tick: Add tick skew boot option

Let the user decide whether power consumption or jitter is the
more important consideration for their machines.

Quoting removal commit af5ab277de:

"Historically, Linux has tried to make the regular timer tick on the
 various CPUs not happen at the same time, to avoid contention on
 xtime_lock.
    
 Nowadays, with the tickless kernel, this contention no longer happens
 since time keeping and updating are done differently. In addition,
 this skew is actually hurting power consumption in a measurable way on
 many-core systems."

Problems:

- Contrary to the above, systems do encounter contention on both
  xtime_lock and RCU structure locks when the tick is synchronized.
  
- Moderate sized RT systems suffer intolerable jitter due to the tick
  being synchronized.

- SGI reports the same for their large systems.

- Fully utilized systems reap no power saving benefit from skew removal,
  but do suffer from resulting induced lock contention.

- 0209f649 rcu: limit rcu_node leaf-level fanout
  This patch was born to combat lock contention which testing showed
  to have been _induced by_ skew removal.  Skew the tick, contention
  disappeared virtually completely.

Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
Link: http://lkml.kernel.org/r/1336472458.21924.78.camel@marge.simpson.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/tick-sched.c

@@ -814,6 +814,8 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
 	return HRTIMER_RESTART;
 }
 
+static int sched_skew_tick;
+
 /**
  * tick_setup_sched_timer - setup the tick emulation timer
  */
@@ -831,6 +833,14 @@ void tick_setup_sched_timer(void)
 	/* Get the next period (per cpu) */
 	hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());
 
+	/* Offset the tick to avert xtime_lock contention. */
+	if (sched_skew_tick) {
+		u64 offset = ktime_to_ns(tick_period) >> 1;
+		do_div(offset, num_possible_cpus());
+		offset *= smp_processor_id();
+		hrtimer_add_expires_ns(&ts->sched_timer, offset);
+	}
+
 	for (;;) {
 		hrtimer_forward(&ts->sched_timer, now, tick_period);
 		hrtimer_start_expires(&ts->sched_timer,
@@ -910,3 +920,11 @@ int tick_check_oneshot_change(int allow_nohz)
 	tick_nohz_switch_to_nohz();
 	return 0;
 }
+
+static int __init skew_tick(char *str)
+{
+	get_option(&str, &sched_skew_tick);
+
+	return 0;
+}
+early_param("skew_tick", skew_tick);