Add support for quantum alignment

2 files changed, 50 insertions, 3 deletions

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 0482229c07db..4f9ba058abdb 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -71,6 +71,11 @@ 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 89aed5933ed4..dcbff7515489 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -686,6 +686,46 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
 }
 
 /**
+ * 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;
+}
+__setup("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:
+                        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
  */
 void tick_setup_sched_timer(void)
@@ -702,9 +742,11 @@ void tick_setup_sched_timer(void)
 
         /* Get the next period (per cpu) */
         hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());
-        offset = ktime_to_ns(tick_period) >> 1;
-        do_div(offset, num_possible_cpus());
-        offset *= smp_processor_id();
+
+        /* 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 (;;) {