Subject: Re: [PATCH RFC 4/5] powerpc:numa Add helper functions to maintain chipid to nid mapping

From: Nishanth Aravamudan <[email-masked]>
Date: Mon, 28 Sep 2015 10:32:45 -0700

On 27.09.2015 [23:59:12 +0530], Raghavendra K T wrote:
> Create arrays that maps serial nids and sparse chipids.
> 
> Note: My original idea had only two arrays of chipid to nid map. Final
> code is inspired by driver/acpi/numa.c that maps a proximity node with
> a logical node by Takayoshi Kochi <[email-masked]>, and thus
> uses an additional chipid_map nodemask. The mask helps in first unused
> nid easily by knowing first unset bit in the mask.
> 
> No change in functionality.
> 
> Signed-off-by: Raghavendra K T <[email-masked]>
> ---
>  arch/powerpc/mm/numa.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 47 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index dd2073b..f015cad 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -63,6 +63,11 @@ static int form1_affinity;
>  static int distance_ref_points_depth;
>  static const __be32 *distance_ref_points;
>  static int distance_lookup_table[MAX_NUMNODES][MAX_DISTANCE_REF_POINTS];
> +static nodemask_t chipid_map = NODE_MASK_NONE;
> +static int chipid_to_nid_map[MAX_NUMNODES]
> +				= { [0 ... MAX_NUMNODES - 1] = NUMA_NO_NODE };

Hrm, conceptually there are *more* chips than nodes, right? So what
guarantees we won't see > MAX_NUMNODES chips?

> +static int nid_to_chipid_map[MAX_NUMNODES]
> +				= { [0 ... MAX_NUMNODES - 1] = NUMA_NO_NODE };
> 
>  /*
>   * Allocate node_to_cpumask_map based on number of available nodes
> @@ -133,6 +138,48 @@ static int __init fake_numa_create_new_node(unsigned long end_pfn,
>  	return 0;
>  }
> 
> +int chipid_to_nid(int chipid)
> +{
> +	if (chipid < 0)
> +		return NUMA_NO_NODE;

Do you really want to support these cases? Or should they be
bugs/warnings indicating that you got an unexpected input? Or at least
WARN_ON_ONCE?

> +	return chipid_to_nid_map[chipid];
> +}
> +
> +int nid_to_chipid(int nid)
> +{
> +	if (nid < 0)
> +		return NUMA_NO_NODE;
> +	return nid_to_chipid_map[nid];
> +}
> +
> +static void __map_chipid_to_nid(int chipid, int nid)
> +{
> +	if (chipid_to_nid_map[chipid] == NUMA_NO_NODE
> +	     || nid < chipid_to_nid_map[chipid])
> +		chipid_to_nid_map[chipid] = nid;
> +	if (nid_to_chipid_map[nid] == NUMA_NO_NODE
> +	    || chipid < nid_to_chipid_map[nid])
> +		nid_to_chipid_map[nid] = chipid;
> +}

chip <-> node mapping is a static (physical) concept, right? Should we
emit some debugging if for some reason we get a runtime call to remap
an already mapped chip to a new node?

> +
> +int map_chipid_to_nid(int chipid)
> +{
> +	int nid;
> +
> +	if (chipid < 0 || chipid >= MAX_NUMNODES)
> +		return NUMA_NO_NODE;
> +
> +	nid = chipid_to_nid_map[chipid];
> +	if (nid == NUMA_NO_NODE) {
> +		if (nodes_weight(chipid_map) >= MAX_NUMNODES)
> +			return NUMA_NO_NODE;

If you create a KVM guest with a bogus topology, doesn't this just start
losing NUMA information for very high-noded guests?

> +		nid = first_unset_node(chipid_map);
> +		__map_chipid_to_nid(chipid, nid);
> +		node_set(nid, chipid_map);
> +	}
> +	return nid;
> +}
> +
>  int numa_cpu_lookup(int cpu)
>  {
>  	return numa_cpu_lookup_table[cpu];
> @@ -264,7 +311,6 @@ out:
>  	return chipid;
>  }
> 
> -

stray change?

>   /* Return the nid from associativity */
>  static int associativity_to_nid(const __be32 *associativity)
>  {
> -- 
> 1.7.11.7
>