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>

include/linux/hrtimer.h

@@ -386,7 +386,7 @@ static inline int hrtimer_restart(struct hrtimer *timer)
 /* Query timers: */
 extern ktime_t hrtimer_get_remaining(const struct hrtimer *timer);
 
-extern ktime_t hrtimer_get_next_event(void);
+extern u64 hrtimer_get_next_event(void);
 
 /*
  * A timer is active, when it is enqueued into the rbtree or the

include/linux/rcupdate.h

@@ -44,6 +44,8 @@
 #include <linux/debugobjects.h>
 #include <linux/bug.h>
 #include <linux/compiler.h>
+#include <linux/ktime.h>
+
 #include <asm/barrier.h>
 
 extern int rcu_expedited; /* for sysctl */
@@ -1154,9 +1156,9 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
 	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
 
 #if defined(CONFIG_TINY_RCU) || defined(CONFIG_RCU_NOCB_CPU_ALL)
-static inline int rcu_needs_cpu(unsigned long *delta_jiffies)
+static inline int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 {
-	*delta_jiffies = ULONG_MAX;
+	*nextevt = KTIME_MAX;
 	return 0;
 }
 #endif /* #if defined(CONFIG_TINY_RCU) || defined(CONFIG_RCU_NOCB_CPU_ALL) */

include/linux/rcutree.h

@@ -32,7 +32,7 @@
 
 void rcu_note_context_switch(void);
 #ifndef CONFIG_RCU_NOCB_CPU_ALL
-int rcu_needs_cpu(unsigned long *delta_jiffies);
+int rcu_needs_cpu(u64 basem, u64 *nextevt);
 #endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
 void rcu_cpu_stall_reset(void);
 

include/linux/timer.h

@@ -187,13 +187,6 @@ extern void set_timer_slack(struct timer_list *time, int slack_hz);
  */
 #define NEXT_TIMER_MAX_DELTA	((1UL << 30) - 1)
 
-/*
- * Return when the next timer-wheel timeout occurs (in absolute jiffies),
- * locks the timer base and does the comparison against the given
- * jiffie.
- */
-extern unsigned long get_next_timer_interrupt(unsigned long now);
-
 /*
  * Timer-statistics info:
  */

kernel/rcu/tree_plugin.h

@@ -1368,9 +1368,9 @@ static void rcu_prepare_kthreads(int cpu)
  * any flavor of RCU.
  */
 #ifndef CONFIG_RCU_NOCB_CPU_ALL
-int rcu_needs_cpu(unsigned long *delta_jiffies)
+int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 {
-	*delta_jiffies = ULONG_MAX;
+	*nextevt = KTIME_MAX;
 	return rcu_cpu_has_callbacks(NULL);
 }
 #endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
@@ -1481,16 +1481,17 @@ static bool __maybe_unused rcu_try_advance_all_cbs(void)
  * The caller must have disabled interrupts.
  */
 #ifndef CONFIG_RCU_NOCB_CPU_ALL
-int rcu_needs_cpu(unsigned long *dj)
+int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 {
 	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
+	unsigned long dj;
 
 	/* Snapshot to detect later posting of non-lazy callback. */
 	rdtp->nonlazy_posted_snap = rdtp->nonlazy_posted;
 
 	/* If no callbacks, RCU doesn't need the CPU. */
 	if (!rcu_cpu_has_callbacks(&rdtp->all_lazy)) {
-		*dj = ULONG_MAX;
+		*nextevt = KTIME_MAX;
 		return 0;
 	}
 
@@ -1504,11 +1505,12 @@ int rcu_needs_cpu(unsigned long *dj)
 
 	/* Request timer delay depending on laziness, and round. */
 	if (!rdtp->all_lazy) {
-		*dj = round_up(rcu_idle_gp_delay + jiffies,
+		dj = round_up(rcu_idle_gp_delay + jiffies,
 			       rcu_idle_gp_delay) - jiffies;
 	} else {
-		*dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies;
+		dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies;
 	}
+	*nextevt = basemono + dj * TICK_NSEC;
 	return 0;
 }
 #endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */

kernel/time/hrtimer.c

@@ -1080,26 +1080,22 @@ EXPORT_SYMBOL_GPL(hrtimer_get_remaining);
 /**
  * hrtimer_get_next_event - get the time until next expiry event
  *
- * Returns the delta to the next expiry event or KTIME_MAX if no timer
- * is pending.
+ * Returns the next expiry time or KTIME_MAX if no timer is pending.
  */
-ktime_t hrtimer_get_next_event(void)
+u64 hrtimer_get_next_event(void)
 {
 	struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
-	ktime_t mindelta = { .tv64 = KTIME_MAX };
+	u64 expires = KTIME_MAX;
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&cpu_base->lock, flags);
 
 	if (!__hrtimer_hres_active(cpu_base))
-		mindelta = ktime_sub(__hrtimer_get_next_event(cpu_base),
-				     ktime_get());
+		expires = __hrtimer_get_next_event(cpu_base).tv64;
 
 	raw_spin_unlock_irqrestore(&cpu_base->lock, flags);
 
-	if (mindelta.tv64 < 0)
-		mindelta.tv64 = 0;
-	return mindelta;
+	return expires;
 }
 #endif
 

kernel/time/tick-internal.h

@@ -137,3 +137,5 @@ extern void tick_nohz_init(void);
 # else
 static inline void tick_nohz_init(void) { }
 #endif
+
+extern u64 get_next_timer_interrupt(unsigned long basej, u64 basem);

