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

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Date:	Tue, 13 Jan 2015 15:19:09 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Ishimatsu, Yasuaki/石松 靖章" 
	<isimatu.yasuaki@...fujitsu.com>,
	Tang Chen <tangchen@...fujitsu.com>,
	"guz.fnst@...fujitsu.com" <guz.fnst@...fujitsu.com>
Subject: Re: [PATCH 1/2] workqueue: update numa affinity info at node hotplug

On 12/26/2014 04:11 AM, Tejun Heo wrote:
> On Wed, Dec 17, 2014 at 01:56:29PM +0900, Kamezawa Hiroyuki wrote:
>> Let me correct my words. Main purpose of this patch 1/2 is handling a case "node disappers" after boot.
>> And try to handle physicall node hotplug caes.
>>
>> Changes of cpu<->node relationship at CPU_ONLINE is handled in patch 2/2.
> 
> Can you please make numa code itself maintain the cpu to nodemask
> maps?  Let's make workqueue a simple consumer of that and we don't
> have proper notification mechanism for node up/down events?
> 
> Thanks.
> 

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.

Thanks
Lai
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