Re: [PATCH] percpu data: only iterate over possible CPUs

[Andrew Morton <akpm@osdl.org>]

Rik van Riel <riel@redhat.com> wrote:
>
> On Sun, 5 Feb 2006, Linux Kernel Mailing List wrote:
> 
>  > [PATCH] percpu data: only iterate over possible CPUs
> 
>  This sched.c bit breaks Xen, and probably also other architectures
>  that have CPU hotplug.  I suspect the reason is that during early 
>  bootup only the boot CPU is online, so nothing initialises the
>  runqueues for CPUs that are brought up afterwards.
> 
>  I suspect we can get rid of this problem quite easily by moving
>  runqueue initialisation to init_idle()...

We've hit this snag with a few architectures.  They're setting up
cpu_possible_map too late.  It's never been clearly defined.

sched_init() is called here:

asmlinkage void __init start_kernel(void)
{
	...
	printk(linux_banner);
	setup_arch(&command_line);
	setup_per_cpu_areas();
	smp_prepare_boot_cpu();
	sched_init();

Users of __GENERIC_PER_CPU definitely need cpu_possible_map to be initialised
by the time setup_per_cpu_areas() is called, so I think it makes sense to
say "thou shalt initialise cpu_possible_map in setup_arch()".

I guess Xen isn't doing that.  Can it be made to?