timers: Cache __next_timer_interrupt result

Each time a cpu goes to sleep on a NOHZ=y system the timer
wheel is searched for the next timer interrupt. It can take
quite a few cycles to find the next pending timer.

This patch adds a field to tvec_base that caches the result of
__next_timer_interrupt.

The hit ratio is around 80% on my thinkpad under normal use, on
a server I've seen hit ratios from 5% to 95% dependent on the
workload.

-v2: jiffies wrap fixes

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Venki Pallipadi <venkatesh.pallipadi@intel.com>
LKML-Reference: <20090721202505.7d56a079@skybase>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 23 insertions, 1 deletions

diff --git a/kernel/timer.c b/kernel/timer.c
index 0b36b9e5cc8b..5c1e49ec2f1b 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -72,6 +72,7 @@ struct tvec_base {
         spinlock_t lock;
         struct timer_list *running_timer;
         unsigned long timer_jiffies;
+        unsigned long next_timer;
         struct tvec_root tv1;
         struct tvec tv2;
         struct tvec tv3;
@@ -622,6 +623,9 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
 
         if (timer_pending(timer)) {
                 detach_timer(timer, 0);
+                if (timer->expires == base->next_timer &&
+                    !tbase_get_deferrable(timer->base))
+                        base->next_timer = base->timer_jiffies;
                 ret = 1;
         } else {
                 if (pending_only)
@@ -663,6 +667,9 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
         }
 
         timer->expires = expires;
+        if (time_before(timer->expires, base->next_timer) &&
+            !tbase_get_deferrable(timer->base))
+                base->next_timer = timer->expires;
         internal_add_timer(base, timer);
 
 out_unlock:
@@ -781,6 +788,9 @@ void add_timer_on(struct timer_list *timer, int cpu)
         spin_lock_irqsave(&base->lock, flags);
         timer_set_base(timer, base);
         debug_timer_activate(timer);
+        if (time_before(timer->expires, base->next_timer) &&
+            !tbase_get_deferrable(timer->base))
+                base->next_timer = timer->expires;
         internal_add_timer(base, timer);
         /*
          * Check whether the other CPU is idle and needs to be
@@ -817,6 +827,9 @@ int del_timer(struct timer_list *timer)
                 base = lock_timer_base(timer, &flags);
                 if (timer_pending(timer)) {
                         detach_timer(timer, 1);
+                        if (timer->expires == base->next_timer &&
+                            !tbase_get_deferrable(timer->base))
+                                base->next_timer = base->timer_jiffies;
                         ret = 1;
                 }
                 spin_unlock_irqrestore(&base->lock, flags);
@@ -850,6 +863,9 @@ int try_to_del_timer_sync(struct timer_list *timer)
         ret = 0;
         if (timer_pending(timer)) {
                 detach_timer(timer, 1);
+                if (timer->expires == base->next_timer &&
+                    !tbase_get_deferrable(timer->base))
+                        base->next_timer = base->timer_jiffies;
                 ret = 1;
         }
 out:
@@ -1134,7 +1150,9 @@ unsigned long get_next_timer_interrupt(unsigned long now)
         unsigned long expires;
 
         spin_lock(&base->lock);
-        expires = __next_timer_interrupt(base);
+        if (time_before_eq(base->next_timer, base->timer_jiffies))
+                base->next_timer = __next_timer_interrupt(base);
+        expires = base->next_timer;
         spin_unlock(&base->lock);
 
         if (time_before_eq(expires, now))
@@ -1523,6 +1541,7 @@ static int __cpuinit init_timers_cpu(int cpu)
                 INIT_LIST_HEAD(base->tv1.vec + j);
 
         base->timer_jiffies = jiffies;
+        base->next_timer = base->timer_jiffies;
         return 0;
 }
 
@@ -1535,6 +1554,9 @@ static void migrate_timer_list(struct tvec_base *new_base, struct list_head *hea
                 timer = list_first_entry(head, struct timer_list, entry);
                 detach_timer(timer, 0);
                 timer_set_base(timer, new_base);
+                if (time_before(timer->expires, new_base->next_timer) &&
+                    !tbase_get_deferrable(timer->base))
+                        new_base->next_timer = timer->expires;
                 internal_add_timer(new_base, timer);
         }
 }