[RFT][PATCH v4 5/7] sched: idle: Select idle state before stopping the tick

[Rafael J. Wysocki]

From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

In order to address the issue with short idle duration predictions
by the idle governor after the tick has been stopped, reorder the
code in cpuidle_idle_call() so that the governor idle state selection
runs before tick_nohz_idle_go_idle() and use the "nohz" hint returned
by cpuidle_select() to decide whether or not to stop the tick.

This isn't straightforward, because menu_select() invokes
tick_nohz_get_sleep_length() to get the time to the next timer
event and the number returned by the latter comes from
__tick_nohz_idle_enter().  Fortunately, however, it is possible
to compute that number without actually stopping the tick and with
the help of the existing code.

Namely, notice that tick_nohz_stop_sched_tick() already computes the
next timer event time to reprogram the scheduler tick hrtimer and
that time can be used as a proxy for the actual next timer event
time in the idle duration predicition.  Moreover, it is possible
to split tick_nohz_stop_sched_tick() into two separate routines,
one computing the time to the next timer event and the other
simply stopping the tick when the time to the next timer event
is known.

Accordingly, split tick_nohz_stop_sched_tick() into
tick_nohz_next_event() and tick_nohz_stop_tick() and use the
former in tick_nohz_get_sleep_length().  Add two new extra fields,
timer_expires and timer_expires_base, to struct tick_sched for
passing data between these two new functions and to indicate that
tick_nohz_next_event() has run and tick_nohz_stop_tick() can be
called now.  Also drop the now redundant sleep_length field from
there.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---

v3 -> v4:
 * Drop the somewhat redundant tick_may_stop flag.
 * Rename timer_expires_basemono to timer_expires_base and use it as the
   cached state validity indicator too instead of tick_may_stop.
 * Document the new fields in struct tick_sched.
 * Drop double underscores from the names of two new functions.
 * Add WARN_ON_ONCE() to tick_nohz_idle_enter/exit() to catch leaks of the
   cached state.
 * Add WARN_ON_ONCE() to tick_nohz_get_sleep_length() to catch invocations
   of it outside of idle.

---
 include/linux/tick.h     |    2 
 kernel/sched/idle.c      |   11 ++-
 kernel/time/tick-sched.c |  156 +++++++++++++++++++++++++++++++----------------
 kernel/time/tick-sched.h |    6 +
 4 files changed, 120 insertions(+), 55 deletions(-)

Index: linux-pm/kernel/time/tick-sched.h
===================================================================
--- linux-pm.orig/kernel/time/tick-sched.h
+++ linux-pm/kernel/time/tick-sched.h
@@ -38,7 +38,8 @@ enum tick_nohz_mode {
  * @idle_exittime:	Time when the idle state was left
  * @idle_sleeptime:	Sum of the time slept in idle with sched tick stopped
  * @iowait_sleeptime:	Sum of the time slept in idle with sched tick stopped, with IO outstanding
- * @sleep_length:	Duration of the current idle sleep
+ * @timer_expires:	Anticipated timer expiration time (in case sched tick is stopped)
+ * @timer_expires_base:	Base time clock monotonic for @timer_expires
  * @do_timer_lst:	CPU was the last one doing do_timer before going idle
  */
 struct tick_sched {
@@ -58,8 +59,9 @@ struct tick_sched {
 	ktime_t				idle_exittime;
 	ktime_t				idle_sleeptime;
 	ktime_t				iowait_sleeptime;
-	ktime_t				sleep_length;
 	unsigned long			last_jiffies;
+	u64				timer_expires;
+	u64				timer_expires_base;
 	u64				next_timer;
 	ktime_t				idle_expires;
 	int				do_timer_last;
Index: linux-pm/kernel/sched/idle.c
===================================================================
--- linux-pm.orig/kernel/sched/idle.c
+++ linux-pm/kernel/sched/idle.c
@@ -190,13 +190,18 @@ static void cpuidle_idle_call(void)
 	} else {
 		bool nohz = true;
 
-		tick_nohz_idle_stop_tick();
-		rcu_idle_enter();
-
 		/*
 		 * Ask the cpuidle framework to choose a convenient idle state.
 		 */
 		next_state = cpuidle_select(drv, dev, &nohz);
+
+		if (nohz)
+			tick_nohz_idle_stop_tick();
+		else
+			tick_nohz_idle_retain_tick();
+
+		rcu_idle_enter();
+
 		entered_state = call_cpuidle(drv, dev, next_state);
 		/*
 		 * Give the governor an opportunity to reflect on the outcome
Index: linux-pm/kernel/time/tick-sched.c
===================================================================
--- linux-pm.orig/kernel/time/tick-sched.c
+++ linux-pm/kernel/time/tick-sched.c
@@ -652,13 +652,10 @@ static inline bool local_timer_softirq_p
 	return local_softirq_pending() & TIMER_SOFTIRQ;
 }
 
-static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
-					 ktime_t now, int cpu)
+static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 {
-	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
 	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 {
@@ -667,6 +664,7 @@ static ktime_t tick_nohz_stop_sched_tick
 		basejiff = jiffies;
 	} while (read_seqretry(&jiffies_lock, seq));
 	ts->last_jiffies = basejiff;
+	ts->timer_expires_base = basemono;
 
