netdev - 2.6.27 problem with shaping using netem/ifb (regression?)

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Date:	Wed, 01 Oct 2008 19:04:45 -0400
From:	Patrick McManus <mcmanus@...ksong.com>
To:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: 2.6.27 problem with shaping using netem/ifb (regression?)

Hi all,

I have a set of scripts I use to emulate certain network conditions on
localhost. (i.e. change the latency, bandwidth, etc..). they use netem
and ifb along with tc and ip to get the job done - they are used to
measure and evaluate changes to userspace applications under a variety
of cell-phone-like network conditions.

It has worked well on 23, 24, 25, and 26.. 

Using the pre-27 kernel in Ubuntu's ibex beta the scripts are pretty
broken. I built a kernel from git (up to date with today) and indeed,
they are broken under that too.

I've distilled the whole thing into a python script, attached, which
sets up the network (160 kbit/s, 750ms latency), starts a python
webserver and then launches wget (which pulls from that server on port
8912) to see how fast it goes.. when it is working I see ~19KB/s of
goodput but on .27 it typically just limps along at less than 1 KB/sec.

I've tried to bisect it, but various interim .27 builds throw OOPS at
the tc commands used in the script (qdisc_create).. hard to narrow
anything down there. The current tip doesn't OOPS, but it doesn't behave
as expected either. I'm happy to try a particular tag if suggested. 

Any thoughts on where to look? I hope attaching the script is helpful -
it is pretty self evident when running it if things are working ok. This
is all on amd64 if that is relevant.

-Patrick

View attachment "shaping-test.py" of type "text/x-python" (5369 bytes)