[PATCH 3/3] clockevents: Switch state to ONESHOT_STOPPED for unused clockevent devices

[Viresh Kumar]

Clockevent device can now be stopped by switching to ONESHOT_STOPPED state, to
avoid getting spurious interrupts on a tickless CPU.

Switch state to ONESHOT_STOPPED at following places:

1.) NOHZ_MODE_LOWRES Mode

Timers & hrtimers are dependent on tick for their working in this mode and the
only place from where clockevent device is programmed is the tick-code.

In LOWRES mode we skip reprogramming the clockevent device in
tick_nohz_stop_sched_tick() if expires == KTIME_MAX. In addition to that we must
also switch the clockevent device to ONESHOT_STOPPED state to avoid all spurious
interrupts that may follow.

2.) NOHZ_MODE_HIGHRES Mode

Tick & timers are dependent on hrtimers for their working in this mode and the
only place from where clockevent device is programmed is the hrtimer-code.

There are two places here from which we reprogram the clockevent device or skip
reprogramming it on expires == KTIME_MAX.

Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 kernel/time/hrtimer.c    | 46 ++++++++++++++++++++++++++++++++++++++++++----
 kernel/time/tick-sched.c |  3 +++
 2 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 045ba7e2be6c..89d4b593dfc0 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -543,8 +543,19 @@ hrtimer_force_reprogram(struct hrtimer_cpu_base *cpu_base, int skip_equal)
 	if (cpu_base->hang_detected)
 		return;
 
-	if (cpu_base->expires_next.tv64 != KTIME_MAX)
+	if (cpu_base->expires_next.tv64 != KTIME_MAX) {
 		tick_program_event(cpu_base->expires_next, 1);
+	} else {
+		struct clock_event_device *dev =
+			__this_cpu_read(tick_cpu_device.evtdev);
+		/*
+		 * Don't need clockevent device anymore, stop it.
+		 *
+		 * We reach here only for NOHZ_MODE_HIGHRES mode and we are
+		 * guaranteed that no timers/hrtimers are enqueued on this cpu.
+		 */
+		clockevents_set_state(dev, CLOCK_EVT_STATE_ONESHOT_STOPPED);
+	}
 }
 
 /*
@@ -1312,9 +1323,36 @@ void hrtimer_interrupt(struct clock_event_device *dev)
 	cpu_base->in_hrtirq = 0;
 	raw_spin_unlock(&cpu_base->lock);
 
-	/* Reprogramming necessary ? */
-	if (expires_next.tv64 == KTIME_MAX ||
-	    !tick_program_event(expires_next, 0)) {
+	if (expires_next.tv64 == KTIME_MAX) {
+		struct clock_event_device *dev =
+			__this_cpu_read(tick_cpu_device.evtdev);
+
+		cpu_base->hang_detected = 0;
+
+		/*
+		 * Don't need clockevent device anymore, stop it.
+		 *
+		 * We reach here only for NOHZ_MODE_HIGHRES mode and we are
+		 * guaranteed that no timers/hrtimers are enqueued on this cpu.
+		 *
+		 * Most of the scenarios will be covered by similar code
+		 * present in hrtimer_force_reprogram(), as we always try to
+		 * evaluate tick requirement on idle/irq exit and cancel
+		 * tick-sched hrtimer when tick isn't required anymore.
+		 *
+		 * It is required here as well as a special case.
+		 *
+		 * Last hrtimer fires on a tickless CPU and doesn't rearm
+		 * itself. tick_nohz_irq_exit() reevaluates next event and it
+		 * gets expires == KTIME_MAX. Because tick was already stopped,
+		 * and last expires == new_expires, we return early. And the
+		 * clockevent device is never stopped.
+		 */
+		clockevents_set_state(dev, CLOCK_EVT_STATE_ONESHOT_STOPPED);
+		return;
+	}
+
+	if (!tick_program_event(expires_next, 0)) {
 		cpu_base->hang_detected = 0;
 		return;
 	}
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 47c04edd07df..ff271a26fa4d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -685,6 +685,9 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 		 if (unlikely(expires.tv64 == KTIME_MAX)) {
 			if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
 				hrtimer_cancel(&ts->sched_timer);
+			else
+				/* stop clock event device */
+				clockevents_set_state(dev, CLOCK_EVT_STATE_ONESHOT_STOPPED);
 			goto out;
 		}
 
-- 
2.3.0.rc0.44.ga94655d

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