Re: Long delays creating a netns after deleting one (possibly RCU related)

[Rolf Neugebauer]

On Thu, Nov 10, 2016 at 9:24 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Nov 10, 2016 at 09:37:47AM -0800, Cong Wang wrote:
>> (Cc'ing Paul)
>>
>> On Wed, Nov 9, 2016 at 7:42 AM, Rolf Neugebauer
>> <rolf.neugebauer@xxxxxxxxxx> wrote:
>> > 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?
>>
>> Not to me.
>>
>>
>> >
>> > [  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
>>
>> This is a worker which holds the net_mutex and is waiting for
>> a RCU grace period to elapse.
>>
>>
>> > [  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
>>
>> This process is apparently waiting for the net_mutex held by the previous one.
>>
>> Either RCU implementation is broken or something else is missing.
>> Do you have more stack traces of related processes? For example,
>> rcu_tasks_kthread. And if anything you can help to narrow down the problem,
>> it would be great.
>
> Did you set the rcu_normal boot parameter?  Doing so would have this effect.
>
> (It is intended for real-time users who don't like expedited grace periods.)

rcu_normal is not set on the kernel command line and
/sys/kernel/rcu_normal and /sys/kernel/rcu_expedited  both show 0.


>
>                                                         Thanx, Paul
>