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

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Date:	Fri, 9 Oct 2015 13:52:55 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	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 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
3) alloc_pages_node(), which may access zero-ed pgdat structure.

I haven't found a mechanism to protect system from above sequence yet,
so puzzled for a long time already:(. Does stop_machine() protect
system from such a execution sequence?
Thanks!
Gerry

> 
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