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Date: Thu, 15 Dec 2011 19:57:08 +0100
From: Ingo Molnar <mingo@elte.hu>
To: "Yu, Fenghua" <fenghua.yu@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, H Peter Anvin <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        linux-kernel <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH v4 0/7] x86: BSP or CPU0 online/offline
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* 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