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

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Message-ID: <20100107000106.GD12742@kryten>
Date:	Thu, 7 Jan 2010 11:01:06 +1100
From:	Anton Blanchard <anton@...ba.org>
To:	David Rientjes <rientjes@...gle.com>
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

 
Hi David,

> 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?

I wasn't using the example to strengthen the case of the -1 behaviour, but to
highlight that a complete fix would be more work and risk not making it back
to -stable.

I'm all for removing the special case as a followon patch.

> > 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.

Yeah I understand it isn't the same thing, but the scheduler oopses in
a number of places with CPUMASK_OFFSTACK and sparse node ids, so things
that can be switched to node iterators should and node_online() checks
should be added elsewhere.

Anton
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