Re: net_ns cleanup / RCU overhead

[Simon Kirby <sim@hostway.ca>]

On Thu, Aug 28, 2014 at 12:24:31PM -0700, Paul E. McKenney wrote:

> On Tue, Aug 19, 2014 at 10:58:55PM -0700, Simon Kirby wrote:
> > 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?
> 
> That is a bit slow for many configurations, but there are some exceptions.
> 
> So, what is your kernel's .config?

I was unable to find a config (or stock kernel) that was any different,
but here's the one we're using: http://0x.ca/sim/ref/3.10/config-3.10.53

How fast does the above test run for you?

We've been running with the attached, which has helped a little, but it's
still quite slow in our particular use case (vsftpd), and with the above
test. Should I enable RCU_TRACE or STALL_INFO with a low timeout or
something?

Simon-

-- >8 --
Subject: [PATCH] netns: use synchronize_rcu_expedited instead of
 synchronize_rcu

Similar to ef323088, with synchronize_rcu(), we are only able to create
and destroy about 4 or 7 net namespaces per second, which really puts a
dent in the performance of programs attempting to use CLONE_NEWNET for
privilege separation (vsftpd, chromium).
---
 net/core/net_namespace.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 85b6269..6dcb4b3 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -296,7 +296,7 @@ static void cleanup_net(struct work_struct *work)
 	 * This needs to be before calling the exit() notifiers, so
 	 * the rcu_barrier() below isn't sufficient alone.
 	 */
-	synchronize_rcu();
+	synchronize_rcu_expedited();
 
 	/* Run all of the network namespace exit methods */
 	list_for_each_entry_reverse(ops, &pernet_list, list)
-- 
1.7.10.4