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

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Message-Id: <20071026120037.7b95a136.pj@sgi.com>
Date:	Fri, 26 Oct 2007 12:00:37 -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:
> If something that was previously unaccepted is now allowed with a 
> newly-introduced semantic, that's an API change.

Agreed, as I wrote earlier:
> It should work with libnuma and be
> fully upward compatible with current code (except perhaps code that
> depends on getting an error from requesting MPOL_INTERLEAVE on a node
> not allowed.)

Without at least this sort of change to MPOL_INTERLEAVE nodemasks,
allowing either empty nodemasks (Lee's proposal) or extending them
outside the current cpuset (what I'm cooking up now), there is no way
for a task that is currently confined to a single node cpuset to say
anything about how it wants be interleaved in the event that it is
subsequently moved to a larger cpuset.  Currently, such a task is only
allowed to pass exactly one particular nodemask to set_mempolicy
MPOL_INTERLEAVE calls, with exactly the one bit corresponding to its
current node.  No useful information can be passed via an API that only
allows a single legal value.

But you knew that ...

You were just correcting my erroneously unqualified statement.  Good.

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