Re: [PATCH v4 0/7] x86: BSP or CPU0 online/offline

[Ingo Molnar <mingo@elte.hu>]

* Yu, Fenghua <fenghua.yu@intel.com> wrote:

> > We need a debug feature for this: 
> > CONFIG_DEBUG_BOOT_CPU_OFF=y or such (disabled by default): 
> > this feature would offline the boot CPU as soon as possible, 
> > and boot up userspace with the boot CPU offlined.
> 
> I would think the earliest time when CPU0 can be offline is 
> right after do_initcalls() because do_initcalls() installs 
> some cpu hotplug notifier.
> 
> But this will change generic code (e.g. add arch hook in 
> init/main.c or add a new initcall level which is after level 
> 7s).
> 
> Another way is to offline CPU0 (or any CPU) in /etc/rc.local. 
> By this way, we don't change kernel. And this is safe, but a 
> bit later than in kernel.
> 
> Any comment?

I think we should try to do this in the kernel, to keep the 
debug feature independent of user-space code. This also helps
us clean init code from boot CPU assumptions.

Not sure we need a new initlevel - why not just offline the CPU? 
I suspect the cpu hotplug notifier installation that is done in 
do_initcalls() should be moved earlier so that the boot CPU can 
be offlined as early as a secondary CPU is up and running.

Thanks,

	Ingo