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

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Message-Id: <20071027013656.4ffadb87.pj@sgi.com>
Date:	Sat, 27 Oct 2007 01:36:56 -0700
From:	Paul Jackson <pj@....com>
To:	Christoph Lameter <clameter@....com>
Cc:	rientjes@...gle.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

> > Are you saying:
> >  1) The kernel continues to default to Choice A, unless
> >     the flag enables Choice B, or
> >  2) The kernel defaults to the new Choice B, unless the
> >     flag reverts to the old Choice A?
> 
> If 2) is keeping the API semantics then 2.

No .. (1) keeps the same API semantics.

> Let everything be as it is today unless
> numactl sets the new.
> ...
> Tough. The API needs to remain stable. 

Good - that I understand.  Your position is clear now.

You have chosen (1) above, which keeps Choice A as the default.

Before I leave this part, there is one more thing I kinda really need,
if you could, Christoph.  Could you describe in your own words what you
think Choices A and B mean?  We seem to be having trouble communicating,
and hence there is some risk right now that we don't mean the same thing
by this new "Choice B".

===

Now ... onto the matter of permanent API warts:

> > I wonder if there might be some way to avoid that permanent ugly wart
> > on each and every set/get mempolicy system call forever afterward.
> 
> Hmmm.. The alternative is to add new set/get mempolicy functions.

Other alternatives include a per-system, per-cpuset or per-process
flag, in addition to the per-system call flag you suggested earlier
(MPOL_MF_RELATIVE), or whatever you mean by "new set/get mempolicy
functions" ... could you elaborate on that one?

So ... the question becomes this:

  How do we migrate to Choice B, without leaving both Choices
  permanently supported, and an ugly mode flag selecting the
  non-default Choice, while not breaking API's too abruptly?

Thanks.

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