 	/*
 	 * Keep the periodic tick, when RCU, architecture or irq_work
@@ -711,31 +709,24 @@ static ktime_t tick_nohz_stop_sched_tick
 		 * next period, so no point in stopping it either, bail.
 		 */
 		if (!ts->tick_stopped) {
-			tick = 0;
+			ts->timer_expires = 0;
 			goto out;
 		}
 	}
 
 	/*
-	 * If this CPU is the one which updates jiffies, then give up
-	 * the assignment and let it be taken by the CPU which runs
-	 * the tick timer next, which might be this CPU as well. If we
-	 * don't drop this here the jiffies might be stale and
-	 * 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_deferment value.
+	 * If this CPU is the one which had the do_timer() duty last, we limit
+	 * the sleep time to the timekeeping max_deferment 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) {
-		delta = KTIME_MAX;
-		ts->do_timer_last = 0;
-	} else if (!ts->do_timer_last) {
-		delta = KTIME_MAX;
+	if (cpu != tick_do_timer_cpu) {
+		if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
+			delta = KTIME_MAX;
+			ts->do_timer_last = 0;
+		} else if (!ts->do_timer_last) {
+			delta = KTIME_MAX;
+		}
 	}
 
 #ifdef CONFIG_NO_HZ_FULL
@@ -750,14 +741,40 @@ static ktime_t tick_nohz_stop_sched_tick
 	else
 		expires = KTIME_MAX;
 
-	expires = min_t(u64, expires, next_tick);
-	tick = expires;
+	ts->timer_expires = min_t(u64, expires, next_tick);
+
+out:
+	return ts->timer_expires;
+}
+
+static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
+{
+	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
+	u64 basemono = ts->timer_expires_base;
+	u64 expires = ts->timer_expires;
+	ktime_t tick = expires;
+
+	/* Make sure we won't be trying to stop it twice in a row. */
+	ts->timer_expires_base = 0;
+
+	/*
+	 * If this CPU is the one which updates jiffies, then give up
+	 * the assignment and let it be taken by the CPU which runs
+	 * the tick timer next, which might be this CPU as well. If we
+	 * don't drop this here the jiffies might be stale and
+	 * do_timer() never invoked. Keep track of the fact that it
+	 * was the one which had the do_timer() duty last.
+	 */
+	if (cpu == tick_do_timer_cpu) {
+		tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+		ts->do_timer_last = 1;
+	}
 
 	/* Skip reprogram of event if its not changed */
 	if (ts->tick_stopped && (expires == ts->next_tick)) {
 		/* Sanity check: make sure clockevent is actually programmed */
 		if (tick == KTIME_MAX || ts->next_tick == hrtimer_get_expires(&ts->sched_timer))
-			goto out;
+			return;
 
 		WARN_ON_ONCE(1);
 		printk_once("basemono: %llu ts->next_tick: %llu dev->next_event: %llu timer->active: %d timer->expires: %llu\n",
@@ -791,7 +808,7 @@ static ktime_t tick_nohz_stop_sched_tick
 	if (unlikely(expires == KTIME_MAX)) {
 		if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
 			hrtimer_cancel(&ts->sched_timer);
-		goto out;
+		return;
 	}
 
 	hrtimer_set_expires(&ts->sched_timer, tick);
@@ -800,15 +817,23 @@ static ktime_t tick_nohz_stop_sched_tick
 		hrtimer_start_expires(&ts->sched_timer, HRTIMER_MODE_ABS_PINNED);
 	else
 		tick_program_event(tick, 1);
-out:
-	/*
-	 * Update the estimated sleep length until the next timer
-	 * (not only the tick).
-	 */
-	ts->sleep_length = ktime_sub(dev->next_event, now);
-	return tick;
 }
 
+static void tick_nohz_retain_tick(struct tick_sched *ts)
+{
+	ts->timer_expires_base = 0;
+}
+
+#ifdef CONFIG_NO_HZ_FULL
+static void tick_nohz_stop_sched_tick(struct tick_sched *ts, int cpu)
+{
+	if (tick_nohz_next_event(ts, cpu))
+		tick_nohz_stop_tick(ts, cpu);
+	else
+		tick_nohz_retain_tick(ts);
+}
+#endif /* CONFIG_NO_HZ_FULL */
+
 static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
 {
 	/* Update jiffies first */
@@ -844,7 +869,7 @@ static void tick_nohz_full_update_tick(s
 		return;
 
 	if (can_stop_full_tick(cpu, ts))
-		tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
+		tick_nohz_stop_sched_tick(ts, cpu);
 	else if (ts->tick_stopped)
 		tick_nohz_restart_sched_tick(ts, ktime_get());
 #endif
@@ -870,10 +895,8 @@ static bool can_stop_idle_tick(int cpu,
 		return false;
 	}
 
