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

[Sasha Levin <sasha.levin@oracle.com>]

On 11/21/2012 01:25 PM, H. Peter Anvin wrote:
> 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.

With ACPI platforms you don't need probe/release because the hardware notifies
on CPU insert/eject - this doesn't exist on mptable which is why you have to
do it manually with probe/release.

The difference is already user visible: you can hotplug on ACPI, but can't on
mptables.

Yes, reading back the subject does sound confusing - a better one would probably
be "provide interface for CPU hotplug on mptable platforms" or something similar.


Thanks,
Sasha