[PATCH v8 08/25] timer: Rework idle logic

[Anna-Maria Behnsen]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

To improve readability of the code, split base->idle calculation and
expires calculation into separate parts.

Thereby the following subtle change happens if the next event is just one
jiffy ahead and the tick was already stopped: Originally base->is_idle
remains true in this situation. Now base->is_idle turns to false. This may
spare an IPI if a timer is enqueued remotely to an idle CPU that is going
to tick on the next jiffy.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Anna-Maria Behnsen <anna-maria@xxxxxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
v4: Change condition to force 0 delta and update commit message (Frederic)
---
 kernel/time/timer.c | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index dc58c479d35a..18f8aac9b19a 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1943,21 +1943,20 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 	 */
 	__forward_timer_base(base, basej);
 
-	if (time_before_eq(nextevt, basej)) {
-		expires = basem;
-		base->is_idle = false;
-	} else {
-		if (base->timers_pending)
-			expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
-		/*
-		 * If we expect to sleep more than a tick, mark the base idle.
-		 * Also the tick is stopped so any added timer must forward
-		 * the base clk itself to keep granularity small. This idle
-		 * logic is only maintained for the BASE_STD base, deferrable
-		 * timers may still see large granularity skew (by design).
-		 */
-		if ((expires - basem) > TICK_NSEC)
-			base->is_idle = true;
+	/*
+	 * Base is idle if the next event is more than a tick away. Also
+	 * the tick is stopped so any added timer must forward the base clk
+	 * itself to keep granularity small. This idle logic is only
+	 * maintained for the BASE_STD base, deferrable timers may still
+	 * see large granularity skew (by design).
+	 */
+	base->is_idle = time_after(nextevt, basej + 1);
+
+	if (base->timers_pending) {
+		/* If we missed a tick already, force 0 delta */
+		if (time_before(nextevt, basej))
+			nextevt = basej;
+		expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
 	}
 	raw_spin_unlock(&base->lock);
 
-- 
2.39.2