sched: tighten up jiffy to sched_clock mapping

The tick code already tracks exact time a tick is expected
to arrive. This can be used to eliminate slack in the jiffy
to sched_clock mapping that aligns windows between a caller
of sched_set_window and the scheduler itself.

Change-Id: I9d47466658d01e6857d7457405459436d504a2ca
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>
[joonwoop@codeaurora.org: fixed minor conflict in include/linux/tick.h]
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index b0741801c4c7..c1f3aca1a01d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -46,6 +46,38 @@ static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);
  */
 static ktime_t last_jiffies_update;
 
+/*
+ * Conversion from ktime to sched_clock is error prone. Use this
+ * as a safetly margin when calculating the sched_clock value at
+ * a particular jiffy as last_jiffies_update uses ktime.
+ */
+#define SCHED_CLOCK_MARGIN 100000
+
+static u64 ns_since_jiffy(void)
+{
+	ktime_t delta;
+
+	delta = ktime_sub(ktime_get(), last_jiffies_update);
+
+	return ktime_to_ns(delta);
+}
+
+u64 jiffy_to_sched_clock(u64 *now, u64 *jiffy_sched_clock)
+{
+	u64 cur_jiffies;
+	unsigned long seq;
+
+	do {
+		seq = read_seqbegin(&jiffies_lock);
+		*now = sched_clock();
+		*jiffy_sched_clock = *now -
+			(ns_since_jiffy() + SCHED_CLOCK_MARGIN);
+		cur_jiffies = get_jiffies_64();
+	} while (read_seqretry(&jiffies_lock, seq));
+
+	return cur_jiffies;
+}
+
 struct tick_sched *tick_get_tick_sched(int cpu)
 {
 	return &per_cpu(tick_cpu_sched, cpu);