xen: sched: reassign vCPUs to pCPUs, when they come back online

xen: sched: reassign vCPUs to pCPUs, when they come back online

When a vcpu that was offline, comes back online, we do want it to either
be assigned to a pCPU, or go into the wait list.

Detecting that a vcpu is coming back online is a bit tricky. Basically,
if the vcpu is waking up, and is neither assigned to a pCPU, nor in the
wait list, it must be coming back from offline.

When this happens, we put it in the waitqueue, and we "tickle" an idle
pCPU (if any), to go pick it up.

Looking at the patch, it seems that the vcpu wakeup code is getting
complex, and hence that it could potentially introduce latencies.
However, all this new logic is triggered only by the case of a vcpu
coming online, so, basically, the overhead during normal operations is
just an additional 'if()'.

Signed-off-by: Dario Faggioli <dario.faggioli@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Message-Id: <156412236222.2385.236340632846050170.stgit@Palanthas>