[PATCH V8 2/3] poserpc/initnodes: Ensure nodes initialized for hotplug

[Nathan Fontenot]

On 11/28/2017 04:58 PM, Michael Bringmann wrote:
> On powerpc systems which allow 'hot-add' of CPU, it may occur that
> the new resources are to be inserted into nodes that were not used
> for memory resources at bootup.  Many different configurations of
> PowerPC resources may need to be supported depending upon the
> environment.  Important characteristics of the nodes and operating
> environment include:
> 
> * Dedicated vs. shared CPUs.  Shared CPUs require information such
>   as the VPHN hcall for CPU assignment to nodes, since shared CPUs
>   have their affinity set to node 0 at boot and when hot-added.
>   Associativity decisions made based on dedicated resource rules,
>   such as associativity properties in the device tree, may vary from
>   decisions made using the values returned by the VPHN hcall.
> * memoryless nodes at boot.  Nodes need to be defined as 'possible'
>   at boot for operation with other code modules.  Previously, the
>   powerpc code would limit the set of possible nodes to those which
>   have memory assigned at boot, and were thus online.  Subsequent
>   add/remove of CPUs or memory would only work with this subset of
>   possible nodes.
> * memoryless nodes with CPUs at boot.  Due to the previous restriction
>   on nodes, nodes that had CPUs but no memory were being collapsed
>   into other nodes that did have memory at boot.  In practice this
>   meant that the node assignment presented by the runtime kernel
>   differed from the affinity and associativity attributes presented
>   by the device tree or VPHN hcalls.  Nodes that might be known to
>   the pHyp were not 'possible' in the runtime kernel because they did
>   not have memory at boot.
> 
> This patch fixes some problems encountered at runtime with
> configurations that support memory-less nodes, or that hot-add CPUs
> into nodes that are memoryless during system execution after boot.
> The problems of interest include,
> 
> * Nodes known to powerpc to be memoryless at boot, but to have
>   CPUs in them are allowed to be 'possible' and 'online'.  Memory
>   allocations for those nodes are taken from another node that does
>   have memory until and if memory is hot-added to the node.
> * Nodes which have no resources assigned at boot, but which may still
>   be referenced subsequently by affinity or associativity attributes,
>   are kept in the list of 'possible' nodes for powerpc.  Hot-add of
>   memory or CPUs to the system can reference these nodes and bring
>   them online instead of redirecting the references to one of the set
>   of nodes known to have memory at boot.
> 
> Note that this software operates under the context of CPU hotplug.
> We are not doing memory hotplug in this code, but rather updating
> the kernel's CPU topology (i.e. arch_update_cpu_topology /
> numa_update_cpu_topology).  We are initializing a node that may be
> used by CPUs or memory before it can be referenced as invalid by a
> CPU hotplug operation.  CPU hotplug operations are protected by a
> range of APIs including cpu_maps_update_begin/cpu_maps_update_done,
> cpus_read/write_lock / cpus_read/write_unlock, device locks, and more.
> Memory hotplug operations, including try_online_node, are protected
> by mem_hotplug_begin/mem_hotplug_done, device locks, and more.  In
> the case of CPUs being hot-added to a previously memoryless node, the
> try_online_node operation occurs wholly within the CPU locks with no
> overlap.  Using HMC hot-add/hot-remove operations, we have been able
> to add and remove CPUs to any possible node without failures.  HMC
> operations involve a degree self-serialization, though.
> 
> Signed-off-by: Michael Bringmann <mwb at linux.vnet.ibm.com>

Reviewed-by: Nathan Fontenot <nfont at linux.vnet.ibm.com>

> ---
> Changes in V8:
>   -- Clarify 'resources' as CPUs in patch description regarding
>      VPHN call.  Add another clause to statement mentioning that
>      shared CPUs start in node 0, and are finally assigned per
>      VPHN information.
>   -- Rename 'find_cpu_nid' to 'find_and_online_cpu_nid' for better
>      clarity of its function.
>   -- Restore '__init' tag to definition of 'setup_node_data'
> ---
>  arch/powerpc/mm/numa.c |   49 ++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 39 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index 735e3fd..6b08dd8 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -551,7 +551,7 @@ static int numa_setup_cpu(unsigned long lcpu)
>  	nid = of_node_to_nid_single(cpu);
> 
>  out_present:
> -	if (nid < 0 || !node_online(nid))
> +	if (nid < 0 || !node_possible(nid))
>  		nid = first_online_node;
> 
>  	map_cpu_to_node(lcpu, nid);
> @@ -910,10 +910,8 @@ static void __init find_possible_nodes(void)
>  		goto out;
> 
>  	for (i = 0; i < numnodes; i++) {
> -		if (!node_possible(i)) {
> -			setup_node_data(i, 0, 0);
> +		if (!node_possible(i))
>  			node_set(i, node_possible_map);
> -		}
>  	}
> 
>  out:
> @@ -1309,6 +1307,42 @@ static long vphn_get_associativity(unsigned long cpu,
>  	return rc;
>  }
> 
> +static inline int find_and_online_cpu_nid(int cpu)
> +{
> +	__be32 associativity[VPHN_ASSOC_BUFSIZE] = {0};
> +	int new_nid;
> +
> +	/* Use associativity from first thread for all siblings */
> +	vphn_get_associativity(cpu, associativity);
> +	new_nid = associativity_to_nid(associativity);
> +	if (new_nid < 0 || !node_possible(new_nid))
> +		new_nid = first_online_node;
> +
> +	if (NODE_DATA(new_nid) == NULL) {
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +		/*
> +		 * Need to ensure that NODE_DATA is initialized
> +		 * for a node from available memory (see
> +		 * memblock_alloc_try_nid).  If unable to init
> +		 * the node, then default to nearest node that
> +		 * has memory installed.
> +		 */
> +		if (try_online_node(new_nid))
> +			new_nid = first_online_node;
> +#else
> +		/*
> +		 * Default to using the nearest node that has
> +		 * memory installed.  Otherwise, it would be 
> +		 * necessary to patch the kernel MM code to deal
> +		 * with more memoryless-node error conditions.
> +		 */
> +		new_nid = first_online_node;
> +#endif
> +	}
> +
> +	return new_nid;
> +}
> +
>  /*
>   * Update the CPU maps and sysfs entries for a single CPU when its NUMA
>   * characteristics change. This function doesn't perform any locking and is
> @@ -1376,7 +1410,6 @@ int numa_update_cpu_topology(bool cpus_locked)
>  {
>  	unsigned int cpu, sibling, changed = 0;
>  	struct topology_update_data *updates, *ud;
> -	__be32 associativity[VPHN_ASSOC_BUFSIZE] = {0};
>  	cpumask_t updated_cpus;
>  	struct device *dev;
>  	int weight, new_nid, i = 0;
> @@ -1414,11 +1447,7 @@ int numa_update_cpu_topology(bool cpus_locked)
>  			continue;
>  		}
> 
> -		/* Use associativity from first thread for all siblings */
> -		vphn_get_associativity(cpu, associativity);
> -		new_nid = associativity_to_nid(associativity);
> -		if (new_nid < 0 || !node_online(new_nid))
> -			new_nid = first_online_node;
> +		new_nid = find_and_online_cpu_nid(cpu);
> 
>  		if (new_nid == numa_cpu_lookup_table[cpu]) {
>  			cpumask_andnot(&cpu_associativity_changes_mask,
>