Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1242720 > unrolled thread

Re: [Intel-wired-lan] [Patch V3 5/9] i40e: Use numa_mem_id() to better support memoryless node

Back to article view | Back to linux.kernel

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  Re: [Intel-wired-lan] [Patch V3 5/9] i40e: Use numa_mem_id() to  better support memoryless node Andrew Morton <akpm@linux-foundation.org> - 2015-10-08 22:30 +0200
    Re: [Intel-wired-lan] [Patch V3 5/9] i40e: Use numa_mem_id() to  better support memoryless node Jiang Liu <jiang.liu@linux.intel.com> - 2015-10-09 08:00 +0200
      Re: [Intel-wired-lan] [Patch V3 5/9] i40e: Use numa_mem_id() to  better support memoryless node Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> - 2015-10-09 11:10 +0200
        Re: [Intel-wired-lan] [Patch V3 5/9] i40e: Use numa_mem_id() to  better support memoryless node Jiang Liu <jiang.liu@linux.intel.com> - 2015-10-09 11:30 +0200

#1242720 — Re: [Intel-wired-lan] [Patch V3 5/9] i40e: Use numa_mem_id() to better support memoryless node

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!

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [next] | [standalone]

#1243046

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

> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [next] | [standalone]

#1243162

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@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

I'm sorry if I misunderstand something.
After commit b0dc3a342af36f95a68fe229b8f0f73552c5ca08, there is no memset().

> 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?

To access pgdat, a pgdat's zone should be on per-pgdat-zonelist.
Now, __build_all_zonelists() is called under stop_machine(). That's the reason
why you're asking what stop_machine() does. And, as you know, stop_machine() is not
protecting anything. The caller may fallback into removed zone.

Then, let's think.

At first, please note "pgdat" is not removed (and cannot be removed),
accessing pgdat's memory will not cause segmentation fault.

Just contents are problem. At removal, zone's page related information
and pgdat's page related information is cleared.

alloc_pages uses zonelist/zoneref/cache to walk each zones without accessing
pgdat itself. I think accessing zonelist is safe because it's an array updated
by stop_machine().

So, the problem is alloc_pages() can work correctly even if zone contains no page.
I think it should work.

(Note: zones are included in pgdat. So, zeroing pgdat means zeroing zone and other
  structures. it will not work.)

So, what problem you see now ?
I'm sorry I can't chase old discusions.

Thanks,
-Kame















--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [next] | [standalone]

#1243184

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@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
> 
> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web