On Wed, Feb 17, 2016 at 11:16:40PM +0100, Andi Kleen wrote: > > Do you have any data to back that up or is that just "believe" ? > > I've seen systems with discontiguous apic ids before. > > It is obvious if you consider setups with node hotplug.
Those systems will also have a num_possible_cpus() that's inflated to account for the possible hotplug of new sockets, right? So while the phys_pkg_id of the present sockets might be 2 and 3 (leaving 0 and 1 available for hotplug), the num_possible_cpus() should be big enough to actually allow for that hotplug to happen. Because if num_possible_cpus() is too small, we could not accommodate the hotplug operation. And if num_possible_cpus() is of the right size, then the computed max_packages() should be of the right size too. Now clearly, BIOS can completely wreck things and indeed report too small an apic_id range or whatever, and in this case we're up a creek without a paddle. But I think you can check for that at boot and report errors/warns whatever, because if you trigger this, your system is not really 'correct' anyway.
