linux-kernel - RE: [PATCH v5 03/12] x86/topology.c: Support functions for CPU0 online/offline

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Date:	Tue, 24 Jan 2012 22:31:23 +0000
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	H Peter Anvin <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>,
	"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>,
	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>
Subject: RE: [PATCH v5 03/12] x86/topology.c: Support functions for CPU0
 online/offline


> On Wed, 2012-01-11 at 09:04 -0800, Fenghua Yu wrote:
> > From: Fenghua Yu <fenghua.yu@...el.com>
> >
> > If CONFIG_BOOTPARAM_HOTPLUG_CPU is turned on, CPU0 hotplug feature is
> enabled
> > by default.
> >
> > If CONFIG_BOOTPARAM_HOTPLUG_CPU is not turned on, CPU0 hotplug
> feature is not
> > enabled by default. The kernel parameter cpu0_hotplug can enable CPU0
> hotplug
> > feature at boot.
> [...]
> >  int __ref arch_register_cpu(int num)
> >  {
> >  	/*
> > -	 * CPU0 cannot be offlined due to several
> > -	 * restrictions and assumptions in kernel. This basically
> > -	 * doesn't add a control file, one cannot attempt to offline
> > -	 * BSP.
> > +	 * Two known BSP/CPU0 dependencies: Resume from suspend/hibernate
> > +	 * depends on BSP. PIC interrupts depend on BSP.
> >  	 *
> > -	 * Also certain PCI quirks require not to enable hotplug control
> > -	 * for all CPU's.
> > +	 * If the BSP depencies are under control, one can tell kernel to
> > +	 * enable BSP hotplug. This basically adds a control file and
> > +	 * one can attempt to offline BSP.
> >  	 */
> > -	if (num)
> > +	if (num || cpu0_hotpluggable)
> >  		per_cpu(cpu_devices, num).cpu.hotpluggable = 1;
> >
> >  	return register_cpu(&per_cpu(cpu_devices, num).cpu, num);
> 
> This change belongs at the end of the series.  It should not be
> possible
> to enable CPU0 hotplug until after the hotplug logic can do it
> correctly, and this might break bisection.

Quote from https://www.linux.com/how-to-participate-in-the-linux-community
"It can be tempting to add a whole new infrastructure with a series of patches, but to leave that infrastructure unused until the final patch in the series enables the whole thing. This temptation should be avoided if possible; if that series adds regressions, bisection will finger the last patch as the one which caused the problem, even though the real bug is elsewhere. Whenever possible, a patch which adds new code should make that code active immediately."

So this patch currently is in the right place in the patch set unless I miss something.

Thanks.

-Fenghua