Re: [RFC] bitmap onto and fold operators for mempolicy extensions

[David Rientjes]

On Sat, 16 Feb 2008, Paul Jackson wrote:

> Let's say an application has specified some mempolicies
> that presume 16 memory nodes, including say a mempolicy that
> specified MPOL_F_RELATIVE_NODES (cpuset relative) nodes 12-15.
> Then lets say that application is crammed into a cpuset that only
> has 8 memory nodes, 0-7.  If one just uses bitmap_onto(), this
> mempolicy, mapped to that cpuset, would ignore the requested
> relative nodes above 7, leaving it empty of nodes.  That's not
> good; better to fold the higher nodes down, so that some nodes
> are included in the resulting mapped mempolicy.  In this case,
> the mempolicy nodes 12-15 are taken modulo 8 (the weight of the
> mems_allowed of the confining cpuset), resulting in a mempolicy
> specifying nodes 4-7.
> 

So what is the MPOL_F_RELATIVE_NODES behavior?  Is it a combination of 
nodes_onto() and nodes_fold()?

In your example, the only way we know to use nodes_fold() is if the 
resultant of nodes_onto() has a weight of 0.  An MPOL_F_RELATIVE_NODES 
nodemask for 4-6, for example, works fine in your case of a cpuset with 
memory nodes 0-7 and no fold is required.

So it's easy enough to do this:

	case MPOL_INTERLEAVE:
		if (flags & MPOL_F_RELATIVE_NODES) {
			nodes_onto(pol->v.nodes, pol->user_nodemask,
				   cpuset_context_nmask);
			if (nodes_empty(pol->v.nodes))
				nodes_fold(pol->v.nodes,
					   pol->user_nodemask,
					   nodes_weight(cpuset_context_nmask));
		} else {
			...
		}
		break;

But what if we require a combination?  Say the user asked for a policy of 
MPOL_INTERLEAVE | MPOL_F_RELATIVE_NODES over nodes 4-8 in a cpuset 
constrained to mems 0-7?  Should the resultant be 0,4-7 (combination of 
nodes_onto() and nodes_fold()) or simply be 4-7 (just nodes_onto())?

And what if the MPOL_INTERLEAVE | MPOL_F_RELATIVE_NODES nodemask is 0,4-8 
in the same cpuset constrained to mems 0-7?  Should the resultant be

 - 0,4-7 (nodes_onto() and nodes_fold()),

 - 0,4-7 (just nodes_onto()), or

 - 0-1,4-7 (nodes_onto(), nodes_fold(), and shift)?

The last option, 0-1,4-7, is the only one that preserves the same weight 
as the relative nodemask.

		David
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