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Date: Thu, 28 Aug 2014 15:33:42 -0500 From: ebiederm@...ssion.com (Eric W. Biederman) To: Simon Kirby <sim@...tway.ca> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, linux-kernel@...r.kernel.org, netdev@...r.kernel.org Subject: Re: net_ns cleanup / RCU overhead Simon Kirby <sim@...tway.ca> writes: > 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 n> test. Should I enable RCU_TRACE or STALL_INFO with a low timeout or > something? I just want to add a little bit more analysis to this. What we desire to be fast is the copy_net_ns, cleanup_net is batched and asynchronous which nothing really cares how long it takes except that cleanup_net holds the net_mutex and thus blocks copy_net_ns. The puzzle is why and which rcu delays Simon is seeing in the network namespace cleanup path, as it seems like the synchronize_rcu is not the only one, and in the case of vsftp with trivail network namespaces where nothing has been done we should not need to delay. Eric > 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) -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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