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SubjectRe: [PATCH 1/5] x86: Change size of node ids from u8 to u16 fixup
On Sat, 19 Jan 2008, Mike Travis wrote:

> > Yeah, NID_INVAL is negative so no unsigned type will work here,
> > unfortunately. And that reduces the intended savings of your change since
> > the smaller type can only be used with a smaller CONFIG_NODES_SHIFT.
> >
>
> Excuse my ignorance but why wouldn't this work:
>
> static numanode_t pxm_to_node_map[MAX_PXM_DOMAINS]
> = { [0 ... MAX_PXM_DOMAINS - 1] = NUMA_NO_NODE };
> ...
> >> int acpi_map_pxm_to_node(int pxm)
> >> {
> > int node = pxm_to_node_map[pxm];
> >
> > if (node < 0)
>
> numanode_t node = pxm_to_node_map[pxm];
>

Because NUMA_NO_NODE is 0xff on x86. That's a valid node id for
configurations with CONFIG_NODES_SHIFT equal to or greater than 8.

> if (node != NUMA_NO_NODE) {

Wrong, this should be

node == NUMA_NO_NODE

> >> if (nodes_weight(nodes_found_map) >= MAX_NUMNODES)
> >> return NID_INVAL;
> >> node = first_unset_node(nodes_found_map);
> >> __acpi_map_pxm_to_node(pxm, node);
> >> node_set(node, nodes_found_map);
> >> }
>

The net result of this is that if a proximity domain is looked up through
acpi_map_pxm_to_node() and already has a mapping to node 255 (legal with
CONFIG_NODES_SHIFT == 8), this function will return NID_INVAL since the
weight of nodes_found_map is equal to MAX_NUMNODES.

You simply can't use valid node id's to signify invalid or unused node
ids.

> or change:
> #define NID_INVAL (-1)
> to
> #define NID_INVAL ((numanode_t)(-1))
> ...
> if (node != NID_INVAL) {

You mean

node == NID_INVAL

> >> if (nodes_weight(nodes_found_map) >= MAX_NUMNODES)
> >> return NID_INVAL;
> >> node = first_unset_node(nodes_found_map);
> >> __acpi_map_pxm_to_node(pxm, node);
> >> node_set(node, nodes_found_map);
> >> }
>

That's the equivalent of your NUMA_NO_NODE code above. The fact remains
that (numanode_t)-1 is still a valid node id for MAX_NUMNODES >= 256.

So, as I said in my initial reply, the only way to get the savings you're
looking for is to use u8 for CONFIG_NODES_SHIFT <= 7 and then convert all
NID_INVAL users to use NUMA_NO_NODE.

Additionally, Linux has always discouraged typedefs when they do not
define an architecture-specific size. The savings from your patch for
CONFIG_NODES_SHIFT == 7 would be 256 bytes for this mapping.

It's simply not worth it.

> And btw, shouldn't the pxm value be sized to numanode_t size as well?
> Will it ever be larger than the largest node id?
>

Section 6.2.9 of ACPI 2.0 states that PXM's return an integer, so that
would be non-conforming to the standard.

Additionally, PXM's are not nodes, so casting them to anything called
numanode_t shows the semantic flaw in your patch.

David


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