timers: Introduce the concept of timer slack for legacy timers

While HR timers have had the concept of timer slack for quite some time
now, the legacy timers lacked this concept, and had to make do with
round_jiffies() and friends.

Timer slack is important for power management; grouping timers reduces the
number of wakeups which in turn reduces power consumption.

This patch introduces timer slack to the legacy timers using the following
pieces:
* A slack field in the timer struct
* An api (set_timer_slack) that callers can use to set explicit timer slack
* A default slack of 0.4% of the requested delay for callers that do not set
  any explicit slack
* Rounding code that is part of mod_timer() that tries to
  group timers around jiffies values every 'power of two'
  (so quick timers will group around every 2, but longer timers
  will group around every 4, 8, 16, 32 etc)

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: johnstul@us.ibm.com
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 9 insertions, 1 deletions

diff --git a/include/linux/timer.h b/include/linux/timer.h
index a2d1eb6cb3f..ea965b857a5 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
 /*