From 05ad941dfa83599208ee11b67c49ad54c4f3b78a Mon Sep 17 00:00:00 2001
From: Felipe Cerqueira <felipec@mpi-sws.org>
Date: Tue, 12 Feb 2013 17:50:12 +0100
Subject: Add quantum aligment code

Enable staggered ticks for PD^2.
---
 include/linux/tick.h     |  5 +++++
 kernel/time/tick-sched.c | 56 ++++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 54 insertions(+), 7 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 9180f4b85e6d..45fd00e4051b 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -77,6 +77,11 @@ extern int tick_is_oneshot_available(void);
 extern struct tick_device *tick_get_device(int cpu);
 
 # ifdef CONFIG_HIGH_RES_TIMERS
+/* LITMUS^RT tick alignment */
+#define LINUX_DEFAULT_TICKS	0
+#define LITMUS_ALIGNED_TICKS	1
+#define	LITMUS_STAGGERED_TICKS	2
+
 extern int tick_init_highres(void);
 extern int tick_program_event(ktime_t expires, int force);
 extern void tick_setup_sched_timer(void);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 0cf1c1453181..2079d6650532 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1117,6 +1117,49 @@ static int __init skew_tick(char *str)
 }
 early_param("skew_tick", skew_tick);
 
+/**
+ * tick_set_quanta_type - get the quanta type as a boot option
+ * Default is standard setup with ticks staggered over first
+ * half of tick period.
+ */
+int quanta_type = LINUX_DEFAULT_TICKS;
+static int __init tick_set_quanta_type(char *str)
+{
+	if (strcmp("aligned", str) == 0) {
+		quanta_type = LITMUS_ALIGNED_TICKS;
+		printk(KERN_INFO "LITMUS^RT: setting aligned quanta\n");
+	}
+	else if (strcmp("staggered", str) == 0) {
+		quanta_type = LITMUS_STAGGERED_TICKS;
+		printk(KERN_INFO "LITMUS^RT: setting staggered quanta\n");
+	}
+	return 1;
+}
+early_param("quanta=", tick_set_quanta_type);
+
+u64 cpu_stagger_offset(int cpu)
+{
+	u64 offset = 0;
+	switch (quanta_type) {
+		case LITMUS_ALIGNED_TICKS:
+			offset = 0;
+			break;
+		case LITMUS_STAGGERED_TICKS:
+			offset = ktime_to_ns(tick_period);
+			do_div(offset, num_possible_cpus());
+			offset *= cpu;
+			break;
+		default:
+			/* default to Linux default behavior */
+			if (sched_skew_tick) {
+				offset = ktime_to_ns(tick_period) >> 1;
+				do_div(offset, num_possible_cpus());
+				offset *= cpu;
+			}
+	}
+	return offset;
+}
+
 /**
  * tick_setup_sched_timer - setup the tick emulation timer
  */
@@ -1124,6 +1167,7 @@ void tick_setup_sched_timer(void)
 {
 	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 	ktime_t now = ktime_get();
+	u64 offset;
 
 	/*
 	 * Emulate tick processing via per-CPU hrtimers:
@@ -1134,13 +1178,11 @@ 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 jiffies_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);
-	}
+	/* Offset must be set correctly to achieve desired quanta type. */
+	offset = cpu_stagger_offset(smp_processor_id());
+
+	/* Add the correct offset to expiration time */
+	hrtimer_add_expires_ns(&ts->sched_timer, offset);
 
 	for (;;) {
 		hrtimer_forward(&ts->sched_timer, now, tick_period);
-- 
cgit v1.2.2