Date: Sat, 11 Feb 2006 15:49:29 +0100
From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: Andi Kleen <ak@muc.de>
Cc: Nathan Lynch <ntl@pobox.com>, Andrew Morton <akpm@osdl.org>,
       Eric Dumazet <dada1@cosmosbay.com>, riel@redhat.com,
       linux-kernel@vger.kernel.org, torvalds@osdl.org, mingo@elte.hu,
       76306.1226@compuserve.com, wli@holomorphy.com,
       Paul Jackson <pj@sgi.com>, jbeulich@novell.com,
       Keir Fraser <Keir.Fraser@cl.cam.ac.uk>
Subject: Re: [PATCH] percpu data: only iterate over possible CPUs
Message-ID: <20060211144929.GA4334@osiris.boeblingen.de.ibm.com>
References: <200602051959.k15JxoHK001630@hera.kernel.org> <20060209173726.GA39278@muc.de> <20060210100521.GA9307@osiris.boeblingen.de.ibm.com> <200602101113.13632.ak@muc.de>
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> > > x86-64 had the same problem, but we now require that you 
> > > boot with additional_cpus=... for how many you want. Default is 0
> > > (used to be half available CPUs but that lead to confusion)
> > 
> > So introducing the additional_cpus kernel parameter seems to be the way
> > to go (for XEN probably too). Even though it seems to be a bit odd if the
> > user specifies both maxcpus=... and additional_cpus=...
> 
> With additional_cpus you don't need maxcpus. They are added together.

How does x86_64 manage to get 'additional_cpus' parsed early enough? As far
as I can see this is done when parse_args() in start_kernel() gets called,
but that's after you need the parameter in prefill_possible_map().
IMHO that should be an early_param and you would need to call
parse_early_param() from setup_arch(). But then again, maybe I got it all
wrong.

But the more interesting question is: what do you do if the command line
contains both additional_cpus and maxcpus. I was just trying to make some
sense of this, but the result is questionable.
I ended up with a cpu_possible_map that has 'present cpus' +
'additional_cpus' bits set. And in smp_prepare_cpus I make sure that
cpu_present_map has not more than max_cpus bits set.

At least that doesn't break the current semantics of the maxcpus parameter.
But we're still wasting memory, since it would make sense that the
cpu_possible_map shouldn't have more than max_cpus bits set.

Heiko
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