Re: localed stuck in recent 3.18 git in copy_net_ns?

[Jay Vosburgh <jay.vosburgh@canonical.com>]

Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

>On Fri, Oct 24, 2014 at 09:33:33PM -0700, Jay Vosburgh wrote:
>> 	Looking at the dmesg, the early boot messages seem to be
>> confused as to how many CPUs there are, e.g.,
>> 
>> [    0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=4, Nodes=1
>> [    0.000000] Hierarchical RCU implementation.
>> [    0.000000]  RCU debugfs-based tracing is enabled.
>> [    0.000000]  RCU dyntick-idle grace-period acceleration is enabled.
>> [    0.000000]  RCU restricting CPUs from NR_CPUS=256 to nr_cpu_ids=4.
>> [    0.000000] RCU: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=4
>> [    0.000000] NR_IRQS:16640 nr_irqs:456 0
>> [    0.000000]  Offload RCU callbacks from all CPUs
>> [    0.000000]  Offload RCU callbacks from CPUs: 0-3.
>> 
>> 	but later shows 2:
>> 
>> [    0.233703] x86: Booting SMP configuration:
>> [    0.236003] .... node  #0, CPUs:      #1
>> [    0.255528] x86: Booted up 1 node, 2 CPUs
>> 
>> 	In any event, the E8400 is a 2 core CPU with no hyperthreading.
>
>Well, this might explain some of the difficulties.  If RCU decides to wait
>on CPUs that don't exist, we will of course get a hang.  And rcu_barrier()
>was definitely expecting four CPUs.
>
>So what happens if you boot with maxcpus=2?  (Or build with
>CONFIG_NR_CPUS=2.) I suspect that this might avoid the hang.  If so,
>I might have some ideas for a real fix.

	Booting with maxcpus=2 makes no difference (the dmesg output is
the same).

	Rebuilding with CONFIG_NR_CPUS=2 makes the problem go away, and
dmesg has different CPU information at boot:

[    0.000000] smpboot: 4 Processors exceeds NR_CPUS limit of 2
[    0.000000] smpboot: Allowing 2 CPUs, 0 hotplug CPUs
 [...]
[    0.000000] setup_percpu: NR_CPUS:2 nr_cpumask_bits:2 nr_cpu_ids:2 nr_node_ids:1
 [...]
[    0.000000] Hierarchical RCU implementation.
[    0.000000] 	RCU debugfs-based tracing is enabled.
[    0.000000] 	RCU dyntick-idle grace-period acceleration is enabled.
[    0.000000] NR_IRQS:4352 nr_irqs:440 0
[    0.000000] 	Offload RCU callbacks from all CPUs
[    0.000000] 	Offload RCU callbacks from CPUs: 0-1.

	-J

---
	-Jay Vosburgh, jay.vosburgh@canonical.com