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, 8 insertions, 0 deletions

diff --git a/arch/s390/include/asm/cputime.h b/arch/s390/include/asm/cputime.h
index 24b1244aadb9..95f3561517c7 100644
--- a/arch/s390/include/asm/cputime.h
+++ b/arch/s390/include/asm/cputime.h
@@ -183,6 +183,7 @@ struct s390_idle_data {
         unsigned long long idle_count;
         unsigned long long idle_enter;
         unsigned long long idle_time;
+        int nohz_delay;
 };
 
 DECLARE_PER_CPU(struct s390_idle_data, s390_idle);
@@ -198,4 +199,11 @@ static inline void s390_idle_check(void)
                 vtime_start_cpu();
 }
 
+static inline int s390_nohz_delay(int cpu)
+{
+        return per_cpu(s390_idle, cpu).nohz_delay != 0;
+}
+
+#define arch_needs_cpu(cpu) s390_nohz_delay(cpu)
+
 #endif /* _S390_CPUTIME_H */