[PATCH] sched: calculate sched_clock_cpu without tick handling during nohz

[Byungchul Park]

Currently, unstable TSCs can be adjusted by sched_clock_tick(),
sched_clock_local() and sched_clock_remote() with the tick handling.
Thus without tick, it does not work properly.

This patch makes it possible to adjust it approximately without the
tick handling so that it would be ok even if the sched_clock_tick()
is not called during nohz.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/sched/clock.c | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index caf4041..064b40a 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -128,6 +128,9 @@ struct sched_clock_data {
 	u64			tick_raw;
 	u64			tick_gtod;
 	u64			clock;
+#ifdef CONFIG_NO_HZ_COMMON
+	unsigned long		update;
+#endif
 };
 
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
@@ -153,6 +156,9 @@ void sched_clock_init(void)
 		scd->tick_raw = 0;
 		scd->tick_gtod = ktime_now;
 		scd->clock = ktime_now;
+#ifdef CONFIG_NO_HZ_COMMON
+		scd->update = READ_ONCE(jiffies);
+#endif
 	}
 
 	sched_clock_running = 1;
@@ -186,6 +192,27 @@ static inline u64 wrap_max(u64 x, u64 y)
 	return (s64)(x - y) > 0 ? x : y;
 }
 
+#ifdef CONFIG_NO_HZ_COMMON
+static inline unsigned long tick_pending(struct sched_clock_data *scd)
+{
+	return READ_ONCE(jiffies) - scd->update;
+}
+
+static inline void sched_clock_tick_update(struct sched_clock_data *scd)
+{
+	scd->update = READ_ONCE(jiffies);
+}
+#else
+static inline unsigned long tick_pending(struct sched_clock_data *scd)
+{
+	return 0;
+}
+
+static inline void sched_clock_tick_update(struct sched_clock_data *scd)
+{
+}
+#endif
+
 /*
  * update the percpu scd from the raw @now value
  *
@@ -194,7 +221,7 @@ static inline u64 wrap_max(u64 x, u64 y)
  */
 static u64 sched_clock_local(struct sched_clock_data *scd)
 {
-	u64 now, clock, old_clock, min_clock, max_clock;
+	u64 now, clock, old_clock, min_clock, max_clock, next_gtod;
 	s64 delta;
 
 again:
@@ -211,9 +238,10 @@ again:
 	 *		      scd->tick_gtod + TICK_NSEC);
 	 */
 
+	next_gtod = scd->tick_gtod + (tick_pending(scd) + 1) * TICK_NSEC;
 	clock = scd->tick_gtod + delta;
 	min_clock = wrap_max(scd->tick_gtod, old_clock);
-	max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
+	max_clock = wrap_max(old_clock, next_gtod);
 
 	clock = wrap_max(clock, min_clock);
 	clock = wrap_min(clock, max_clock);
@@ -333,6 +361,7 @@ void sched_clock_tick(void)
 
 	scd->tick_raw = now;
 	scd->tick_gtod = now_gtod;
+	sched_clock_tick_update(scd);
 	sched_clock_local(scd);
 }
 
-- 
1.9.1