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.
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.