[PATCH] Extend next_timer_interrupt() to use a reference jiffie

For CONFIG_NO_HZ we need to calculate the next timer wheel event based on a
given jiffie value.  Extend the existing code to allow the extra 'now'
argument.  Provide a compability function for the existing implementations to
call the function with now == jiffies.  (This also solves the racyness of the
original code vs.  jiffies changing during the iteration.)

No functional changes to existing users of this infrastructure.

[ remove WARN_ON() that triggered on s390, by Carsten Otte <cotte@de.ibm.com> ]
[ made new helper static, Adrian Bunk <bunk@stusta.de> ]
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/timer.c

diff --git a/kernel/timer.c b/kernel/timer.c
index 201bee0..6d843e1 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -591,7 +591,7 @@
 	spin_unlock_irq(&base->lock);
 }
 
-#ifdef CONFIG_NO_IDLE_HZ
+#if defined(CONFIG_NO_IDLE_HZ) || defined(CONFIG_NO_HZ)
 /*
  * Find out when the next timer event is due to happen. This
  * is used on S/390 to stop all activity when a cpus is idle.
@@ -687,10 +687,10 @@
 /**
  * next_timer_interrupt - return the jiffy of the next pending timer
  */
-unsigned long next_timer_interrupt(void)
+unsigned long get_next_timer_interrupt(unsigned long now)
 {
 	tvec_base_t *base = __get_cpu_var(tvec_bases);
-	unsigned long expires, now = jiffies;
+	unsigned long expires;
 
 	spin_lock(&base->lock);
 	expires = __next_timer_interrupt(base);
@@ -701,6 +701,14 @@
 
 	return cmp_next_hrtimer_event(now, expires);
 }
+
+#ifdef CONFIG_NO_IDLE_HZ
+unsigned long next_timer_interrupt(void)
+{
+	return get_next_timer_interrupt(jiffies);
+}
+#endif
+
 #endif
 
 /******************************************************************/