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. ]
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