timers: Add accounting of non deferrable timers

The code in get_next_timer_interrupt() is suboptimal as it has to run
through the cascade to find the next expiring timer. On a completely
idle core we should only do that when there is an active timer
enqueued and base->next_timer does not give us a fast answer.

Add accounting of the active timers to the now consolidated
attach/detach code. I deliberately avoided sanity checks because the
code is fully symetric and any fiddling with timers w/o using the API
functions will lead to cute explosions anyway. ulong is big enough
even on 32bit and if we really run into the situation to have more
than 1<<32 timers enqueued there, then we are definitely not in a
state to go idle and run through that code.

This allows us to fix another shortcoming of get_next_timer_interrupt().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20120525214819.236377028@linutronix.de

1 files changed, 23 insertions, 8 deletions

diff --git a/kernel/timer.c b/kernel/timer.c
index 7207690b535..7fada698bd1 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -77,6 +77,7 @@ struct tvec_base {
         struct timer_list *running_timer;
         unsigned long timer_jiffies;
         unsigned long next_timer;
+        unsigned long active_timers;
         struct tvec_root tv1;
         struct tvec tv2;
         struct tvec tv3;
@@ -377,11 +378,13 @@ static void internal_add_timer(struct tvec_base *base, struct timer_list *timer)
 {
         __internal_add_timer(base, timer);
         /*
-         * Update base->next_timer if this is the earliest one.
+         * Update base->active_timers and base->next_timer
          */
-        if (time_before(timer->expires, base->next_timer) &&
-            !tbase_get_deferrable(timer->base))
-                base->next_timer = timer->expires;
+        if (!tbase_get_deferrable(timer->base)) {
+                if (time_before(timer->expires, base->next_timer))
+                        base->next_timer = timer->expires;
+                base->active_timers++;
+        }
 }
 
 #ifdef CONFIG_TIMER_STATS
@@ -678,6 +681,14 @@ static inline void detach_timer(struct timer_list *timer, bool clear_pending)
         entry->prev = LIST_POISON2;
 }
 
+static inline void
+detach_expired_timer(struct timer_list *timer, struct tvec_base *base)
+{
+        detach_timer(timer, true);
+        if (!tbase_get_deferrable(timer->base))
+                timer->base->active_timers--;
+}
+
 static int detach_if_pending(struct timer_list *timer, struct tvec_base *base,
                              bool clear_pending)
 {
@@ -685,9 +696,11 @@ static int detach_if_pending(struct timer_list *timer, struct tvec_base *base,
                 return 0;
 
         detach_timer(timer, clear_pending);
-        if (timer->expires == base->next_timer &&
-            !tbase_get_deferrable(timer->base))
-                base->next_timer = base->timer_jiffies;
+        if (!tbase_get_deferrable(timer->base)) {
+                timer->base->active_timers--;
+                if (timer->expires == base->next_timer)
+                        base->next_timer = base->timer_jiffies;
+        }
         return 1;
 }
 
@@ -1175,7 +1188,7 @@ static inline void __run_timers(struct tvec_base *base)
                         timer_stats_account_timer(timer);
 
                         base->running_timer = timer;
-                        detach_timer(timer, true);
+                        detach_expired_timer(timer, base);
 
                         spin_unlock_irq(&base->lock);
                         call_timer_fn(timer, fn, data);
@@ -1701,6 +1714,7 @@ static int __cpuinit init_timers_cpu(int cpu)
 
         base->timer_jiffies = jiffies;
         base->next_timer = base->timer_jiffies;
+        base->active_timers = 0;
         return 0;
 }
 
@@ -1711,6 +1725,7 @@ static void migrate_timer_list(struct tvec_base *new_base, struct list_head *hea
 
         while (!list_empty(head)) {
                 timer = list_first_entry(head, struct timer_list, entry);
+                /* We ignore the accounting on the dying cpu */
                 detach_timer(timer, false);
                 timer_set_base(timer, new_base);
                 internal_add_timer(new_base, timer);