Re: [PATCH 1/5] x86, gfp: Cache best near node for memory allocation.

[Tang Chen <tangchen@cn.fujitsu.com>]

Hi Liu,

Have you posted your new patches ?
(I mean memory-less node support patches.)

If you are going to post them, please cc me.

And BTW, how did you reproduce the memory-less node problem ?
Do you have a real memory-less node on your machine ?

Thanks. :)

On 08/04/2015 04:05 PM, Jiang Liu wrote:
> On 2015/8/4 11:36, Tang Chen wrote:
>> Hi TJ,
>>
>> Sorry for the late reply.
>>
>> On 07/16/2015 05:48 AM, Tejun Heo wrote:
>>> ......
>>> so in initialization pharse makes no sense any more. The best near online
>>> node for each cpu should be cached somewhere.
>>> I'm not really following.  Is this because the now offline node can
>>> later come online and we'd have to break the constant mapping
>>> invariant if we update the mapping later?  If so, it'd be nice to
>>> spell that out.
>> Yes. Will document this in the next version.
>>
>>>> ......
>>>>    +int get_near_online_node(int node)
>>>> +{
>>>> +    return per_cpu(x86_cpu_to_near_online_node,
>>>> +               cpumask_first(&node_to_cpuid_mask_map[node]));
>>>> +}
>>>> +EXPORT_SYMBOL(get_near_online_node);
>>> Umm... this function is sitting on a fairly hot path and scanning a
>>> cpumask each time.  Why not just build a numa node -> numa node array?
>> Indeed. Will avoid to scan a cpumask.
>>
>>> ......
>>>
>>>>      static inline struct page *alloc_pages_exact_node(int nid, gfp_t
>>>> gfp_mask,
>>>>                            unsigned int order)
>>>>    {
>>>> -    VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES || !node_online(nid));
>>>> +    VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
>>>> +
>>>> +#if IS_ENABLED(CONFIG_X86) && IS_ENABLED(CONFIG_NUMA)
>>>> +    if (!node_online(nid))
>>>> +        nid = get_near_online_node(nid);
>>>> +#endif
>>>>          return __alloc_pages(gfp_mask, order, node_zonelist(nid,
>>>> gfp_mask));
>>>>    }
>>> Ditto.  Also, what's the synchronization rules for NUMA node
>>> on/offlining.  If you end up updating the mapping later, how would
>>> that be synchronized against the above usages?
>> I think the near online node map should be updated when node online/offline
>> happens. But about this, I think the current numa code has a little
>> problem.
>>
>> As you know, firmware info binds a set of CPUs and memory to a node. But
>> at boot time, if the node has no memory (a memory-less node) , it won't
>> be online.
>> But the CPUs on that node is available, and bound to the near online node.
>> (Here, I mean numa_set_node(cpu, node).)
>>
>> Why does the kernel do this ? I think it is used to ensure that we can
>> allocate memory
>> successfully by calling functions like alloc_pages_node() and
>> alloc_pages_exact_node().
>> By these two fuctions, any CPU should be bound to a node who has memory
>> so that
>> memory allocation can be successful.
>>
>> That means, for a memory-less node at boot time, CPUs on the node is
>> online,
>> but the node is not online.
>>
>> That also means, "the node is online" equals to "the node has memory".
>> Actually, there
>> are a lot of code in the kernel is using this rule.
>>
>>
>> But,
>> 1) in cpu_up(), it will try to online a node, and it doesn't check if
>> the node has memory.
>> 2) in try_offline_node(), it offlines CPUs first, and then the memory.
>>
>> This behavior looks a little wired, or let's say it is ambiguous. It
>> seems that a NUMA node
>> consists of CPUs and memory. So if the CPUs are online, the node should
>> be online.
> Hi Chen,
> 	I have posted a patch set to enable memoryless node on x86,
> will repost it for review:) Hope it help to solve this issue.
> Thanks!
> Gerry
>
>> And also,
>> The main purpose of this patch-set is to make the cpuid <-> nodeid
>> mapping persistent.
>> After this patch-set, alloc_pages_node() and alloc_pages_exact_node()
>> won't depend on
>> cpuid <-> nodeid mapping any more. So the node should be online if the
>> CPUs on it are
>> online. Otherwise, we cannot setup interfaces of CPUs under /sys.
>>
>>
>> Unfortunately, since I don't have a machine a with memory-less node, I
>> cannot reproduce
>> the problem right now.
>>
>> How do you think the node online behavior should be changed ?
>>
>> Thanks.
>>
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