linux-kernel - Re: [patch 6/6] x86: cpumask_of_node() should handle -1 as a node

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Message-ID: <alpine.DEB.2.00.1001061545210.19882@chino.kir.corp.google.com>
Date:	Wed, 6 Jan 2010 15:51:06 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Anton Blanchard <anton@...ba.org>
cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 6/6] x86: cpumask_of_node() should handle -1 as a node

On Thu, 7 Jan 2010, Anton Blanchard wrote:

> I don't like the use of -1 as a node, but it's much more widespread than
> x86; including sh, powerpc, sparc and the generic topology code. eg:
> 
> 
> #fdef CONFIG_PCI
> extern int pcibus_to_node(struct pci_bus *pbus);
> #else
> static inline int pcibus_to_node(struct pci_bus *pbus)
> {
>         return -1;
> }

This seems to be the same semantics that NUMA_NO_NODE was defined for, 
it's not necessarily a special case.

Regardless, the result of cpumask_of_node(NUMA_NO_NODE) should be 
undefined as it currently is unless you want to obsolete NUMA_NO_NODE 
entirely which is much more work.  In other words, special-casing a nid of 
-1 to mean no affinity is inappropriate if NUMA_NO_NODE represents an 
invalid nid.

If x86 pci buses want to use -1 to imply that meaning, that's fine, but it 
shouldn't be coded in a generic interface such as cpumask_of_node().  Does 
that make sense?

> Speaking of invalid node ids, I also noticed the scheduler isn't using
> node iterators:
> 
>         for (i = 0; i < nr_node_ids; i++) {
> 
> which should be fixed at some stage too since it doesn't allow us to
> allocate the node structures sparsely.
> 

That loop has nothing to do with the allocation of a node structure, it's 
quite plausible that it checks for various states such as node_online(i) 
while looping and doing something else interesting for those that are 
offline.  Keep in mind that this isn't equivalent to using for_each_node() 
since that only iterates over N_POSSIBLE which is architecture specific.
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