Re: [patch 2/2] cpusets: add interleave_over_allowed option

[David Rientjes <rientjes@google.com>]

On Sun, 28 Oct 2007, Paul Jackson wrote:

> > If we can't identify any applications that would be broken by this, what's 
> > the difference in simply implementing Choice B and then, if we hear 
> > complaints, add your hack to revert back to Choice A behavior based on the 
> > get_mempolicy() call you specified is always part of libnuma?
> 
> I'll probably reply to other parts of your message later, but this
> one catches my eye right now.
> 
> "if we hear complaints, add your hack ... back"  -- this doesn't seem
> like a good idea to me.  Maybe inside Google you don't see it, but
> for those of us shipping computer systems using major distributions
> such as SUSE or Red Hat, there can be a year lag between when I send a
> feature patch to Andrew, and when my customers send their first
> feedback to me resulting from using that new feature.
> 

Let's add a Choice C:

	Any nodemask that is passed to set_mempolicy() is saved as the
	intent of the application in struct mempolicy.  All policies
	are effected on a contextualized per-allocation basis.

	Policies such as MPOL_INTERLEAVE always get AND'd with 
	pol->cpuset_mems_allowed.  If that yields numa_no_nodes,
	MPOL_DEFAULT is used instead.

	Policies such as MPOL_PREFERRED are respected if the node is set
	in pol->cpuset_mems_allowed, otherwise MPOL_DEFAULT is used.	

	If an application attempts to setup a memory policy for an
	MPOL_PREFERRED node that it doesn't have access to or an
	MPOL_INTERLEAVE nodemask that is empty when AND'd with
	pol->cpuset_mems_allowed, -EINVAL is returned and no new policy
	is effected.

	If an application gains nodes in pol->cpuset_mems_allowed that
	now include the nodes from MPOL_INTERLEAVE or MPOL_PREFERRED,
	that policy is then effected once again.  Otherwise,
	MPOL_DEFAULT is still used.
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