Subject: Re: [Next] CPU Hotplug test failures on powerpc

From: Sachin Sant <[email-masked]>
Date: Tue, 15 Dec 2009 15:14:11 +0530

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

-- 

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