Re: [PATCH] workqueue: Ensure that cpumask set for pools created after boot

[Michael Bringmann <mwb@linux.vnet.ibm.com>]

To confirm, you want the WARN_ON(cpumask_any(pool->attrs->cpumask) >= NR_CPUS)
at the point where I place my current patch?

On 05/23/2017 02:49 PM, Tejun Heo wrote:
> Hello, Michael.
> 
> On Tue, May 23, 2017 at 02:44:23PM -0500, Michael Bringmann wrote:
>> On 05/16/2017 10:55 AM, Tejun Heo wrote:
>>> Hello, Michael.
>>>
>>> On Mon, May 15, 2017 at 10:48:04AM -0500, Michael Bringmann wrote:
>>>>>> --- a/kernel/workqueue.c
>>>>>> +++ b/kernel/workqueue.c
>>>>>> @@ -3366,6 +3366,8 @@ static struct worker_pool *get_unbound_pool(const struct workqueue_attrs *attrs)
>>>>>>  	copy_workqueue_attrs(pool->attrs, attrs);
>>>>>>  	pool->node = target_node;
>>>>>>  
>>>>>> +	cpumask_copy(pool->attrs->cpumask, cpumask_of(smp_processor_id()));
>>>>>
>>>>> What prevents a cpu getting added right here tho?
>>>>
>>>> PowerPC has only one control path to add/remove CPUs via DLPAR operations.
>>>> Even so, the underlying code is protected through multiple locks.
>>>
>>> The more I look at the patch, the less sense it seems to make.  So,
>>> whenever we create a new pool, we ignore the requested cpumask and
>>> override it with the cpumask of the current thread?
>>
>> No.  As I mentioned previously, the operation/problem occurs within a DLPAR
>> hotplug add/remove operation.  This is happening to a node which previously
> 
> But that's what the code is doing.  Whenever it creates a new unbound
> pool, it ends up ignoring the requested cpumask and overwrites it with
> the cpumask containing self.
> 
>> did not have any CPUs associated to it -- we are trying to add more resources
>> to an LPAR / partition.  At this point, the cpumask for the node is empty / zero.
>> Sorry for not being more clear on this point earlier.
> ...
>>> A new unbound workqueue and thus unbound pool can also be created from
>>> paths outside cpu hotplug, so get_unbound_pool() can race against
>>> hotplug.  Can you please explain the failures that you see in more
>>> detail?  I'm sure your patch works around the issue somehow but it
>>> doesn't look like the right fix.
>>
>> We fill in an empty cpumask field with a guaranteed non-empty value.
>> I verified that the incoming cpumask in the attrs was zero at this point
>> preceding the failure.  If we proceed without putting in a useful value,
>> we go to 'wake_up_process()' (kernel/sched/core.c) next to wakeup the new
>> worker for the new unbound pool.  While there, the code runs through
>> 'select_task_rq()' and invokes cpumask_any() on a copy of the cpumask.
>> Unfortunately, running that function over an empty/non-initialized cpumask
>> returns an index beyond the end of the list, resulting shortly thereafter
>> in an instruction/data fetch exception.
>>
>> If you have a suggestion for an alternate non-empty value to use, I would
>> be happy to try it.
> 
> Can you please post the backtrace of the problematic worker pool being
> created (WARN_ON empty cpumask while creating a new pool)?
> 
> Thanks.
> 

-- 
Michael W. Bringmann
Linux Technology Center
IBM Corporation
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mwb@linux.vnet.ibm.com