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

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 [off-list ref]

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).