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

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Date:	Thu, 15 Dec 2011 19:57:08 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Yu, Fenghua" <fenghua.yu@...el.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	H Peter Anvin <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	"Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"Brown, Len" <len.brown@...el.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v4 0/7] x86: BSP or CPU0 online/offline


* Yu, Fenghua <fenghua.yu@...el.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
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