nohz: Introduce arch_needs_cpu

nohz: Introduce arch_needs_cpu

commit 3c5d92a0cfb5103c0d5ab74d4ae6373d3af38148 upstream.

Allow the architecture to request a normal jiffy tick when the system
goes idle and tick_nohz_stop_sched_tick is called . On s390 the hook is
used to prevent the system going fully idle if there has been an
interrupt other than a clock comparator interrupt since the last wakeup.

On s390 the HiperSockets response time for 1 connection ping-pong goes
down from 42 to 34 microseconds. The CPU cost decreases by 27%.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
LKML-Reference: <20090929122533.402715150@de.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Jolly <jjolly@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>