From: Prarit Bhargava
Date: Thu Sep 08 2016 - 09:45:55 EST

On 09/08/2016 03:45 AM, Peter Zijlstra wrote:
> On Wed, Sep 07, 2016 at 07:44:17AM -0400, Prarit Bhargava wrote:
>> The information in /sys/devices/system/cpu/cpuX/topology
>> directory is useful for userspace monitoring applications and in-tree
>> utilities like cpupower & turbostat.
>> When down'ing a CPU the /sys/devices/system/cpu/cpuX/topology directory is
>> removed during the CPU_DEAD hotplug callback in the kernel. The problem
>> with this model is that the CPU has not been physically removed and the
>> data in the topology directory is still valid and the cpu's location is
>> now lost to userspace.
>> This patch adds CONFIG_PERMANENT_CPU_TOPOLOGY, and is Y by default for
>> x86, an N for all other arches. When enabled the kernel is modified so
>> that the topology directory is added to the core cpu sysfs files so that
>> the topology directory exists while the CPU is physically present. When
>> disabled, the behavior of the current kernel is maintained (that is, the
>> topology directory is removed on a soft down and added on an soft up).
>> Adding CONFIG_PERMANENT_CPU_TOPOLOGY may require additional architecture
>> so that the cpumask data the CPU's topology is not cleared during a CPU
>> down.
> So how do you physically remove things and how does the information then
> get removed?

Physical hotremove was done with a toggle switch enabled test box.

The topology directory gets removed when either common_cpu_attr_groups
(!CONFIG_HOTPLUG_CPU) or hotplugable_cpu_attr_groups (CONFIG_HOTPLUG_CPU) are
destroyed. The cpu device's attributes are set in register_cpu() and the device
is destroyed in unregister_cpu().

IOW, the patch expands the lifetime of the topology directories to match that of
the device.

> Also, why is that an x86 only feature?

The only arch that I could test on was x86 and I wasn't able to test to see if
topology fields are valid for other arches on a soft remove. So x86 by default
for now and other arches can make their decisions.