Re: [patch x86/core] x86: allow number of additional hotplug CPUs to be set at compile time

[Chuck Ebbert <cebbert@redhat.com>]

On Thu, 2 Oct 2008 21:44:09 +0200
Andi Kleen <andi@firstfloor.org> wrote:

> On Thu, Oct 02, 2008 at 03:25:21PM -0400, Chuck Ebbert wrote:
> > On Thu, 02 Oct 2008 11:12:51 +0200
> > Andi Kleen <andi@firstfloor.org> wrote:
> > 
> > > Chuck Ebbert <cebbert@redhat.com> writes:
> > > 
> > > > The default number of additional CPU IDs for hotplugging is determined
> > > > by asking ACPI or mptables how many "disabled" CPUs there are in the
> > > > system, but many systems get this wrong so that e.g. a uniprocessor
> > > > machine gets an extra CPU allocated and never switches to single CPU
> > > > mode.
> 
> What do you mean with single cpu mode?
> 
> e.g. the lock prefix rewriting is only determined by online CPUs,
> not possible CPUs. And this only affects the possible ones.
> 


The prefix rewriting doesn't happen unless I boot with additional_cpus=0,
maxcpus=1, or with this patch applied and the config option set. I think
the rules for when/if the rewriting happens changed a while ago to avoid
multiple switches and now it's not happening at all on this machine by
default.

Oh, and with NR_CPUS=512 I am seeing 1.6MB per-cpu data (I'll have to
check that, but I remember being surprised at how big the number was.)