timers: Introduce the concept of timer slack for legacy timers

[safe/jmp/linux-2.6] / kernel / timer.c

diff --git a/kernel/timer.c b/kernel/timer.c
index 7e12e7b..49773f3 100644 (file)
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -318,6 +318,24 @@ unsigned long round_jiffies_up_relative(unsigned long j)
 }
 EXPORT_SYMBOL_GPL(round_jiffies_up_relative);
 
+/**
+ * set_timer_slack - set the allowed slack for a timer
+ * @slack_hz: the amount of time (in jiffies) allowed for rounding
+ *
+ * Set the amount of time, in jiffies, that a certain timer has
+ * in terms of slack. By setting this value, the timer subsystem
+ * will schedule the actual timer somewhere between
+ * the time mod_timer() asks for, and that time plus the slack.
+ *
+ * By setting the slack to -1, a percentage of the delay is used
+ * instead.
+ */
+void set_timer_slack(struct timer_list *timer, int slack_hz)
+{
+       timer->slack = slack_hz;
+}
+EXPORT_SYMBOL_GPL(set_timer_slack);
+
 
 static inline void set_running_timer(struct tvec_base *base,
                                        struct timer_list *timer)
@@ -549,6 +567,7 @@ static void __init_timer(struct timer_list *timer,
 {
        timer->entry.next = NULL;
        timer->base = __raw_get_cpu_var(tvec_bases);
+       timer->slack = -1;
 #ifdef CONFIG_TIMER_STATS
        timer->start_site = NULL;
        timer->start_pid = -1;
@@ -714,6 +733,41 @@ int mod_timer_pending(struct timer_list *timer, unsigned long expires)
 }
 EXPORT_SYMBOL(mod_timer_pending);
 
+/*
+ * Decide where to put the timer while taking the slack into account
+ *
+ * Algorithm:
+ *   1) calculate the maximum (absolute) time
+ *   2) calculate the highest bit where the expires and new max are different
+ *   3) use this bit to make a mask
+ *   4) use the bitmask to round down the maximum time, so that all last
+ *      bits are zeros
+ */
+static inline
+unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
+{
+       unsigned long expires_limit, mask;
+       int bit;
+
+       expires_limit = expires + timer->slack;
+
+       if (timer->slack < 0) /* auto slack: use 0.4% */
+               expires_limit = expires + (expires - jiffies)/256;
+
+       mask = expires ^ expires_limit;
+
+       if (mask == 0)
+               return expires;
+
+       bit = find_last_bit(&mask, BITS_PER_LONG);
+
+       mask = (1 << bit) - 1;
+
+       expires_limit = expires_limit & ~(mask);
+
+       return expires_limit;
+}
+
 /**
  * mod_timer - modify a timer's timeout
  * @timer: the timer to be modified
@@ -744,6 +798,8 @@ int mod_timer(struct timer_list *timer, unsigned long expires)
        if (timer_pending(timer) && timer->expires == expires)
                return 1;
 
+       expires = apply_slack(timer, expires);
+
        return __mod_timer(timer, expires, false, TIMER_NOT_PINNED);
 }
 EXPORT_SYMBOL(mod_timer);