netdev - Re: VLAN I/F's and TX queue.

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Date:	Mon, 10 May 2010 16:33:00 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
CC:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: VLAN I/F's and TX queue.

Joakim Tjernlund wrote:
> Eric Dumazet <eric.dumazet@...il.com> wrote on 2010/05/07 10:53:23:
>>> 3) I would expect lost pkgs to be accounted on eth0 instead of
>>>    the VLAN interface(s) since that is where the pkg is lost, why
>>>    isn't it so?
>> You try to send packets on eth0.XXX, some are dropped, and accounted for
>> on eth0.XXX stats. What is wrong with this ?
> 
> In this case one lost pkg is accounted for twice, once on eth0.1 and
> once more on eth0.1.1. Note that eth0.1.1 is stacked on
> top of eth0.1
> 
> I would at least expect eth0 to also account lost pkgs too.
> I was confused by the current accounting as I knew that
> the underlying HW I/F should be the only I/F that could
> drop pkgs.

In case of NET_XMIT_CN, the packet is dropped by the qdisc before
it reaches eth0, so its only accounted on the upper devices.

>> If you want to avoid this, just add queues to your vlans
>>
>> ip link add link eth0 eth0.103 txqueuelen 100 type vlan id 103
> 
>>>From memory now, but that didn't help. Still accounts pgks
> as described. Why would where to account pkgs be affected by
> queue or no queue?

If a queue is used on the vlan device, it will queue the packet
until the lower device is able to transmit it (unless its own
queue overflows).
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