At 10/10/2012 07:27 AM, David Rientjes Wrote:
> On Tue, 9 Oct 2012, Peter Zijlstra wrote:
>
>> Well the code they were patching is in the wakeup path. As I think Tang
>> said, we leave !runnable tasks on whatever cpu they ran on last, even if
>> that cpu is offlined, we try and fix up state when we get a wakeup.
>>
>> On wakeup, it tries to find a cpu to run on and will try a cpu of the
>> same node first.
>>
>> Now if that node's entirely gone away, it appears the cpu_to_node() map
>> will not return a valid node number.
>>
>> I think that's a change in behaviour, it didn't used to do that afaik.
>> Certainly this code hasn't change in a while.
>>
>
> If cpu_to_node() always returns a valid node id even if all cpus on the
> node are offline, then the cpumask_of_node() implementation, which the
> sched code is using, should either return an empty cpumask (if
> node_to_cpumask_map[nid] isn't freed) or cpu_online_mask. The change in
> behavior here occurred because
> cpu_hotplug-unmap-cpu2node-when-the-cpu-is-hotremoved.patch in -mm doesn't
> return a valid node id and forces it to return -1 so a kzalloc_node(...,
> -1) fallsback to allocate anywhere.
>
> But if you only need cpu_to_node() when waking up to find a runnable cpu
> for this NUMA information, then I think you can just change the
> kzalloc_node() in alloc_{fair,rt}_sched_group() to do
> kzalloc(..., cpu_online(cpu) ? cpu_to_node(cpu) : NUMA_NO_NODE).
>
> [ The changelog here is confusing because it's fixing a problem in
> linux-next without saying so. ]
>
I don't agree with this way. Because it only fix the code which causes a
problem, and we can't say there is no any similar problem. So it is
why I clear the cpu-to-node mapping.
What about the following solution:
1. clear the cpu-to-node mapping when the node is offlined
2. tang's patch is still necessary because we leave !runnable tasks on
whatever cpu they ran on last. If cpu's node is NUMA_NO_NODE, it means
the entire node is offlined, and we must migrate the task to the other
node.
Thanks
Wen Congyang
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