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, 5 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 7378e2c71ca6..3840f6dff7eb 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -264,12 +264,15 @@ void tick_nohz_stop_sched_tick(int inidle)
                 last_jiffies = jiffies;
         } while (read_seqretry(&xtime_lock, seq));
 
-        /* Get the next timer wheel timer */
-        next_jiffies = get_next_timer_interrupt(last_jiffies);
-        delta_jiffies = next_jiffies - last_jiffies;
-
-        if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu))
+        if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
+            arch_needs_cpu(cpu)) {
+                next_jiffies = last_jiffies + 1;
                 delta_jiffies = 1;
+        } else {
+                /* Get the next timer wheel timer */
+                next_jiffies = get_next_timer_interrupt(last_jiffies);
+                delta_jiffies = next_jiffies - last_jiffies;
+        }
         /*
          * Do not stop the tick, if we are only one off
          * or if the cpu is required for rcu