Re: [PATCH v3 1/2] cpumask: introduce cpumask for hotpluggable CPUs

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2011-08-11 at 12:54 -0700, Mike Turquette wrote:
> On some platforms it is possible to have some CPUs which support CPU
> hotplug and some which do not.  Currently the prescence of an 'online'
> sysfs entry in userspace is adequate for applications to know that a CPU
> supports hotplug, but there is no convenient way to make the same
> determination in the kernel.
> 
> To better model this relationship this patch introduces a new cpumask to
> track CPUs that support CPU hotplug operations.
> 
> This new cpumask is populated at boot-time and remains static for the
> life of the machine.  Bits set in the mask indicate a CPU which supports
> hotplug, but make no guarantees about whether that CPU is currently
> online or not.  Likewise a cleared bit in the mask indicates either a
> CPU which cannot hotplug or a lack of a populated CPU.
> 
> The purpose of this new cpumask is to aid kernel code which uses CPU to
> take CPUs online and offline.  Possible uses are as a thermal event
> mitigation technique or as a power capping mechanism. 

Nacked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

the kernel really shouldn't be using hotplug for this (nor should
userspace really). hot-unplugging random cpus wrecks things like
cpusets. Furthermore hotplug does way too much work to use as a simple
means to idle a cpu.

Even the availability of this mask is wrong, since that implies the
information is useful, which per the above it is not, the kernel
shouldn't care about this full-stop.

The only reason for the OS to unplug a CPU is imminent and unavoidable
hardware failure. Thermal capping is not that (and yes ACPI-4.0 is a
broken piece of shit).