[Xen-devel] [PATCH] NUMA: Fix computation of needed nodes

[Andre Przywara <andre.przywara@xxxxxxx>]

Hi,

as Jan Beulich pointed out:
> I'm afraid this change isn't really correct:
>
> > +                cores_per_node = info['nr_cpus'] / info['nr_nodes']
> > +                nodes_required = (self.info['VCPUs_max'] + cores_per_node - 1) 
> > / cores_per_node
> Simply using cores_per_node (as calculated here) as a divisor is bound
> to cause division-by-zero issues, namely when limiting the number of
> CPUs on the Xen command line (maxcpus=).
Actually Jan's proposed method of getting additional nodes is more 
elegant, so I implemented: enumerating the best nodes and adding CPU 
affinity until all VCPUs can be backed by at least on physical core. 
This should fix problems with asymmetric NUMA configurations and cropped 
number of CPUs in Xen.
Regards,
Andre.

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>

--
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 488-3567-12
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diff -r 13d4e78ede97 tools/python/xen/xend/XendDomainInfo.py
--- a/tools/python/xen/xend/XendDomainInfo.py   Wed Jan 13 08:33:34 2010 +0000
+++ b/tools/python/xen/xend/XendDomainInfo.py   Wed Jan 13 22:55:56 2010 +0100
@@ -2724,13 +2724,12 @@
                         candidate_node_list.append(i)
                 best_node = find_relaxed_node(candidate_node_list)[0]
                 cpumask = info['node_to_cpu'][best_node]
-                cores_per_node = info['nr_cpus'] / info['nr_nodes']
-                nodes_required = (self.info['VCPUs_max'] + cores_per_node - 1) 
/ cores_per_node
-                if nodes_required > 1:
-                    log.debug("allocating %d NUMA nodes", nodes_required)
-                    best_nodes = find_relaxed_node(filter(lambda x: x != 
best_node, range(0,info['nr_nodes'])))
-                    for i in best_nodes[:nodes_required - 1]:
-                        cpumask = cpumask + info['node_to_cpu'][i]
+                best_nodes = find_relaxed_node(filter(lambda x: x != 
best_node, range(0,info['nr_nodes'])))
+                for node_idx in best_nodes:
+                    if len(cpumask) >= self.info['VCPUs_max']:
+                        break
+                    cpumask = cpumask + info['node_to_cpu'][node_idx]
+                    log.debug("allocating additional NUMA node %d", node_idx)
                 for v in range(0, self.info['VCPUs_max']):
                     xc.vcpu_setaffinity(self.domid, v, cpumask)
         return index

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