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

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Message-Id: <20071027142239.0ded1e42.pj@sgi.com>
Date:	Sat, 27 Oct 2007 14:22:39 -0700
From:	Paul Jackson <pj@....com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Lee.Schermerhorn@...com, clameter@....com,
	akpm@...ux-foundation.org, ak@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] cpusets: add interleave_over_allowed option

David wrote:
> I prefer Choice B because it does not force mempolicies to have any 
> dependence on cpusets with regard to what nodemask is passed.

Yes, well said.

> It would be very good to store the passed nodemask to set_mempolicy in 
> struct mempolicy, 

Yes - that's what I'm intending to do.

> If the cpuset has fewer than four nodes, the behavior 
> should be undefined (probably implemented to just cycle the set of 
> mems_allowed until you reach the fourth entry).

I do intend to implement it as you suggest.  See the lib/bitmap.c
routines bitmap_remap() and bitmap_bitremap(), and the nodemask
wrappers for these, nodes_remap() and node_remap().  They will
define the cycling, or I sometimes call it folding.

I would have tended to make this folding a defined part of the API,
though I will grant that the possibility of being lazy and forgetting
to document it seems attractive (less to document ;).

> That [running in a cpuset with fewer nodes than used in a memory policy
> mask] is the result of constraining a task to a cpuset that obviously
> wants access to more nodes -- it's a userspace mistake and abusing
> cpusets so that the task does not get what it expects.

Nah - I wouldn't put it that way.  It's no mistake or abuse.  It's just
one more example of a kernel making too few resources look sufficient
by sharing, multiplexing and virtualizing them.  That's what kernels do.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.925.600.0401
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