Re: [PATCH 1/2] workqueue: update numa affinity info at node hotplug

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 01/13/2015 11:22 PM, Tejun Heo wrote:
> Hello,
> 
> On Tue, Jan 13, 2015 at 03:19:09PM +0800, Lai Jiangshan wrote:
>> The Mapping of the *online* cpus to nodes is already maintained by numa code.
>>
>> What the workqueue needs is a special Mapping:
>> 	The Mapping of the *possible* cpus to nodes
>>
>> But this mapping (if the numa code maintain it) is a trouble:
>> 	"possible" implies the mapping is stable/constant/immutable, it is hard to
>> 	ensure it in the numa code.
>>
>> if mutability of this mapping is acceptable, we just move 20~40 LOC
>> of code from workqueue to numa code, all the other complexities
>> about it are still in workqueue.c.
> 
> Make numa code maintain the mapping to the best of its knowledge and
> invoke notification callbacks when it changes.  

The best of its knowledge is the physical onlined nodes and CPUs.
The cpu_present_mask can represent this knowledge. But it lacks of
the per-node cpu_present_mask and the notification callbacks.

> Even if that involves slightly more code, that's the right thing to do at this point.

Right, but in currently, the workqueue will be the only user, and I don't known
asking who to do it, so I may keep it in the workqueue.c.

> This puts the logic which is complicated by the fact that the mapping may
> change where it's caused not some random unrelated place.  


> It'd be
> awesome if somebody more familiar with the numa side can chime in and
> explain why this mapping change can't be avoided.

I'm also looking for someone answer it.

> 
> Thanks.
>