netdev - Re: twice past the taps, thence out to net?

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Date:	Thu, 15 Dec 2011 18:43:18 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vijay Subramanian <subramanian.vijay@...il.com>
Cc:	Rick Jones <rick.jones2@...com>, tcpdump-workers@...ts.tcpdump.org,
	netdev@...r.kernel.org
Subject: Re: twice past the taps, thence out to net?

Le mercredi 14 décembre 2011 à 18:12 -0800, Vijay Subramanian a écrit :
> On 14 December 2011 11:27, Rick Jones <rick.jones2@...com> wrote:
> > While looking at "something else" with tcpdump/tcptrace, tcptrace emitted
> > lots of notices about hardware duplicated packets being detected (same TCP
> > sequence number and IP datagram ID).  Sure enough, if I go into the tcpdump
> > trace (taken on the sender) I can find instances of what it was talking
> > about, separated in time by rather less than I would expect to be the RTO,
> > and often as not with few if any intervening arriving ACKs to trigger
> > anything like fast retransmit.  And besides, those would have a different IP
> > datagram ID no?
> >
> > I did manage to reproduce the issue with plain netperf tcp_stream tests. I
> > had one sending system with 30 concurrent netperf tcp_stream tests to 30
> > other receiving systems.  There are "hardware duplicates" in the sending
> > trace, but no duplicate segments (that I can find thus far) in the two
> > receiver side traces I took.  Of course that doesn't mean "conclusively"
> > there were two actual sends but it suggests there werent.
> >
> > While I work through the "obtain permission" path to post the packet traces
> > (don't ask...) I thought I would ask if anyone else has seen something
> > similar.
> >
> > In this case, all the systems are running a 2.6.38-8 Ubuntu kernel (the same
> > sorts of issues which delay my just putting the traces up on netperf.org
> > preclude a later kernel, and I've no other test systems :( ), with Intel
> > 82576 interfaces being driven by:
> >
> > $ sudo ethtool -i eth0
> > driver: igb
> > version: 2.1.0-k2
> > firmware-version: 1.8-2
> > bus-info: 0000:05:00.0
> >
> > All the systems were connected to the same switch.
> >
> 
> Rick,
> This may be of help.
> http://www.tcptrace.org/faq_ans.html#FAQ%2021

More exactly, we call dev_queue_xmit_nit() from dev_hard_start_xmit()
_before_ giving skb to device driver.

If device driver returns NETDEV_TX_BUSY, and a qdisc was setup on the
device, packet is requeued.

Later, when queue is allowed to send again packets, packet is
retransmitted (and traced a second time in dev_queue_xmit_nit())

You can see the 'requeues' counter from "tc -s -d qdisc" output :

qdisc mq 0: dev eth2 root 
 Sent 29421597369 bytes 20301716 pkt (dropped 0, overlimits 0 requeues 371) 
 backlog 0b 0p requeues 371 


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