Re: [Next] CPU Hotplug test failures on powerpc

[Sachin Sant]

Benjamin Herrenschmidt wrote:
> > static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
> > {
> >         int dest_cpu;
> >         const struct cpumask *nodemask = cpumask_of_node(cpu_to_node(dead_cpu));
> >
> > again:
> >         /* Look for allowed, online CPU in same node. */
> >         for_each_cpu_and(dest_cpu, nodemask, cpu_active_mask)
> >                 if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
> >                         goto move;
> >
> >         /* Any allowed, online CPU? */
> >         dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
> >         if (dest_cpu < nr_cpu_ids)
> >                 goto move;
> >
> >         /* No more Mr. Nice Guy. */
> >         if (dest_cpu >= nr_cpu_ids) {
> >                 cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
> > ====>           dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed);
> >
> >                 /*
> >                  * Don't tell them about moving exiting tasks or
> >                  * kernel threads (both mm NULL), since they never
> >                  * leave kernel.
> >                  */
> >                 if (p->mm && printk_ratelimit()) {
> >                         pr_info("process %d (%s) no longer affine to cpu%d\n",
> >                                 task_pid_nr(p), p->comm, dead_cpu);
> >                 }
> >         }
> >
> > move:
> >         /* It can have affinity changed while we were choosing. */
> >         if (unlikely(!__migrate_task_irq(p, dead_cpu, dest_cpu)))
> >                 goto again;
> > }
> >
> > Both masks, p->cpus_allowed and cpu_active_mask are stable in that p
> > won't go away since we hold the tasklist_lock (in migrate_list_tasks),
> > and cpu_active_mask is static storage, so WTH is it going funny on?
> >     
I added some debug statements within the above code. 
This is a 2 cpu machine.

XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
XMON dest_cpu = 1024 
XMON dest_cpu = 1024 . dead_cpu = 1
XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
XMON dest_cpu = 1024 
XMON dest_cpu = 1024 . dead_cpu = 1
XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
XMON dest_cpu = 1024 
XMON dest_cpu = 1024 . dead_cpu = 1

Seems to me that the control is stuck in an infinite loop and hence the
machine appears to be in hung state. The dest_cpu value is always 1024
and never changes, which result in an infinite loop.

In working scenario the o/p is something on the following lines

XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
XMON dest_cpu = 0 
XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
XMON dest_cpu = 0 
XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
XMON dest_cpu = 0 

Let me know if i should try to record any specific value ?

Thanks
-Sachin

--

---------------------------------
Sachin Sant
IBM Linux Technology Center
India Systems and Technology Labs
Bangalore, India
---------------------------------

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