linux-kernel - Re: [PATCH v6 04/12] x86/smpboot.c: Don't offline CPU0 if any irq can not be migrated out of it and remove CPU0 check in smp

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Message-ID: <1337294829.1997.96.camel@sbsiddha-desk.sc.intel.com>
Date:	Thu, 17 May 2012 15:47:09 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Tony Luck <tony.luck@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...e.hu>,
	H Peter Anvin <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Asit K Mallick <asit.k.mallick@...el.com>,
	Arjan Dan De Ven <arjan@...ux.intel.com>,
	Len Brown <len.brown@...el.com>,
	"Srivatssa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Chen Gong <gong.chen@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...r.kernel.org>, x86 <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v6 04/12] x86/smpboot.c: Don't offline CPU0 if any irq
 can not be migrated out of it and remove CPU0 check in smp_callin()

On Mon, 2012-05-14 at 14:17 +0200, Ingo Molnar wrote:
> * Tony Luck <tony.luck@...el.com> wrote:
> 
> > Biggest code impact of that is the extra code to bring cpu0 
> > back online using NMI instead of INIT. We can't use INIT 
> > because if cpu0 gets one, it just resets the whole machine :-( 
> > But obviously we'd like to avoid special cases where there is 
> > a sane way to do so.
> 
> Could we just standardize on NMI bringup during regular bootup?

May be we can use the NMI or even better monitor/mwait based bringup to
re-online an AP that is currently offline in play_dead(), specifically
if we haven't lost that AP processor state because of suspend/resume
event etc.

This can speed-up online if we want to use light weight online/offline
of the AP's for saving power on mobile platforms.

thanks,
suresh

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