[PATCH 2/3] clockevents: Restart clockevent device before using it again

[Viresh Kumar]

Next commit would update clockevents core to switch clockevent devices to
ONESHOT_STOPPED state to avoid getting spurious interrupts on a tickless CPU.

Before programming the clockevent device for next event, we must switch its
state to ONESHOT.

Switch state back to ONESHOT from 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.

Two routines can restart ticks in LOWRES mode: tick_nohz_restart() and
tick_nohz_stop_sched_tick().

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.

Only hrtimer_reprogram() is responsible for programming the clockevent device
for next event, if the clockevent device is stopped earlier. And updating that
alone is sufficient here.

To make sure we haven't missed any corner case, add a WARN() for the case where
we try to reprogram clockevent device while we aren't configured in
ONESHOT_STOPPED state.

Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 kernel/time/clockevents.c |  4 ++++
 kernel/time/hrtimer.c     |  5 +++++
 kernel/time/tick-sched.c  | 14 +++++++++++++-
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index 04f6c3433f8e..e95896fd21fd 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -335,6 +335,10 @@ int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
 	if (dev->state == CLOCK_EVT_STATE_SHUTDOWN)
 		return 0;
 
+	/* We must be in ONESHOT state here */
+	WARN_ONCE(dev->state != CLOCK_EVT_STATE_ONESHOT, "Current state: %d\n",
+		  dev->state);
+
 	/* Shortcut for clockevent devices that can deal with ktime. */
 	if (dev->features & CLOCK_EVT_FEAT_KTIME)
 		return dev->set_next_ktime(expires, dev);
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index bee0c1f78091..045ba7e2be6c 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -566,6 +566,7 @@ static int hrtimer_reprogram(struct hrtimer *timer,
 {
 	struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
 	ktime_t expires = ktime_sub(hrtimer_get_expires(timer), base->offset);
+	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
 	int res;
 
 	WARN_ON_ONCE(hrtimer_get_expires_tv64(timer) < 0);
@@ -610,6 +611,10 @@ static int hrtimer_reprogram(struct hrtimer *timer,
 	if (cpu_base->hang_detected)
 		return 0;
 
+	/* Switchback to ONESHOT state */
+	if (unlikely(dev->state == CLOCK_EVT_STATE_ONESHOT_STOPPED))
+		clockevents_set_state(dev, CLOCK_EVT_STATE_ONESHOT);
+
 	/*
 	 * Clockevents returns -ETIME, when the event was in the past.
 	 */
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a4c4edac4528..47c04edd07df 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -694,8 +694,14 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 			/* Check, if the timer was already in the past */
 			if (hrtimer_active(&ts->sched_timer))
 				goto out;
-		} else if (!tick_program_event(expires, 0))
+		} else {
+			/* Switchback to ONESHOT state */
+			if (unlikely(dev->state == CLOCK_EVT_STATE_ONESHOT_STOPPED))
+				clockevents_set_state(dev, CLOCK_EVT_STATE_ONESHOT);
+
+			if (!tick_program_event(expires, 0))
 				goto out;
+		}
 		/*
 		 * We are past the event already. So we crossed a
 		 * jiffie boundary. Update jiffies and raise the
@@ -873,6 +879,8 @@ ktime_t tick_nohz_get_sleep_length(void)
 
 static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
 {
+	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
+
 	hrtimer_cancel(&ts->sched_timer);
 	hrtimer_set_expires(&ts->sched_timer, ts->last_tick);
 
@@ -887,6 +895,10 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
 			if (hrtimer_active(&ts->sched_timer))
 				break;
 		} else {
+			/* Switchback to ONESHOT state */
+			if (likely(dev->state == CLOCK_EVT_STATE_ONESHOT_STOPPED))
+				clockevents_set_state(dev, CLOCK_EVT_STATE_ONESHOT);
+
 			if (!tick_program_event(
 				hrtimer_get_expires(&ts->sched_timer), 0))
 				break;
-- 
2.3.0.rc0.44.ga94655d

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