Re: [Qemu-devel] [RFC PATCH v0 0/9] Generic cpu-core device

[Igor Mammedov]

On Wed, 16 Dec 2015 18:22:20 +0100
Igor Mammedov <address@hidden> wrote:

> On Wed, 16 Dec 2015 16:57:54 +0100
> Andreas Färber <address@hidden> wrote:
[...]
> > 
> > Attendees in Seattle said that thread-level hot-plug were dangerous
> > for Linux guests due to assumptions in the (guest's) scheduler
> > breaking for any incompletely filled cores or sockets. No one
> > present
> There is not such thing as cpu hotplug at socket level in x86 linux
> so far. CPUs are plugged at logical(thread) cpu level, one at a time.
> And ACPI spec does the same (describes logical CPUs) and hotplug
> notification in guest handled per one logical cpu at a time.
BTW:
from my experience with CPU hotplug, I've had to fix only one
topology+sched related bug in kernel but more bugs were related to fast
CPU online/offline especially if host is over-committed.

And plugging whole socket would lead to a rapid sequence of hotplugging
CPUs which has been causing non deterministic kernel crash/lock up
much more often than topology issues, there are still bugs to fix in
that area.