tick: Nohz: Rework next timer evaluation

The evaluation of the next timer in the nohz code is based on jiffies
while all the tick internals are nano seconds based. We have also to
convert hrtimer nanoseconds to jiffies in the !highres case. That's
just wrong and introduces interesting corner cases.

Turn it around and convert the next timer wheel timer expiry and the
rcu event to clock monotonic and base all calculations on
nanoseconds. That identifies the case where no timer is pending
clearly with an absolute expiry value of KTIME_MAX.

Makes the code more readable and gets rid of the jiffies magic in the
nohz code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Link: http://lkml.kernel.org/r/20150414203502.184198593@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 49 insertions, 60 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 4c5f4a9dcc0a5f..753c211f619517 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -582,39 +582,46 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
 static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 					 ktime_t now, int cpu)
 {
-	unsigned long seq, last_jiffies, next_jiffies, delta_jiffies;
-	ktime_t last_update, expires, ret = { .tv64 = 0 };
-	unsigned long rcu_delta_jiffies;
 	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
-	u64 time_delta;
-
-	time_delta = timekeeping_max_deferment();
+	u64 basemono, next_tick, next_tmr, next_rcu, delta, expires;
+	unsigned long seq, basejiff;
+	ktime_t	tick;
 
 	/* Read jiffies and the time when jiffies were updated last */
 	do {
 		seq = read_seqbegin(&jiffies_lock);
-		last_update = last_jiffies_update;
-		last_jiffies = jiffies;
+		basemono = last_jiffies_update.tv64;
+		basejiff = jiffies;
 	} while (read_seqretry(&jiffies_lock, seq));
+	ts->last_jiffies = basejiff;
 
-	if (rcu_needs_cpu(&rcu_delta_jiffies) ||
+	if (rcu_needs_cpu(basemono, &next_rcu) ||
 	    arch_needs_cpu() || irq_work_needs_cpu()) {
-		next_jiffies = last_jiffies + 1;
-		delta_jiffies = 1;
+		next_tick = basemono + TICK_NSEC;
 	} else {
-		/* Get the next timer wheel timer */
-		next_jiffies = get_next_timer_interrupt(last_jiffies);
-		delta_jiffies = next_jiffies - last_jiffies;
-		if (rcu_delta_jiffies < delta_jiffies) {
-			next_jiffies = last_jiffies + rcu_delta_jiffies;
-			delta_jiffies = rcu_delta_jiffies;
-		}
+		/*
+		 * Get the next pending timer. If high resolution
+		 * timers are enabled this only takes the timer wheel
+		 * timers into account. If high resolution timers are
+		 * disabled this also looks at the next expiring
+		 * hrtimer.
+		 */
+		next_tmr = get_next_timer_interrupt(basejiff, basemono);
+		ts->next_timer = next_tmr;
+		/* Take the next rcu event into account */
+		next_tick = next_rcu < next_tmr ? next_rcu : next_tmr;
 	}
 
-	if ((long)delta_jiffies <= 1) {
+	/*
+	 * If the tick is due in the next period, keep it ticking or
+	 * restart it proper.
+	 */
+	delta = next_tick - basemono;
+	if (delta <= (u64)TICK_NSEC) {
+		tick.tv64 = 0;
 		if (!ts->tick_stopped)
 			goto out;
-		if (delta_jiffies == 0) {
+		if (delta == 0) {
 			/* Tick is stopped, but required now. Enforce it */
 			tick_nohz_restart(ts, now);
 			goto out;
@@ -629,54 +636,39 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 	 * do_timer() never invoked. Keep track of the fact that it
 	 * was the one which had the do_timer() duty last. If this cpu
 	 * is the one which had the do_timer() duty last, we limit the
-	 * sleep time to the timekeeping max_deferement value which we
-	 * retrieved above. Otherwise we can sleep as long as we want.
+	 * sleep time to the timekeeping max_deferement value.
+	 * Otherwise we can sleep as long as we want.
 	 */
+	delta = timekeeping_max_deferment();
 	if (cpu == tick_do_timer_cpu) {
 		tick_do_timer_cpu = TICK_DO_TIMER_NONE;
 		ts->do_timer_last = 1;
 	} else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
-		time_delta = KTIME_MAX;
+		delta = KTIME_MAX;
 		ts->do_timer_last = 0;
 	} else if (!ts->do_timer_last) {
-		time_delta = KTIME_MAX;
+		delta = KTIME_MAX;
 	}
 
 #ifdef CONFIG_NO_HZ_FULL
+	/* Limit the tick delta to the maximum scheduler deferment */
 	if (!ts->inidle)
-		time_delta = min(time_delta, scheduler_tick_max_deferment());
+		delta = min(delta, scheduler_tick_max_deferment());
 #endif
 
-	/*
-	 * calculate the expiry time for the next timer wheel
-	 * timer. delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
-	 * there is no timer pending or at least extremely far into
-	 * the future (12 days for HZ=1000). In this case we set the
-	 * expiry to the end of time.
-	 */
-	if (likely(delta_jiffies < NEXT_TIMER_MAX_DELTA)) {
-		/*
-		 * Calculate the time delta for the next timer event.
-		 * If the time delta exceeds the maximum time delta
-		 * permitted by the current clocksource then adjust
-		 * the time delta accordingly to ensure the
-		 * clocksource does not wrap.
-		 */
-		time_delta = min_t(u64, time_delta,
-				   tick_period.tv64 * delta_jiffies);
-	}
-
-	if (time_delta < KTIME_MAX)
-		expires = ktime_add_ns(last_update, time_delta);
+	/* Calculate the next expiry time */
+	if (delta < (KTIME_MAX - basemono))
+		expires = basemono + delta;
 	else
-		expires.tv64 = KTIME_MAX;
+		expires = KTIME_MAX;
+
+	expires = min_t(u64, expires, next_tick);
+	tick.tv64 = expires;
 
 	/* Skip reprogram of event if its not changed */
-	if (ts->tick_stopped && ktime_equal(expires, dev->next_event))
+	if (ts->tick_stopped && (expires == dev->next_event.tv64))
 		goto out;
 
-	ret = expires;
-
 	/*
 	 * nohz_stop_sched_tick can be called several times before
 	 * the nohz_restart_sched_tick is called. This happens when
@@ -694,26 +686,23 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 	}
 
 	/*
-	 * If the expiration time == KTIME_MAX, then
-	 * in this case we simply stop the tick timer.
+	 * If the expiration time == KTIME_MAX, then we simply stop
+	 * the tick timer.
 	 */
-	if (unlikely(expires.tv64 == KTIME_MAX)) {
+	if (unlikely(expires == KTIME_MAX)) {
 		if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
 			hrtimer_cancel(&ts->sched_timer);
 		goto out;
 	}
 
 	if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
-		hrtimer_start(&ts->sched_timer, expires,
-			      HRTIMER_MODE_ABS_PINNED);
+		hrtimer_start(&ts->sched_timer, tick, HRTIMER_MODE_ABS_PINNED);
 	else
-		tick_program_event(expires, 1);
+		tick_program_event(tick, 1);
 out:
-	ts->next_jiffies = next_jiffies;
-	ts->last_jiffies = last_jiffies;
+	/* Update the estimated sleep length */
 	ts->sleep_length = ktime_sub(dev->next_event, now);
-
-	return ret;
+	return tick;
 }
 
 static void tick_nohz_full_stop_tick(struct tick_sched *ts)