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

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Message-ID: <4EEA3D58.5010101@hp.com>
Date:	Thu, 15 Dec 2011 10:32:56 -0800
From:	Rick Jones <rick.jones2@...com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Vijay Subramanian <subramanian.vijay@...il.com>,
	tcpdump-workers@...ts.tcpdump.org, netdev@...r.kernel.org
Subject: Re: twice past the taps, thence out to net?


> 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())

Is this then an unintended consequence bug, or a known feature?

rick

> 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

Sure enough:

$ tc -s -d qdisc
qdisc mq 0: dev eth0 root
  Sent 2212158799862 bytes 1938268098 pkt (dropped 0, overlimits 0 
requeues 4975139)
  backlog 0b 0p requeues 4975139

rick jones
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