timers: Spare timer softirq until next expiry

Now that the core timer infrastructure doesn't depend anymore on
periodic base->clk increments, even when the CPU is not in NO_HZ mode,
timer softirqs can be skipped until there are timers to expire.

Some spurious softirqs can still remain since base->next_expiry doesn't
keep track of canceled timers but this still reduces the number of softirqs
significantly: ~15 times less for HZ=1000 and ~5 times less for HZ=100.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200717140551.29076-11-frederic@kernel.org

kernel/time/timer.c

@@ -1458,10 +1458,10 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
 	}
 }
 
-static int __collect_expired_timers(struct timer_base *base,
-				    struct hlist_head *heads)
+static int collect_expired_timers(struct timer_base *base,
+				  struct hlist_head *heads)
 {
-	unsigned long clk = base->clk;
+	unsigned long clk = base->clk = base->next_expiry;
 	struct hlist_head *vec;
 	int i, levels = 0;
 	unsigned int idx;
@@ -1684,40 +1684,6 @@ void timer_clear_idle(void)
 	 */
 	base->is_idle = false;
 }
-
-static int collect_expired_timers(struct timer_base *base,
-				  struct hlist_head *heads)
-{
-	unsigned long now = READ_ONCE(jiffies);
-
-	/*
-	 * NOHZ optimization. After a long idle sleep we need to forward the
-	 * base to current jiffies. Avoid a loop by searching the bitfield for
-	 * the next expiring timer.
-	 */
-	if ((long)(now - base->clk) > 2) {
-		/*
-		 * If the next timer is ahead of time forward to current
-		 * jiffies, otherwise forward to the next expiry time:
-		 */
-		if (time_after(base->next_expiry, now)) {
-			/*
-			 * The call site will increment base->clk and then
-			 * terminate the expiry loop immediately.
-			 */
-			base->clk = now;
-			return 0;
-		}
-		base->clk = base->next_expiry;
-	}
-	return __collect_expired_timers(base, heads);
-}
-#else
-static inline int collect_expired_timers(struct timer_base *base,
-					 struct hlist_head *heads)
-{
-	return __collect_expired_timers(base, heads);
-}
 #endif
 
 /*
@@ -1750,7 +1716,7 @@ static inline void __run_timers(struct timer_base *base)
 	struct hlist_head heads[LVL_DEPTH];
 	int levels;
 
-	if (!time_after_eq(jiffies, base->clk))
+	if (time_before(jiffies, base->next_expiry))
 		return;
 
 	timer_base_lock_expiry(base);
@@ -1763,7 +1729,8 @@ static inline void __run_timers(struct timer_base *base)
 	 */
 	base->must_forward_clk = false;
 
-	while (time_after_eq(jiffies, base->clk)) {
+	while (time_after_eq(jiffies, base->clk) &&
+	       time_after_eq(jiffies, base->next_expiry)) {
 
 		levels = collect_expired_timers(base, heads);
 		base->clk++;
@@ -1798,12 +1765,12 @@ void run_local_timers(void)
 
 	hrtimer_run_queues();
 	/* Raise the softirq only if required. */
-	if (time_before(jiffies, base->clk)) {
+	if (time_before(jiffies, base->next_expiry)) {
 		if (!IS_ENABLED(CONFIG_NO_HZ_COMMON))
 			return;
 		/* CPU is awake, so check the deferrable base. */
 		base++;
-		if (time_before(jiffies, base->clk))
+		if (time_before(jiffies, base->next_expiry))
 			return;
 	}
 	raise_softirq(TIMER_SOFTIRQ);