Re: Hotplug CPU Boot Changes: BEWARE

[ebiederm@xmission.com (Eric W. Biederman)]

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