linux-kernel - Re: [PATCH 1/5] x86: Change size of node ids from u8 to u16 fixup

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Date:	Sat, 19 Jan 2008 14:33:04 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Mike Travis <travis@....com>
cc:	Yinghai Lu <yhlu.kernel@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, mingo@...e.hu,
	Christoph Lameter <clameter@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Eric Dumazet <dada1@...mosbay.com>
Subject: Re: [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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