x86/apic/x2apic: Use multiple cluster members for the irq destination only with the explicit affinity

During boot or driver load etc, interrupt destination is setup
using default target cpu's. Later the user (irqbalance etc) or
the driver (irq_set_affinity/ irq_set_affinity_hint) can request
the interrupt to be migrated to some specific set of cpu's.

In the x2apic cluster routing, for the default scenario use
single cpu as the interrupt destination and when there is an
explicit interrupt affinity request, route the interrupt to
multiple members of a x2apic cluster specified in the cpumask of
the migration request.

This will minmize the vector pressure when there are lot of
interrupt sources and relatively few x2apic clusters (for
example a single socket server). This will allow the performance
critical interrupts to be routed to multiple cpu's in the x2apic
cluster (irqbalance for example uses the cache siblings etc
while specifying the interrupt destination) and allow
non-critical interrupts to be serviced by a single logical cpu.

Signed-off-by: Suresh Siddha <[email protected]>
Acked-by: Yinghai Lu <[email protected]>
Acked-by: Alexander Gordeev <[email protected]>
Acked-by: Cyrill Gorcunov <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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