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.
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?
> 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.
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