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SubjectRe: [Intel-wired-lan] [Patch V3 5/9] i40e: Use numa_mem_id() to better support memoryless node
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On 2015/10/9 4:20, Andrew Morton wrote:
> On Wed, 19 Aug 2015 17:18:15 -0700 (PDT) David Rientjes <rientjes@google.com> wrote:
>
>> On Wed, 19 Aug 2015, Patil, Kiran wrote:
>>
>>> Acked-by: Kiran Patil <kiran.patil@intel.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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