-	if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) {
-		ts->sleep_length = NSEC_PER_SEC / HZ;
+	if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
 		return false;
-	}
 
 	if (need_resched())
 		return false;
@@ -913,25 +936,33 @@ static void __tick_nohz_idle_stop_tick(s
 	ktime_t expires;
 	int cpu = smp_processor_id();
 
-	if (can_stop_idle_tick(cpu, ts)) {
+	/*
+	 * If tick_nohz_get_sleep_length() ran tick_nohz_next_event(), the
+	 * tick timer expiration time is known already.
+	 */
+	if (ts->timer_expires_base)
+		expires = ts->timer_expires;
+	else if (can_stop_idle_tick(cpu, ts))
+		expires = tick_nohz_next_event(ts, cpu);
+	else
+		return;
+
+	ts->idle_calls++;
+
+	if (expires > 0LL) {
 		int was_stopped = ts->tick_stopped;
 
-		ts->idle_calls++;
+		tick_nohz_stop_tick(ts, cpu);
 
-		/*
-		 * The idle entry time should be a sufficient approximation of
-		 * the current time at this point.
-		 */
-		expires = tick_nohz_stop_sched_tick(ts, ts->idle_entrytime, cpu);
-		if (expires > 0LL) {
-			ts->idle_sleeps++;
-			ts->idle_expires = expires;
-		}
+		ts->idle_sleeps++;
+		ts->idle_expires = expires;
 
 		if (!was_stopped && ts->tick_stopped) {
 			ts->idle_jiffies = ts->last_jiffies;
 			nohz_balance_enter_idle(cpu);
 		}
+	} else {
+		tick_nohz_retain_tick(ts);
 	}
 }
 
@@ -945,6 +976,11 @@ void tick_nohz_idle_stop_tick(void)
 	__tick_nohz_idle_stop_tick(this_cpu_ptr(&tick_cpu_sched));
 }
 
+void tick_nohz_idle_retain_tick(void)
+{
+	tick_nohz_retain_tick(this_cpu_ptr(&tick_cpu_sched));
+}
+
 /**
  * tick_nohz_idle_enter - prepare for entering idle on the current CPU
  *
@@ -957,7 +993,7 @@ void tick_nohz_idle_enter(void)
 	lockdep_assert_irqs_enabled();
 	/*
 	 * Update the idle state in the scheduler domain hierarchy
-	 * when tick_nohz_stop_sched_tick() is called from the idle loop.
+	 * when tick_nohz_stop_tick() is called from the idle loop.
 	 * State will be updated to busy during the first busy tick after
 	 * exiting idle.
 	 */
@@ -966,6 +1002,9 @@ void tick_nohz_idle_enter(void)
 	local_irq_disable();
 
 	ts = this_cpu_ptr(&tick_cpu_sched);
+
+	WARN_ON_ONCE(ts->timer_expires_base);
+
 	ts->inidle = 1;
 	tick_nohz_start_idle(ts);
 
@@ -991,15 +1030,31 @@ void tick_nohz_irq_exit(void)
 }
 
 /**
- * tick_nohz_get_sleep_length - return the length of the current sleep
+ * tick_nohz_get_sleep_length - return the expected length of the current sleep
  *
  * Called from power state control code with interrupts disabled
  */
 ktime_t tick_nohz_get_sleep_length(void)
 {
+	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
+	int cpu = smp_processor_id();
+	/*
+	 * The idle entry time is expected to be a sufficient approximation of
+	 * the current time at this point.
+	 */
+	ktime_t now = ts->idle_entrytime;
+
+	WARN_ON_ONCE(!ts->inidle);
+
+	if (can_stop_idle_tick(cpu, ts)) {
+		ktime_t next_event = tick_nohz_next_event(ts, cpu);
+
+		if (next_event)
+			return ktime_sub(next_event, now);
+	}
 
-	return ts->sleep_length;
+	return ktime_sub(dev->next_event, now);
 }
 
 /**
@@ -1063,6 +1118,7 @@ void tick_nohz_idle_exit(void)
 	local_irq_disable();
 
 	WARN_ON_ONCE(!ts->inidle);
+	WARN_ON_ONCE(ts->timer_expires_base);
 
 	ts->inidle = 0;
 
Index: linux-pm/include/linux/tick.h
===================================================================
--- linux-pm.orig/include/linux/tick.h
+++ linux-pm/include/linux/tick.h
@@ -115,6 +115,7 @@ enum tick_dep_bits {
 extern bool tick_nohz_enabled;
 extern int tick_nohz_tick_stopped(void);
 extern void tick_nohz_idle_stop_tick(void);
+extern void tick_nohz_idle_retain_tick(void);
 extern void tick_nohz_idle_enter(void);
 extern void tick_nohz_idle_exit(void);
 extern void tick_nohz_irq_exit(void);
@@ -127,6 +128,7 @@ extern u64 get_cpu_iowait_time_us(int cp
 #define tick_nohz_enabled (0)
 static inline int tick_nohz_tick_stopped(void) { return 0; }
 static inline void tick_nohz_idle_stop_tick(void) { }
+static inline void tick_nohz_idle_retain_tick(void) { }
 static inline void tick_nohz_idle_enter(void) { }
 static inline void tick_nohz_idle_exit(void) { }