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)

kernel/time/tick-sched.h

@@ -57,7 +57,7 @@ struct tick_sched {
 	ktime_t				iowait_sleeptime;
 	ktime_t				sleep_length;
 	unsigned long			last_jiffies;
-	unsigned long			next_jiffies;
+	u64				next_timer;
 	ktime_t				idle_expires;
 	int				do_timer_last;
 };

kernel/time/timer.c

@@ -49,6 +49,8 @@
 #include <asm/timex.h>
 #include <asm/io.h>
 
+#include "tick-internal.h"
+
 #define CREATE_TRACE_POINTS
 #include <trace/events/timer.h>
 
@@ -1311,54 +1313,48 @@ cascade:
  * Check, if the next hrtimer event is before the next timer wheel
  * event:
  */
-static unsigned long cmp_next_hrtimer_event(unsigned long now,
-					    unsigned long expires)
+static u64 cmp_next_hrtimer_event(u64 basem, u64 expires)
 {
-	ktime_t hr_delta = hrtimer_get_next_event();
-	struct timespec tsdelta;
-	unsigned long delta;
-
-	if (hr_delta.tv64 == KTIME_MAX)
-		return expires;
+	u64 nextevt = hrtimer_get_next_event();
 
 	/*
-	 * Expired timer available, let it expire in the next tick
+	 * If high resolution timers are enabled
+	 * hrtimer_get_next_event() returns KTIME_MAX.
 	 */
-	if (hr_delta.tv64 <= 0)
-		return now + 1;
-
-	tsdelta = ktime_to_timespec(hr_delta);
-	delta = timespec_to_jiffies(&tsdelta);
+	if (expires <= nextevt)
+		return expires;
 
 	/*
-	 * Limit the delta to the max value, which is checked in
-	 * tick_nohz_stop_sched_tick():
+	 * If the next timer is already expired, return the tick base
+	 * time so the tick is fired immediately.
 	 */
-	if (delta > NEXT_TIMER_MAX_DELTA)
-		delta = NEXT_TIMER_MAX_DELTA;
+	if (nextevt <= basem)
+		return basem;
 
 	/*
-	 * Take rounding errors in to account and make sure, that it
-	 * expires in the next tick. Otherwise we go into an endless
-	 * ping pong due to tick_nohz_stop_sched_tick() retriggering
-	 * the timer softirq
+	 * Round up to the next jiffie. High resolution timers are
+	 * off, so the hrtimers are expired in the tick and we need to
+	 * make sure that this tick really expires the timer to avoid
+	 * a ping pong of the nohz stop code.
+	 *
+	 * Use DIV_ROUND_UP_ULL to prevent gcc calling __divdi3
 	 */
-	if (delta < 1)
-		delta = 1;
-	now += delta;
-	if (time_before(now, expires))
-		return now;
-	return expires;
+	return DIV_ROUND_UP_ULL(nextevt, TICK_NSEC) * TICK_NSEC;
 }
 
 /**
- * get_next_timer_interrupt - return the jiffy of the next pending timer
- * @now: current time (in jiffies)
+ * get_next_timer_interrupt - return the time (clock mono) of the next timer
+ * @basej:	base time jiffies
+ * @basem:	base time clock monotonic
+ *
+ * Returns the tick aligned clock monotonic time of the next pending
+ * timer or KTIME_MAX if no timer is pending.
  */
-unsigned long get_next_timer_interrupt(unsigned long now)
+u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 {
 	struct tvec_base *base = __this_cpu_read(tvec_bases);
-	unsigned long expires = now + NEXT_TIMER_MAX_DELTA;
+	u64 expires = KTIME_MAX;
+	unsigned long nextevt;
 
 	/*
 	 * Pretend that there is no timer pending if the cpu is offline.
@@ -1371,14 +1367,15 @@ unsigned long get_next_timer_interrupt(unsigned long now)
 	if (base->active_timers) {
 		if (time_before_eq(base->next_timer, base->timer_jiffies))
 			base->next_timer = __next_timer_interrupt(base);
-		expires = base->next_timer;
+		nextevt = base->next_timer;
+		if (time_before_eq(nextevt, basej))
+			expires = basem;
+		else
+			expires = basem + (nextevt - basej) * TICK_NSEC;
 	}
 	spin_unlock(&base->lock);
 
-	if (time_before_eq(expires, now))
-		return now;
-
-	return cmp_next_hrtimer_event(now, expires);
+	return cmp_next_hrtimer_event(basem, expires);
 }
 #endif
 

kernel/time/timer_list.c

@@ -191,7 +191,7 @@ static void print_cpu(struct seq_file *m, int cpu, u64 now)
 		P_ns(idle_sleeptime);
 		P_ns(iowait_sleeptime);
 		P(last_jiffies);
-		P(next_jiffies);
+		P(next_timer);
 		P_ns(idle_expires);
 		SEQ_printf(m, "jiffies: %Lu\n",
 			   (unsigned long long)jiffies);
@@ -289,7 +289,7 @@ static void timer_list_show_tickdevices_header(struct seq_file *m)
 
 static inline void timer_list_header(struct seq_file *m, u64 now)
 {
-	SEQ_printf(m, "Timer List Version: v0.7\n");
+	SEQ_printf(m, "Timer List Version: v0.8\n");
 	SEQ_printf(m, "HRTIMER_MAX_CLOCK_BASES: %d\n", HRTIMER_MAX_CLOCK_BASES);
 	SEQ_printf(m, "now at %Ld nsecs\n", (unsigned long long)now);
 	SEQ_printf(m, "\n");