To: Rusty Russell <rusty@rustcorp.com.au>
Cc: linux-kernel@vger.kernel.org, ralf@gnu.org, rhw@memalpha.cx,
        mingo@redhat.com, paulus@samba.org, anton@samba.org,
        schwidefsky@de.ibm.com, bh@sgi.com, davem@redhat.com, ak@suse.de,
        torvalds@transmeta.com
Subject: Re: Hotplug CPU Boot Changes: BEWARE
In-Reply-To: <E17GHB3-0000gD-00@wagner.rustcorp.com.au>
From: ebiederm@xmission.com (Eric W. Biederman)
Date: 07 Jun 2002 08:51:32 -0600
Message-ID: <m1elfjw39n.fsf@frodo.biederman.org>
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Sender: linux-kernel-owner@vger.kernel.org

Rusty Russell <rusty@rustcorp.com.au> writes:

> Hi all (esp port maintainers),
> 
> 	In writing the hotplug CPU stuff, Linus asked me to alter the
> boot sequence to "plug in" CPUs.  I am shortly going to be sending
> these patches to him now I have got my x86 box to boot with the
> changes.

If to the general SMP case is added the ability to dynamically enable
and disable cpus at runtime, this infrastructure work appears to have
general applicability now.  Allowing for example dynamic
enable/disable of HT on P4-Xeons at runtime for example.


> There are two ways to transition: one is to do the minimal hacks so
> that the new boot code works (as per my x86 patch).  The other is to
> take into account that the next stage (optional by arch) is to
> actually bring cpus up and down on the fly, and hence actually write
> code that will work after boot as well (as per my ppc patch).

Thinking in terms of physically hot-plugging cpus has me doubt the
actual utility of this code.  Instead thinking of dynamically enabling
and disabling processors for debugging sounds very reasonable.

But for the latter something just a little more than minimal hacks
must be implemented.  But dynamic cpu enable/disable is definitely
worth it.

Eric

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