linux-kernel - Re: [PATCH] x86, smpboot: allow manual hotplug of CPUs

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Date:	Wed, 21 Nov 2012 10:25:58 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Sasha Levin <sasha.levin@...cle.com>
CC:	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, penberg@...nel.org
Subject: Re: [PATCH] x86, smpboot: allow manual hotplug of CPUs

On 11/21/2012 10:22 AM, Sasha Levin wrote:
> So far CPU hotplug was ignored for mptable implementations which support it by
> having the hotpluggable CPUs marked as disabled during boot.
> 
> The current kernel code detects that behaviour and actually deals with it
> properly:
> 
> 	[    0.000000] Intel MultiProcessor Specification v1.4
> 	[    0.000000] MPTABLE: OEM ID: KVMCPU00
> 	[    0.000000] MPTABLE: Product ID: 0.1
> 	[    0.000000] MPTABLE: APIC at: 0xFEE00000
> 	[    0.000000] Processor #0 (Bootup-CPU)
> 	[    0.000000] Processor #1
> 	[    0.000000] Processor #2
> 	[    0.000000] IOAPIC[0]: apic_id 4, version 17, address 0xfec00000, GSI 0-23
> 	[    0.000000] Processors: 3
> 	[    0.000000] smpboot: Allowing 3 CPUs, 1 hotplug CPUs
> 
> The problem begins when a user might actually want to online such CPU; there
> is no interface for him to tell the kernel that the CPU is now present and
> can be used.
> 
> Luckily, the kernel provides a generic interface in the form of 'probe' and
> 'release' sysfs files which are used on different architectures exactly for
> that - to probe and release CPUs. On x86 however this was unimplemented
> until now.
> 
> This patch adds code into the x86 implementation of probe and release to allow
> adding and removing CPUs. This allows machines that use mptable to hotplug
> CPUs:
> 

Reading between the lines, this sounds like would cause a user-visible
difference between mptable platforms and ACPI platforms?  If so, that is
totally unacceptable.  If not, the description is confusing.

	-hpa


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