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, 2 insertions, 0 deletions

diff --git a/arch/s390/kernel/s390_ext.c b/arch/s390/kernel/s390_ext.c
index 0de305b598ce..59618bcd99b7 100644
--- a/arch/s390/kernel/s390_ext.c
+++ b/arch/s390/kernel/s390_ext.c
@@ -126,6 +126,8 @@ void __irq_entry do_extint(struct pt_regs *regs, unsigned short code)
                 /* Serve timer interrupts first. */
                 clock_comparator_work();
         kstat_cpu(smp_processor_id()).irqs[EXTERNAL_INTERRUPT]++;
+        if (code != 0x1004)
+                __get_cpu_var(s390_idle).nohz_delay = 1;
         index = ext_hash(code);
         for (p = ext_int_hash[index]; p; p = p->next) {
                 if (likely(p->code == code))