netdev - Re: [PATCH] netem: Fixes byte backlog accounting for the first of two chained netem instances

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Message-Id: <20150407.183442.1733318471114947035.davem@davemloft.net>
Date:	Tue, 07 Apr 2015 18:34:42 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	jdb109120@...allas.edu
Cc:	hagen@...u.net, stephen@...workplumber.org,
	netem@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH] netem: Fixes byte backlog accounting for the first of
 two chained netem instances

From: "Beshay, Joseph" <jdb109120@...allas.edu>
Date: Mon, 6 Apr 2015 18:00:56 +0000

> Fixes byte backlog accounting for the first of two chained netem instances.
> Bytes backlog reported now corresponds to the number of queued packets.
> 
> When two netem instances are chained, for instance to apply rate and queue
> limitation followed by packet delay, the number of backlogged bytes reported
> by the first netem instance is wrong. It reports the sum of bytes in the queues
> of the first and second netem. The first netem reports the correct number of
> backlogged packets but not bytes. This is shown in the example below.
> 
> Consider a chain of two netem schedulers created using the following commands:
> 
> $ tc -s qdisc replace dev veth2 root handle 1:0 netem rate 10000kbit limit 100
> $ tc -s qdisc add dev veth2 parent 1:0 handle 2: netem delay 50ms
> 
> Start an iperf session to send packets out on the specified interface and
> monitor the backlog using tc:
> 
> $ tc -s qdisc show dev veth2
> 
> Output using unpatched netem:
> 	qdisc netem 1: root refcnt 2 limit 100 rate 10000Kbit
> 	 Sent 98422639 bytes 65434 pkt (dropped 123, overlimits 0 requeues 0)
> 	 backlog 172694b 73p requeues 0
> 	qdisc netem 2: parent 1: limit 1000 delay 50.0ms
> 	 Sent 98422639 bytes 65434 pkt (dropped 0, overlimits 0 requeues 0)
> 	 backlog 63588b 42p requeues 0
> 	 
> The interface used to produce this output has an MTU of 1500. The output for
> backlogged bytes behind netem 1 is 172694b. This value is not correct. Consider
> the total number of sent bytes and packets. By dividing the number of sent
> bytes by the number of sent packets, we get an average packet size of ~=1504.
> If we divide the number of backlogged bytes by packets, we get ~=2365. This is
> due to the first netem incorrectly counting the 63588b which are in netem 2's
> queue as being in its own queue. To verify this is the case, we subtract them
> from the reported value and divide by the number of packets as follows: 
> 	172694 - 63588 = 109106 bytes actualled backlogged in netem 1
> 	109106 / 73 packets ~= 1494 bytes (which matches our MTU)
> 	
> The root cause is that the byte accounting is not done at the
> same time with packet accounting. The solution is to update the backlog value
> every time the packet queue is updated.
> 
> Signed-off-by: Joseph D Beshay <joseph.beshay@...allas.edu>
> Acked-by: Hagen Paul Pfeifer <hagen@...u.net>

Looks good, applied, thanks.
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