I don’t know if we want to call this an issue, but...

I’m seeing a lockup with cake (and also sfq, but not either pfifo or fq_codel), when run over veth devices. Two network namespaces are created, one for client and one for server, each with one veth device. Netem is added as the root qdisc with a delay of 1ms, and a leaf qdisc may be added. Lockups occur on my box when the leaf qdisc is either cake or sfq, and I'm running flent’s tcp_ndown test with >= 4 download streams. Note that I happen to be running on a quad-core.

- If no leaf qdisc is added below netem, no lockup occurs.
- If either pfifo or fq_codel is added below netem, no lockup occurs.
- If either cake or sfq is the leaf, the lockup occurs.

The symptoms (lockup with >= 4 streams on a quad-core box), and the fact that it occurs with both cake and sfq, make me think that it may simply have to do with the code not being re-entrant, which may be the case for veth, and this is just by design? maybe something that we should consider fixing but wouldn’t be a show-stopper? But that should be confirmed.

I’ll keep investigating, but am sharing the scripts I’m running meanwhile in case anyone else wants to look. See README.txt in the attached...

Pete