Date:	Thu, 2 Apr 2015 11:54:19 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Gu Zheng <guz.fnst@...fujitsu.com>, Tejun Heo <tj@...nel.org>
CC:	<linux-kernel@...r.kernel.org>, <laijs@...fujitsu.com>,
	<isimatu.yasuaki@...fujitsu.com>, <tangchen@...fujitsu.com>,
	<izumi.taku@...fujitsu.com>
Subject: Re: [PATCH 0/2] workqueue: fix a bug when numa mapping is changed

On 2015/04/02 10:36, Gu Zheng wrote:
> Hi Kame, TJ,
>
> On 04/01/2015 04:30 PM, Kamezawa Hiroyuki wrote:
>
>> On 2015/04/01 12:02, Tejun Heo wrote:
>>> On Wed, Apr 01, 2015 at 11:55:11AM +0900, Kamezawa Hiroyuki wrote:
>>>> Now, hot-added cpus will have the lowest free cpu id.
>>>>
>>>> Because of this, in most of systems which has only cpu-hot-add, cpu-ids are always
>>>> contiguous even after cpu hot add.
>>>> In enterprise, this would be considered as imcompatibility.
>>>>
>>>> determining cpuid <-> lapicid at boot will make cpuids sparse. That may corrupt
>>>> exisiting script or configuration/resource management software.
>>>
>>> Ugh... so, cpu number allocation on hot-add is part of userland
>>> interface that we're locked into?
>>
>> We checked most of RHEL7 packages and didn't find a problem yet.
>> But, for examle, we know some performance test team's test program assumed contiguous
>> cpuids and it failed. It was an easy case because we can ask them to fix the application
>> but I guess there will be some amount of customers that cpuids are contiguous.
>>
>>> Tying hotplug and id allocation
>>> order together usually isn't a good idea.  What if the cpu up fails
>>> while running the notifiers?  The ID is already allocated and the next
>>> cpu being brought up will be after a hole anyway.  Is this even
>>> actually gonna affect userland?
>>>
>>
>> Maybe. It's not fail-safe but....
>>
>> In general, all kernel engineers (and skilled userland engineers) knows that
>> cpuids cannot be always contiguous and cpuids/nodeids should be checked before
>> running programs. I think most of engineers should be aware of that but many
>> users have their own assumption :(
>>
>> Basically, I don't have strong objections, you're right technically.
>>
>> In summary...
>>   - users should not assume cpuids are contiguous.
>>   - all possible ids should be fixed at boot time.
>>   - For uses, some clarification document should be somewhere in Documenatation.
>
> Fine to me.
>
>>
>> So, Gu-san
>>   1) determine all possible ids at boot.
>>   2) clarify cpuid/nodeid can have hole because of 1) in Documenation.
>>   3) It would be good if other guys give us ack.
>
> Also fine.
> But before this going, could you please reconsider determining the ids when firstly
> present (the implementation on this patchset)?
> Though it is not the perfect one in some words, but we can ignore the doubts that
> mentioned above as the cpu/node hotplug is not frequent behaviours, and there seems
> not anything harmful to us if we go this way.
>

Is it so heavy work ?  Hmm. My requests are

Implement your patches as
- Please don't change current behavior at boot.
- Remember all possible apicids and give them future cpuids if not assigned.
as step 1.

Please fix dynamic pxm<->node detection in step2.

In future, memory-less node handling in x86 should be revisited.

Thanks,
-Kame







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