net_ns cleanup / RCU overhead

[Simon Kirby]

Hello!

In trying to figure out what happened to a box running lots of vsftpd
since we deployed a CONFIG_NET_NS=y kernel to it, we found that the
(wall) time needed for cleanup_net() to complete, even on an idle box,
can be quite long:

#!/bin/bash

ip netns delete test >&/dev/null
while ip netns add test; do
        echo hi
        ip netns delete test
done

On my desktop and typical hosts, this prints at only around 4 or 6 per
second. While this is happening, "vmstat 1" reports 100% idle, and there
there are D-state processes with stacks similar to:

30566 [kworker/u16:1] D wait_rcu_gp+0x48, synchronize_sched+0x2f, cleanup_net+0xdb, process_one_work+0x175, worker_thread+0x119, kthread+0xbb, ret_from_fork+0x7c, 0xffffffffffffffff

32220 ip              D copy_net_ns+0x68, create_new_namespaces+0xfc, unshare_nsproxy_namespaces+0x66, SyS_unshare+0x159, system_call_fastpath+0x16, 0xffffffffffffffff

copy_net_ns() is waiting on net_mutex which is held by cleanup_net().

vsftpd uses CLONE_NEWNET to set up privsep processes. There is a comment
about it being really slow before 2.6.35 (it avoids CLONE_NEWNET in that
case). I didn't find anything that makes 2.6.35 any faster, but on Debian
2.6.36-5-amd64, I notice it does seem to be a bit faster than 3.2, 3.10,
3.16, though still not anything I'd ever want to rely on per connection.

C implementation of the above: http://0x.ca/sim/ref/tools/netnsloop.c

Kernel stack "top": http://0x.ca/sim/ref/tools/pstack

What's going on here?

Simon-
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