Messages in this thread | | | Date | Mon, 27 Apr 2015 18:44:48 +0900 | From | Kamezawa Hiroyuki <> | Subject | Re: [RESEND RFC PATCH 2/2] gfp: use the best near online node if the target node is offline |
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On 2015/04/25 5:01, Andrew Morton wrote: > On Fri, 24 Apr 2015 17:58:33 +0800 Gu Zheng <guz.fnst@cn.fujitsu.com> 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);
In above VM_BUG_ON(), !node_online(nid) is the bug.
>> + >> + 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.
I agree. the results should be cached. If necessary, in per-cpu line.
Thanks, -Kame
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