tick/nohz: Don't shutdown the lowres tick from itself

In lowres dynticks mode, just like in highres dynticks mode, when there
is no tick to program in the future, the tick eventually gets
deactivated either:

  * From the idle loop if in idle mode.
  * From the IRQ exit if in full dynticks mode.

Therefore there is no need to deactivate it from the tick itself. This
just just brings more overhead in the idle tick path for no reason.

Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Joel Fernandes (Google) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Frederic Weisbecker

kernel/time/tick-sched.c

@@ -1403,18 +1403,16 @@ static void tick_nohz_lowres_handler(struct clock_event_device *dev)
 	tick_sched_do_timer(ts, now);
 	tick_sched_handle(ts, regs);
 
-	if (unlikely(ts->tick_stopped)) {
-		/*
-		 * The clockevent device is not reprogrammed, so change the
-		 * clock event device to ONESHOT_STOPPED to avoid spurious
-		 * interrupts on devices which might not be truly one shot.
-		 */
-		tick_program_event(KTIME_MAX, 1);
-		return;
+	/*
+	 * In dynticks mode, tick reprogram is deferred:
+	 * - to the idle task if in dynticks-idle
+	 * - to IRQ exit if in full-dynticks.
+	 */
+	if (likely(!ts->tick_stopped)) {
+		hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
+		tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
 	}
 
-	hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
-	tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
 }
 
 static inline void tick_nohz_activate(struct tick_sched *ts, int mode)
@@ -1519,7 +1517,11 @@ static enum hrtimer_restart tick_nohz_highres_handler(struct hrtimer *timer)
 	else
 		ts->next_tick = 0;
 
-	/* No need to reprogram if we are in idle or full dynticks mode */
+	/*
+	 * In dynticks mode, tick reprogram is deferred:
+	 * - to the idle task if in dynticks-idle
+	 * - to IRQ exit if in full-dynticks.
+	 */
 	if (unlikely(ts->tick_stopped))
 		return HRTIMER_NORESTART;