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

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Message-ID: <20111215104440.1eef9e47@s6510.linuxnetplumber.net>
Date:	Thu, 15 Dec 2011 10:44:40 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	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?

On Thu, 15 Dec 2011 10:32:56 -0800
Rick Jones <rick.jones2@...com> wrote:

> 
> > 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

Device's work better if the driver proactively manages stop_queue/wake_queue.
Old devices used TX_BUSY, but newer devices tend to manage the queue
themselves.

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