Hi

We noticed some long delays starting docker containers on some newer
kernels (starting with 4.5.x and still present in 4.9-rc4, 4.4.x is
fine). We narrowed this down to the creation of a network namespace
being delayed directly after removing another one (details and
reproduction below). We have seen delays of up to 60s on some systems.

- The delay is proportional to the number of CPUs (online or offline).
We first discovered it with a Hyper-V Linux VM. Hyper-V advertises up
to 240 offline vCPUs even if one configures the VM with only, say 2
vCPUs. We see linear increase in delay when we change NR_CPUS in the
kernel config.

- The delay is also dependent on some tunnel network interfaces being
present (which we had compiled in in one of our kernel configs).

- We can reproduce this issue with stock kernels from
http://kernel.ubuntu.com/~kernel-ppa/mainline/running in Hyper-V VMs
as well as other hypervisors like qemu and hyperkit where we have good
control over the number of CPUs.

A simple test is:
modprobe ipip
moprobe  ip_gre
modprobe ip_vti
echo -n "add netns foo ===> "; /usr/bin/time -f "%E" ip netns add foo
echo -n "del netns foo ===> "; /usr/bin/time -f "%E" ip netns delete foo
echo -n "add netns bar ===> "; /usr/bin/time -f "%E" ip netns add bar
echo -n "del netns bar ===> "; /usr/bin/time -f "%E" ip netns delete bar

with an output like:
add netns foo ===> 0:00.00
del netns foo ===> 0:00.01
add netns bar ===> 0:08.53
del netns bar ===> 0:00.01

This is on a 4.9-rc4 kernel from the above URL configured with
NR_CPUS=256 running in a Hyper-V VM (kernel config attached).

Below is a dump of the work queues while the second 'ip add netns' is
hanging. The state of the work queues does not seem to change while
the command is delayed and the pattern shown is consistent across
different kernel versions.

Is this a known issue and/or is someone working on a fix?

Thanks
Rolf
[I'm not subscribed to linux-kernel@ nor netdev@, please cc me on replies]


[  610.356272] sysrq: SysRq : Show Blocked State
[  610.356742]   task                        PC stack   pid father
[  610.357252] kworker/u480:1  D    0  1994      2 0x00000000
[  610.357752] Workqueue: netns cleanup_net
[  610.358239]  ffff9892f1065800 0000000000000000 ffff9892ee1e1e00
ffff9892f8e59340
[  610.358705]  ffff9892f4526900 ffffbf0104b5ba88 ffffffffbe486df3
ffffbf0104b5ba60
[  610.359168]  00ffffffbdcbe663 ffff9892f8e59340 0000000100012e70
ffff9892ee1e1e00
[  610.359677] Call Trace:
[  610.360169]  [<ffffffffbe486df3>] ? __schedule+0x233/0x6e0
[  610.360723]  [<ffffffffbe4872d6>] schedule+0x36/0x80
[  610.361194]  [<ffffffffbe48a9ca>] schedule_timeout+0x22a/0x3f0
[  610.361789]  [<ffffffffbe486dfb>] ? __schedule+0x23b/0x6e0
[  610.362260]  [<ffffffffbe487d24>] wait_for_completion+0xb4/0x140
[  610.362736]  [<ffffffffbdcb05a0>] ? wake_up_q+0x80/0x80
[  610.363306]  [<ffffffffbdceb528>] __wait_rcu_gp+0xc8/0xf0
[  610.363782]  [<ffffffffbdceea5c>] synchronize_sched+0x5c/0x80
[  610.364137]  [<ffffffffbdcf0010>] ? call_rcu_bh+0x20/0x20
[  610.364742]  [<ffffffffbdceb440>] ?
trace_raw_output_rcu_utilization+0x60/0x60
[  610.365337]  [<ffffffffbe3696bc>] synchronize_net+0x1c/0x30
[  610.365846]  [<ffffffffbe369803>] netif_napi_del+0x23/0x80
[  610.367494]  [<ffffffffc057f6f8>] ip_tunnel_dev_free+0x68/0xf0 [ip_tunnel]
[  610.368007]  [<ffffffffbe372c10>] netdev_run_todo+0x230/0x330
[  610.368454]  [<ffffffffbe37eb4e>] rtnl_unlock+0xe/0x10
[  610.369001]  [<ffffffffc057f4df>] ip_tunnel_delete_net+0xdf/0x120 [ip_tunnel]
[  610.369500]  [<ffffffffc058b92c>] ipip_exit_net+0x2c/0x30 [ipip]
[  610.369997]  [<ffffffffbe362688>] ops_exit_list.isra.4+0x38/0x60
[  610.370636]  [<ffffffffbe363674>] cleanup_net+0x1c4/0x2b0
[  610.371130]  [<ffffffffbdc9e4ac>] process_one_work+0x1fc/0x4b0
[  610.371812]  [<ffffffffbdc9e7ab>] worker_thread+0x4b/0x500
[  610.373074]  [<ffffffffbdc9e760>] ? process_one_work+0x4b0/0x4b0
[  610.373622]  [<ffffffffbdc9e760>] ? process_one_work+0x4b0/0x4b0
[  610.374100]  [<ffffffffbdca4b09>] kthread+0xd9/0xf0
[  610.374574]  [<ffffffffbdca4a30>] ? kthread_park+0x60/0x60
[  610.375198]  [<ffffffffbe48c2b5>] ret_from_fork+0x25/0x30
[  610.375678] ip              D    0  2149   2148 0x00000000
[  610.376185]  ffff9892f0a99000 0000000000000000 ffff9892f0a66900
[  610.376185]  ffff9892f8e59340
[  610.376185]  ffff9892f4526900 ffffbf0101173db8 ffffffffbe486df3
[  610.376753]  00000005fecffd76
[  610.376762]  00ff9892f11d9820 ffff9892f8e59340 ffff989200000000
ffff9892f0a66900
[  610.377274] Call Trace:
[  610.377789]  [<ffffffffbe486df3>] ? __schedule+0x233/0x6e0
[  610.378306]  [<ffffffffbe4872d6>] schedule+0x36/0x80
[  610.378992]  [<ffffffffbe48756e>] schedule_preempt_disabled+0xe/0x10
[  610.379514]  [<ffffffffbe489199>] __mutex_lock_slowpath+0xb9/0x130
[  610.380031]  [<ffffffffbde0fce2>] ? __kmalloc+0x162/0x1e0
[  610.380556]  [<ffffffffbe48922f>] mutex_lock+0x1f/0x30
[  610.381135]  [<ffffffffbe3637ff>] copy_net_ns+0x9f/0x170
[  610.381647]  [<ffffffffbdca5e6b>] create_new_namespaces+0x11b/0x200
[  610.382249]  [<ffffffffbdca60fa>] unshare_nsproxy_namespaces+0x5a/0xb0
[  610.382818]  [<ffffffffbdc82dcd>] SyS_unshare+0x1cd/0x360
[  610.383319]  [<ffffffffbe48c03b>] entry_SYSCALL_64_fastpath+0x1e/0xad