Re: [PATCH] Do not use cpu_to_node() to find an offlined cpu's node.

[Tang Chen]

Hi David,

Thanks for reviewing this patch. :)

On 10/09/2012 04:34 PM, Wen Congyang wrote:
> At 10/09/2012 02:21 PM, David Rientjes Wrote:
> > On Mon, 8 Oct 2012, Tang Chen wrote:
> > > +	/* If the cpu has been offlined, its nid was set to -1. */
> > > +	if (nid != -1) {
> >
> > NUMA_NO_NODE.

Yes, NUMA_NO_NODE is better. I'll improve it, thanks. :)

> > Eek, the nid shouldn't be -1 yet, though, for cpu hotplug since this
> > should be called at CPU_DYING level and migrate_tasks() still sees a valid
> > cpu.

As Wen said below, nid is now set to -1 when cpu is hotremoved.
I reproduce this problem in this situation:

all cpus are online, and hot remove a system board directorily, without
offlining any cpu.

As a result, the removed cpu's nid is set to -1, and this causes
problems.

> the cpu's node is set when the cpu is hotpluged(not online), and it will
> be cleared when the cpu is hotremoved(This patch is in akpm tree):
> https://lkml.org/lkml/2012/9/3/39
>
> I guess the task is in sleep state when the cpu is offlined, and it doesn't
> be migrated to another cpu.
>
> Thanks
> Wen Congyang
>
> > On x86, cpumask_of_node() is always guaranteed to return a valid cpumask
> > after boot so presumably this is a problem in some non-x86 arch code and
> > isn't actually a sched problem.

BTW, my box is x86. I think for the reason I said above,
cpumask_of_node() will no longer guaranteed anything even if on x86.

Thanks. :)

> > > +		nodemask = cpumask_of_node(nid);
> > > +
> > > +		/* Look for allowed, online CPU in same node. */
> > > +		for_each_cpu(dest_cpu, nodemask) {
> > > +			if (!cpu_online(dest_cpu))
> > > +				continue;
> > > +			if (!cpu_active(dest_cpu))
> > > +				continue;
> > > +			if (cpumask_test_cpu(dest_cpu, tsk_cpus_allowed(p)))
> > > +				return dest_cpu;
> > > +		}
> > >   	}
> > >
> > >   	for (;;) {
>
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