On Tue, Jan 22, 2013 at 10:04:07AM +0100, Peter Lieven wrote:
>>>> On 23.11.2012 12:01, Michael S. Tsirkin wrote:
>>>>> 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.
>>>>
>>>> After some time I again have a vServer in this state. It seems not like there
>>>> are no TX errors.
>>>>
>>>> # ifconfig tap10
>>>> tap10 Link encap:Ethernet HWaddr 7a:59:20:6f:e7:e5
>>>> inet6 addr: fe80::7859:20ff:fe6f:e7e5/64 Scope:Link
>>>> UP BROADCAST RUNNING PROMISC MULTICAST MTU:1500 Metric:1
>>>> RX packets:197431 errors:0 dropped:0 overruns:0 frame:0
>>>> TX packets:264309 errors:0 dropped:0 overruns:2 carrier:0
>>>> collisions:0 txqueuelen:500
>>>> RX bytes:13842063 (13.8 MB) TX bytes:35092821 (35.0 MB)
>>>>
>>>> It seems like the bridge is not forwarding any packets to the tap device anymore altough it has learnt
>>>> the MAC-Adresses and there are also broadcast packets coming in.
>>>>
>>>> Any more ideas where I could debug?
>>>>
>>>> Peter
>>>>
>>>>>
>>>>>>>
>>>>>>> Stefan
>>>
>>> Hmm. So there are two overrun errors that triggered, so
>>> it's possible after the second one the queue got stuck in an xoff state.
>>> You'd have to use something like systemtap or kdb to poke at the
>>> queue state to see whether xoff flag is set and/or look
>>> at the receive queue length.
>>>
>>> For future, we can try to set TUN_ONE_QUEUE flag on the interface,
>>> or try applying this patch
>>> 5d097109257c03a71845729f8db6b5770c4bbedc
>>> in kernel see if this helps.
>>>
>>
>> If have set this option for 2 weeks now and not seen this problem again.
>> How does this flag work with the recently added tap multiqueue support?
>>
>> Peter
>
> This will be the only option in 3.8.
Ok, but wouldn`t it be good to set it in qemu for kernels <3.8?
Peter
>
> --
> MST
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