linux-kernel - Re: [PATCH] Assign CPUs to nodes in round-robin manner on Fake NUMA.

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Date:	Fri, 8 Oct 2010 11:23:32 +0530
From:	Nikanth Karthikesan <knikanth@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Assign CPUs to nodes in round-robin manner on Fake NUMA.

On Thursday 07 October 2010 13:12:52 Andrew Morton wrote:
> On Thu, 30 Sep 2010 17:34:10 +0530 Nikanth Karthikesan <knikanth@...e.de> 
wrote:
> > Assign CPUs to nodes in round-robin manner on Fake NUMA.
> >
> > commit d9c2d5ac6af87b4491bff107113aaf16f6c2b2d9
> > "x86, numa: Use near(er) online node instead of roundrobin for NUMA"
> > changed NUMA initialization on Intel to choose the nearest online node or
> > first node. Fake NUMA would be better of with round-robin initialization,
> > instead of the all CPUS on first node. Change the choice of first node,
> > back to round-robin.
> 
> Why would fake NUMA "be better off with round-robin initialization"?
> 

For testing NUMA kernel behaviour without cpusets and NUMA aware applications, 
it would be better to have cpus in different nodes, rather than all in a 
single node. With cpusets migration of tasks scenarios cannot not be tested.

I guess having it round-robin shouldn't affect the use cases for all cpus on 
the first node.

>From the code comments in arch/x86/mm/numa_64.c:759 looks like this used to be 
the case, which was changed by commit d9c2d5ac6. It changed from roundrobin to 
nearer or first node. And I couldn't find any reason for this change in its 
changelog.

Thanks
Nikanth

> > ---
> >
> > diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> > index 85f69cd..47dd171 100644
> > --- a/arch/x86/kernel/cpu/intel.c
> > +++ b/arch/x86/kernel/cpu/intel.c
> > @@ -283,9 +283,7 @@ static void __cpuinit srat_detect_node(struct
> > cpuinfo_x86 *c) /* Don't do the funky fallback heuristics the AMD version
> > employs for now. */
> >  	node = apicid_to_node[apicid];
> > -	if (node == NUMA_NO_NODE)
> > -		node = first_node(node_online_map);
> > -	else if (!node_online(node)) {
> > +	if (node == NUMA_NO_NODE || !node_online(node)) {
> >  		/* reuse the value from init_cpu_to_node() */
> >  		node = cpu_to_node(cpu);
> >  	}
> > --
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