linux-kernel - Re: [patch 2/2] cpusets: add interleave_over

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Date:	Sat, 27 Oct 2007 10:50:22 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Paul Jackson <pj@....com>
cc:	Christoph Lameter <clameter@....com>, Lee.Schermerhorn@...com,
	akpm@...ux-foundation.org, ak@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] cpusets: add interleave_over_allowed option

On Fri, 26 Oct 2007, Paul Jackson wrote:

> > Yes. We should default to Choice B. Add an option MPOL_MF_RELATIVE to 
> > enable that functionality? A new version of numactl can then enable
> > that by default for newer applications.
> 
> I'm confused.  If B is the default, then we don't need a flag to
> enable it, rather we need a flag to go back to the old choice A.
> 

I think there's a mixup in the flag name there, but I actually would 
recommend against any flag to effect Choice A.  It's simply going to be 
too complex to describe and is going to be a headache to code and support.  

The MPOL_PREFERRED behavior when constrained by cpusets was previously, to 
my knowledge, undocumented; you're in the position to make the behavior do 
what you want it to do and then release documentation so we'll finally 
have a complete and unambiguous API for it.  Right now it should be 
considered undefined and thus you are free to implement it as you choose.  
Then all callers of set_mempolicy(MPOL_PREFERRED) will standardize on that 
and not have to worry about the machine's 
mpol_preferred_relative_to_cpuset setting.

Then, any task that is attached to a cpuset and expecting the fourth node 
in their set_mempolicy(MPOL_PREFERRED) call to mean system node 3 if 
it's in the cpuset's mems_allowed will be broken.  If you want that, 
you'll need your task to be attached to a cpuset with at least mems 0-3; 
programmers will pick that up quickly enough if it's clearly documented.
I think Choice B is correct and makes more sense in terms of the semantics 
and at least allows mempolicies and cpusets to play nicely together 
without a bidirectional dependency on one another.

		David
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