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

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Date:	Wed, 7 Dec 2011 08:40:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Yu, Fenghua" <fenghua.yu@...el.com>
Cc:	Borislav Petkov <bp@...64.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	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>,
	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>,
	Tejun Heo <tj@...nel.org>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>
Subject: Re: [PATCH v4 0/7] x86: BSP or CPU0 online/offline


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

> > When you take it down for maintenance eventually, you don't 
> > need to suspend but simply poweroff.
> 
> Agree with you. To maintain a system with a bad CPU, either 
> you hot plug or hot replace the CPU, or you power off then 
> replace the CPU. Replacing the CPU between suspend and resume 
> doesn't seem a normal RAS behavior.

More importantly, you generally *cannot* realistically continue 
with a bad CPU anyway - the system will crash or will show signs 
of corruptions and you *want* a full powerdown and a clean 
reboot.

The usecases for real CPU hotplug look pretty limited to me:

 - Special hardware environments that are deeply redundant and 
   can warn about 'soft' failures well before hard failures
   which gives a realistic window of time for a maintenance
   hot-swap. [Such hardware actually exists, i even worked with
   an x86 one eons ago.]

 - Swapping slower CPUs for a faster CPUs, without any downtime.
   Given that mixed steppings and mixed frequencies are
   generally pretty unpredictable even with no hotswap in the
   picture, i can see hw designers (and qa test matrix
   engineers) cringe at the idea.

Thanks,

	Ingo
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