tick/sched: Optimize tick_do_update_jiffies64() further

Now that it's clear that there is always one tick to account, simplify the
calculations some more.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201117132006.565663056@linutronix.de

kernel/time/tick-sched.c

@@ -55,7 +55,7 @@ static ktime_t last_jiffies_update;
  */
 static void tick_do_update_jiffies64(ktime_t now)
 {
-	unsigned long ticks = 0;
+	unsigned long ticks = 1;
 	ktime_t delta;
 
 	/*
@@ -93,20 +93,21 @@ static void tick_do_update_jiffies64(ktime_t now)
 
 	write_seqcount_begin(&jiffies_seq);
 
-	last_jiffies_update = ktime_add(last_jiffies_update, tick_period);
-
 	delta = ktime_sub(now, tick_next_period);
 	if (unlikely(delta >= tick_period)) {
 		/* Slow path for long idle sleep times */
 		s64 incr = ktime_to_ns(tick_period);
 
-		ticks = ktime_divns(delta, incr);
+		ticks += ktime_divns(delta, incr);
 
 		last_jiffies_update = ktime_add_ns(last_jiffies_update,
 						   incr * ticks);
+	} else {
+		last_jiffies_update = ktime_add(last_jiffies_update,
+						tick_period);
 	}
 
-	do_timer(++ticks);
+	do_timer(ticks);
 
 	/*
 	 * Keep the tick_next_period variable up to date.  WRITE_ONCE()