2 files changed, 65 insertions, 1 deletions

diff --git a/include/linux/timer.h b/include/linux/timer.h
index a2d1eb6cb3f0..ea965b857a50 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -10,13 +10,19 @@
 struct tvec_base;
 
 struct timer_list {
+        /*
+         * All fields that change during normal runtime grouped to the
+         * same cacheline
+         */
         struct list_head entry;
         unsigned long expires;
+        struct tvec_base *base;
 
         void (*function)(unsigned long);
         unsigned long data;
 
-        struct tvec_base *base;
+        int slack;
+
 #ifdef CONFIG_TIMER_STATS
         void *start_site;
         char start_comm[16];
@@ -165,6 +171,8 @@ extern int mod_timer(struct timer_list *timer, unsigned long expires);
 extern int mod_timer_pending(struct timer_list *timer, unsigned long expires);
 extern int mod_timer_pinned(struct timer_list *timer, unsigned long expires);
 
+extern void set_timer_slack(struct timer_list *time, int slack_hz);
+
 #define TIMER_NOT_PINNED        0
 #define TIMER_PINNED            1
 /*
diff --git a/kernel/timer.c b/kernel/timer.c
index 7e12e7bc7ce6..49773f38c9bc 100644
--- 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);