Hi Andrew,
On 04/25/2015 04:01 AM, Andrew Morton wrote:
> On Fri, 24 Apr 2015 17:58:33 +0800 Gu Zheng <guz.fnst@xxxxxxxxxxxxxx> wrote:
>
>> Since the change to the cpu <--> mapping (map the cpu to the physical
>> node for all possible at the boot), the node of cpu may be not present,
>> so we use the best near online node if the node is not present in the low
>> level allocation APIs.
>>
>> ...
>>
>> --- a/include/linux/gfp.h
>> +++ b/include/linux/gfp.h
>> @@ -298,9 +298,31 @@ __alloc_pages(gfp_t gfp_mask, unsigned int order,
>> return __alloc_pages_nodemask(gfp_mask, order, zonelist, NULL);
>> }
>>
>> +static int find_near_online_node(int node)
>> +{
>> + int n, val;
>> + int min_val = INT_MAX;
>> + int best_node = -1;
>> +
>> + for_each_online_node(n) {
>> + val = node_distance(node, n);
>> +
>> + if (val < min_val) {
>> + min_val = val;
>> + best_node = n;
>> + }
>> + }
>> +
>> + return best_node;
>> +}
>
> This should be `inline' if it's in a header file.
>
> But it is far too large to be inlined anyway - please move it to a .c file.
Agree.
>
> And please document it. A critical thing to describe is how we
> determine whether a node is "near". There are presumably multiple ways
> in which we could decide that a node is "near" (number of hops, minimum
> latency, ...). Which one did you choose, and why?
It just reuse the dropped code in PATCH 1/2, based on the node_distance table,
which is a arch special defined one, and the data mostly comes from the
firmware info, e.g. SLIT table.
>
>> static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
>> unsigned int order)
>> {
>> + /* Offline node, use the best near online node */
>> + if (!node_online(nid))
>> + nid = find_near_online_node(nid);
>> +
>> /* Unknown node is current node */
>> if (nid < 0)
>> nid = numa_node_id();
>> @@ -311,7 +333,11 @@ static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
>> 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));
>> + /* Offline node, use the best near online node */
>> + if (!node_online(nid))
>> + nid = find_near_online_node(nid);
>> +
>> + VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
>>
>> return __alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask));
>> }
>
> Ouch. These functions are called very frequently, and adding overhead
> to them is a big deal. And the patch even adds overhead to non-x86
> architectures which don't benefit from it!
>
> Is there no way this problem can be fixed somewhere else? Preferably
> by fixing things up at hotplug time.
As Kame suggested, maintaining a per-cpu cache about the alternative-node
only for x86 arch seems a good choice.
Regards,
Gu
> .
>
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