Re: [PATCH 0/2 v3] cpu hotplug: Preserve topology directory after soft remove event

From: Prarit Bhargava
Date: Tue Sep 27 2016 - 07:46:14 EST

On 09/26/2016 07:57 AM, Borislav Petkov wrote:
> On Mon, Sep 26, 2016 at 07:45:37AM -0400, Prarit Bhargava wrote:
>> When offline, /sys/devices/system/cpuX/cpu/online is 0. The problem is that
>> when online is 0, topology disappears so there is no way to determine _the
>> location_ of the offline'd thread.
> What does "the location" mean exactly?
>> cpupower should still print out all asterisks for down'd threads. It does not
>> because the topology directory is incorrectly removed.
>> IOW how does userspace know the _location_ of the thread? The topology
>> directory no longer exists when the thread is downed, so core_id and
>> physical_package_id (both of which would be effectively static) do not exist.
>> The whole point of this patchset is to know where the offline'd thread actually is.
> What do you mean "where"?

The socket and core location.

Look at it this way (and let's get the terminology straight at the same time).

You have a socket CPU. That socket has cores on it. Each core (at least on
Intel) has two threads.

I down a thread (as you did):

> $ echo 0 > /sys/devices/system/cpu/cpu2/online

This results in the topology directory being destroyed. It shouldn't be -- the
socket and core are still there. If you could open up your computer you could
touch them. This is similar to downing a PCI device, or removing !kernel memory
DIMM from a system. The device is still physically there.

> $ cat /sys/devices/system/cpu/online
> 0-1,3-7
> So core 2 is right between 1 and 3.

Yes. But *where* is it relative to the cores and socket(s)?

> If you need to show the package id, you still iterate over the core
> numbers in an increasing order and show '*' for the offlined ones.

Explain this in more detail please?