Paul E. McKenney <paul...@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.vosbu...@canonical.com
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