Re: [RFC workqueue/driver-core PATCH 1/5] workqueue: Provide queue_work_near to queue work near a given NUMA node

Alexander Duyck
2018-10-01T21:55:01Z

On 10/1/2018 9:01 AM, Tejun Heo wrote:
> Hello,
>
> On Wed, Sep 26, 2018 at 03:19:21PM -0700, Alexander Duyck wrote:
>> On 9/26/2018 3:09 PM, Tejun Heo wrote:
>> I could just use queue_work_on probably, but is there any issue if I
>> am passing CPU values that are not in the wq_unbound_cpumask? That
>
> That should be fine. If it can't find any available cpu, it'll fall
> back to round-robin. We probably can improve it so that it can
> consider the numa distance when falling back.
>
>> was mostly my concern. Also for an unbound queue do I need to worry
>> about the hotplug lock? I wasn't sure if that was the case or not as
>
> Issuers don't need to worry about them.
>
>> I know it is called out as something to be concerned with using
>> queue_work_on, but in __queue_work the value is just used to
>> determine which node to grab a work queue from.
>
> It might be better to leave queue_work_on() to be used for per-cpu
> workqueues and introduce queue_work_near() as you suggseted. I just
> don't want it to duplicate the node selection code in it. Would that
> work?

So if I understand what you are saying correctly we default to
round-robin on a given node has no CPUs attached to it. I could probably
work with that if that is the default behavior instead of adding much of
the complexity I already have.

The question I have then is what should I do about workqueues that
aren't WQ_UNBOUND if they attempt to use queue_work_near? In that case I
should be looking for some way to go from a node to a CPU shouldn't I?
If so should I look at doing something like wq_select_unbound_cpu that
uses the node cpumask instead of the wq_unbound_cpumask?

- Alex