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Date: Sat, 19 Jan 2008 16:41:19 -0800 From: Mike Travis <travis@....com> To: David Rientjes <rientjes@...gle.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 David Rientjes wrote: > 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. Perhaps numanode_t should be set to u16 if MAX_NUMNODES > 255 to allow for an invalid value of 255? #if MAX_NUMNODES > 255 typedef u16 numanode_t; #else typedef u8 numanode_t; #endif > >> if (node != NUMA_NO_NODE) { > > Wrong, this should be > > node == NUMA_NO_NODE Oops, yes you're right. >>>> 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. Yes, I agree. I'll do the changes you're suggesting. > 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. So are you saying that I should just use u16 for all node ids whether CONFIG_NODES_SHIFT > 7 or not? Othersise, I would think that defining a typedef is a fairly clean solution. A quick grep shows that there are 35 arrays defined by MAX_NUMNODES in x86_64, 38 in X86_32 (not verified.) So it's not exactly a trivial amount of memory. > >> 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. Thanks for the info. I wasn't sure exactly what the PXM value represents. > > David Thanks again, Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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