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Date: Wed, 7 Dec 2011 08:40:35 +0100
From: Ingo Molnar <mingo@elte.hu>
To: "Yu, Fenghua" <fenghua.yu@intel.com>
Cc: Borislav Petkov <bp@amd64.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:

> > 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