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SubjectRe: [PATCH 1/2] cpuset: fix cpuset_cpus_allowed_fallback() don't update tsk->rt.nr_cpus_allowed
On Sun, May 15, 2011 at 11:55:47AM -0700, Paul E. McKenney wrote:
> > Seems rcu_spawn_one_cpu_kthread() call wake_up_process() directly,
> > which is under hotplug event CPU_UP_PREPARE. Maybe it should be
> > under CPU_ONLINE.
>
> Sorry, but this does not work. The kthread must be running by the time

Yup, I also noticed that from the comments of rcu_spawn_one_cpu_kthread();

> the CPU appears, otherwise RCU grace periods in CPU_ONLINE notifiers
> will never complete.
>
> This did turn out to be a scheduler bug -- see Cheng Xu's recent patch.
> (chengxu@linux.vnet.ibm.com)

Hmmm, I don't think they are same issue.

The patch sent by Cheng Xu is about rt runtime normalization during
CPU hotplug.

But this one is about too earlier rt_rq load balance.

Let consider this:

on boot CPU
...
kernel_init();
smp_init();
_cpu_up();
__cpu_notify(CPU_UP_PREPARE | mod, hcpu, -1, &nr_calls);
rcu_cpu_notify();
rcu_online_kthreads();
rcu_spawn_one_node_kthread();
wake_up_process();
try_to_wake_up();
select_task_rq();
select_fallback_rq();
cpuset_cpus_allowed_fallback();
/* here the rcu_thread's cpus_allowed will be set
to cpu_possible_mask, but now we only have the
boot cpu online, so it will run on the boot cpu
So if we have KOSAKI's patch applied,
p->rt.nr_cpus_allowed will be set to
cpumask_weight(cpu_possible_mask);
*/
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
__sched_setscheduler();
check_class_changed();
p->sched_class->switched_to(rq, p); /* rt_class */
push_rt_task();
find_lock_lowest_rq();
find_lowest_rq();
/* crash here because local_cpu_mask is uninitialized */


Thanks,
Yong


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