timers: Recalculate next timer interrupt only when necessary

The nohz tick code recalculates the timer wheel's next expiry on each idle
loop iteration.

On the other hand, the base next expiry is now always cached and updated
upon timer enqueue and execution. Only timer dequeue may leave
base->next_expiry out of date (but then its stale value won't ever go past
the actual next expiry to be recalculated).

Since recalculating the next_expiry isn't a free operation, especially when
the last wheel level is reached to find out that no timer has been enqueued
at all, reuse the next expiry cache when it is known to be reliable, which
it is most of the time.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200723151641.12236-1-frederic@kernel.org

kernel/time/timer.c

@@ -204,6 +204,7 @@ struct timer_base {
 	unsigned long		clk;
 	unsigned long		next_expiry;
 	unsigned int		cpu;
+	bool			next_expiry_recalc;
 	bool			is_idle;
 	DECLARE_BITMAP(pending_map, WHEEL_SIZE);
 	struct hlist_head	vectors[WHEEL_SIZE];
@@ -593,6 +594,7 @@ static void enqueue_timer(struct timer_base *base, struct timer_list *timer,
 		 * can reevaluate the wheel:
 		 */
 		base->next_expiry = bucket_expiry;
+		base->next_expiry_recalc = false;
 		trigger_dyntick_cpu(base, timer);
 	}
 }
@@ -836,8 +838,10 @@ static int detach_if_pending(struct timer_list *timer, struct timer_base *base,
 	if (!timer_pending(timer))
 		return 0;
 
-	if (hlist_is_singular_node(&timer->entry, base->vectors + idx))
+	if (hlist_is_singular_node(&timer->entry, base->vectors + idx)) {
 		__clear_bit(idx, base->pending_map);
+		base->next_expiry_recalc = true;
+	}
 
 	detach_timer(timer, clear_pending);
 	return 1;
@@ -1571,6 +1575,9 @@ static unsigned long __next_timer_interrupt(struct timer_base *base)
 		clk >>= LVL_CLK_SHIFT;
 		clk += adj;
 	}
+
+	base->next_expiry_recalc = false;
+
 	return next;
 }
 
@@ -1631,9 +1638,11 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 		return expires;
 
 	raw_spin_lock(&base->lock);
-	nextevt = __next_timer_interrupt(base);
+	if (base->next_expiry_recalc)
+		base->next_expiry = __next_timer_interrupt(base);
+	nextevt = base->next_expiry;
 	is_max_delta = (nextevt == base->clk + NEXT_TIMER_MAX_DELTA);
-	base->next_expiry = nextevt;
+
 	/*
 	 * We have a fresh next event. Check whether we can forward the
 	 * base. We can only do that when @basej is past base->clk
@@ -1725,6 +1734,12 @@ static inline void __run_timers(struct timer_base *base)
 	while (time_after_eq(jiffies, base->clk) &&
 	       time_after_eq(jiffies, base->next_expiry)) {
 		levels = collect_expired_timers(base, heads);
+		/*
+		 * The only possible reason for not finding any expired
+		 * timer at this clk is that all matching timers have been
+		 * dequeued.
+		 */
+		WARN_ON_ONCE(!levels && !base->next_expiry_recalc);
 		base->clk++;
 		base->next_expiry = __next_timer_interrupt(base);