On Fri, 2016-07-15 at 19:02 +0100, George Dunlap wrote:
> The generic domain creation logic in
> xen/common/domctl.c:default_vcpu0_location() attempts to try to do
> initial placement load-balancing by placing vcpu 0 on the least-busy
> non-primary hyperthread available.  Unfortunately, the logic can end
> up picking a pcpu that's not in the online mask.  When this is passed
> to a scheduler such which assumes that the initial assignment is
> valid, it causes a null pointer dereference looking up the runqueue.
> 
> Furthermore, this initial placement doesn't take into account hard or
> soft affinity, or any scheduler-specific knowledge (such as historic
> runqueue load, as in credit2).
> 
> To solve this, when inserting a vcpu, always call the per-scheduler
> "pick" function to revise the initial placement.  This will
> automatically take all knowledge the scheduler has into account.
> 
> Signed-off-by: George Dunlap <george.dunlap@citrix.com>
> ---
> CC: Dario Faggioli <dario.faggioli@citrix.com>
> CC: Anshul Makkar <anshul.makkar@citrix.com>
> CC: Meng Xu <mengxu@cis.upenn.edu>
> CC: Jan Beulich <jbeulich@suse.com>
>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
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Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)