nohz: Introduce arch_needs_cpu

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>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/s390/cio/cio.c b/drivers/s390/cio/cio.c
index 138124fcfcad..126f240715a4 100644
--- a/drivers/s390/cio/cio.c
+++ b/drivers/s390/cio/cio.c
@@ -618,6 +618,7 @@ void __irq_entry do_IRQ(struct pt_regs *regs)
         old_regs = set_irq_regs(regs);
         s390_idle_check();
         irq_enter();
+        __get_cpu_var(s390_idle).nohz_delay = 1;
         if (S390_lowcore.int_clock >= S390_lowcore.clock_comparator)
                 /* Serve timer interrupts first. */
                 clock_comparator_work();