netdev - Re: [Intel-wired-lan] [Patch V3 5/9] i40e: Use numa_mem

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Date:	Fri, 9 Oct 2015 17:25:36 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>
Cc:	"Patil, Kiran" <kiran.patil@...el.com>,
	Mel Gorman <mgorman@...e.de>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
	Tang Chen <tangchen@...fujitsu.com>, Tejun Heo <tj@...nel.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
	"Nelson, Shannon" <shannon.nelson@...el.com>,
	"Wyborny, Carolyn" <carolyn.wyborny@...el.com>,
	"Skidmore, Donald C" <donald.c.skidmore@...el.com>,
	"Vick, Matthew" <matthew.vick@...el.com>,
	"Ronciak, John" <john.ronciak@...el.com>,
	"Williams, Mitch A" <mitch.a.williams@...el.com>,
	"Luck, Tony" <tony.luck@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-hotplug@...r.kernel.org" <linux-hotplug@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [Intel-wired-lan] [Patch V3 5/9] i40e: Use numa_mem_id() to
 better support memoryless node

On 2015/10/9 17:08, Kamezawa Hiroyuki wrote:
> On 2015/10/09 14:52, Jiang Liu wrote:
>> On 2015/10/9 4:20, Andrew Morton wrote:
>>> On Wed, 19 Aug 2015 17:18:15 -0700 (PDT) David Rientjes
>>> <rientjes@...gle.com> wrote:
>>>
>>>> On Wed, 19 Aug 2015, Patil, Kiran wrote:
>>>>
>>>>> Acked-by: Kiran Patil <kiran.patil@...el.com>
>>>>
>>>> Where's the call to preempt_disable() to prevent kernels with
>>>> preemption
>>>> from making numa_node_id() invalid during this iteration?
>>>
>>> David asked this question twice, received no answer and now the patch
>>> is in the maintainer tree, destined for mainline.
>>>
>>> If I was asked this question I would respond
>>>
>>>    The use of numa_mem_id() is racy and best-effort.  If the unlikely
>>>    race occurs, the memory allocation will occur on the wrong node, the
>>>    overall result being very slightly suboptimal performance.  The
>>>    existing use of numa_node_id() suffers from the same issue.
>>>
>>> But I'm not the person proposing the patch.  Please don't just ignore
>>> reviewer comments!
>> Hi Andrew,
>>     Apologize for the slow response due to personal reasons!
>> And thanks for answering the question from David. To be honest,
>> I didn't know how to answer this question before. Actually this
>> question has puzzled me for a long time when dealing with memory
>> hot-removal. For normal cases, it only causes sub-optimal memory
>> allocation if schedule event happens between querying NUMA node id
>> and calling alloc_pages_node(). But what happens if system run into
>> following execution sequence?
>> 1) node = numa_mem_id();
>> 2) memory hot-removal event triggers
>> 2.1) remove affected memory
>> 2.2) reset pgdat to zero if node becomes empty after memory removal
> 
> I'm sorry if I misunderstand something.
> After commit b0dc3a342af36f95a68fe229b8f0f73552c5ca08, there is no
> memset().
Hi Kamezawa,
	Thanks for the information. The commit solved the issue what
I was puzzling about. With this change applied, thing should work
as expected. Seems it would be better to enhance __build_all_zonelists()
to handle those offlined empty nodes too, but that really doesn't
make to much difference:)
	Thanks for the info again!
Thanks!
Gerry
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