On Tue, 23 Feb 2010, Miao Xie wrote:
> >> /*
> >> @@ -1391,11 +1393,10 @@ static void cpuset_attach(struct cgroup_subsys *ss, struct cgroup *cont,
> >>
> >> if (cs == &top_cpuset) {
> >> cpumask_copy(cpus_attach, cpu_possible_mask);
> >> - to = node_possible_map;
> >> } else {
> >> guarantee_online_cpus(cs, cpus_attach);
> >> - guarantee_online_mems(cs, &to);
> >> }
> >> + guarantee_online_mems(cs, &to);
> >>
> >> /* do per-task migration stuff possibly for each in the threadgroup */
> >> cpuset_attach_task(tsk, &to, cs);
> >
> > Do we need to set cpus_attach to cpu_possible_mask? Why won't
> > cpu_active_mask suffice?
>
> If we set cpus_attach to cpu_possible_mask, we needn't do anything for tasks in the top_cpuset when
> doing cpu hotplug. If not, we will update cpus_allowed of all tasks in the top_cpuset.
>
Cpu hotplug sets top_cpuset's cpus_allowed to cpu_active_mask by default,
regardless of what was onlined or offlined. cpus_attach in the context of
your patch (in cpuset_attach()) passes cpu_possible_mask to
set_cpus_allowed_ptr() if the task is being attached to top_cpuset, my
question was why don't we pass cpu_active_mask instead? In other words, I
think we should do
cpumask_copy(cpus_attach, cpu_active_mask);
when attached to top_cpuset like my patch did.
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