xen: sched: leave CPUs doing tasklet work alone.

xen: sched: leave CPUs doing tasklet work alone.

In both Credit1 and Credit2, stop considering a pCPU idle,
if the reason why the idle vCPU is being selected, is to
do tasklet work.

Not doing so means that the tickling and load balancing
logic, seeing the pCPU as idle, considers it a candidate
for picking up vCPUs. But the pCPU won't actually pick
up or schedule any vCPU, which would then remain in the
runqueue, which is bad, especially if there were other,
truly idle pCPUs, that could execute it.

The only drawback is that we can't assume that a pCPU is
in always marked as idle when being removed from an
instance of the Credit2 scheduler (csched2_deinit_pdata).
In fact, if we are in stop-machine (i.e., during suspend
or shutdown), the pCPUs are running the stopmachine_tasklet
and hence are actually marked as busy. On the other hand,
when removing a pCPU from a Credit2 pool, it will indeed
be idle. The only thing we can do, therefore, is to
remove the BUG_ON() check.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>