timers: Split out "get next timer interrupt" functionality

The functionality for getting the next timer interrupt in
get_next_timer_interrupt() is split into a separate function
fetch_next_timer_interrupt() to be usable by other call sites.

This is preparatory work for the conversion of the NOHZ timer
placement to a pull at expiry time model. No functional change.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20240221090548.36600-13-anna-maria@linutronix.de

Author: anna-marialx

kernel/time/timer.c

@@ -2033,30 +2033,13 @@ static unsigned long next_timer_interrupt(struct timer_base *base,
 	return base->next_expiry;
 }
 
-static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
-					     bool *idle)
+static unsigned long fetch_next_timer_interrupt(unsigned long basej, u64 basem,
+						struct timer_base *base_local,
+						struct timer_base *base_global,
+						struct timer_events *tevt)
 {
-	struct timer_events tevt = { .local = KTIME_MAX, .global = KTIME_MAX };
 	unsigned long nextevt, nextevt_local, nextevt_global;
-	struct timer_base *base_local, *base_global;
 	bool local_first;
-	u64 expires;
-
-	/*
-	 * Pretend that there is no timer pending if the cpu is offline.
-	 * Possible pending timers will be migrated later to an active cpu.
-	 */
-	if (cpu_is_offline(smp_processor_id())) {
-		if (idle)
-			*idle = true;
-		return tevt.local;
-	}
-
-	base_local = this_cpu_ptr(&timer_bases[BASE_LOCAL]);
-	base_global = this_cpu_ptr(&timer_bases[BASE_GLOBAL]);
-
-	raw_spin_lock(&base_local->lock);
-	raw_spin_lock_nested(&base_global->lock, SINGLE_DEPTH_NESTING);
 
 	nextevt_local = next_timer_interrupt(base_local, basej);
 	nextevt_global = next_timer_interrupt(base_global, basej);
@@ -2074,8 +2057,8 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
 		/* If we missed a tick already, force 0 delta */
 		if (time_before(nextevt, basej))
 			nextevt = basej;
-		tevt.local = basem + (u64)(nextevt - basej) * TICK_NSEC;
-		goto forward;
+		tevt->local = basem + (u64)(nextevt - basej) * TICK_NSEC;
+		return nextevt;
 	}
 
 	/*
@@ -2085,12 +2068,41 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
 	 * ignored. If the global queue is empty, nothing to do either.
 	 */
 	if (!local_first && base_global->timers_pending)
-		tevt.global = basem + (u64)(nextevt_global - basej) * TICK_NSEC;
+		tevt->global = basem + (u64)(nextevt_global - basej) * TICK_NSEC;
 
 	if (base_local->timers_pending)
-		tevt.local = basem + (u64)(nextevt_local - basej) * TICK_NSEC;
+		tevt->local = basem + (u64)(nextevt_local - basej) * TICK_NSEC;
+
+	return nextevt;
+}
+
+static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
+					     bool *idle)
+{
+	struct timer_events tevt = { .local = KTIME_MAX, .global = KTIME_MAX };
+	struct timer_base *base_local, *base_global;
+	unsigned long nextevt;
+	u64 expires;
+
+	/*
+	 * Pretend that there is no timer pending if the cpu is offline.
+	 * Possible pending timers will be migrated later to an active cpu.
+	 */
+	if (cpu_is_offline(smp_processor_id())) {
+		if (idle)
+			*idle = true;
+		return tevt.local;
+	}
+
+	base_local = this_cpu_ptr(&timer_bases[BASE_LOCAL]);
+	base_global = this_cpu_ptr(&timer_bases[BASE_GLOBAL]);
+
+	raw_spin_lock(&base_local->lock);
+	raw_spin_lock_nested(&base_global->lock, SINGLE_DEPTH_NESTING);
+
+	nextevt = fetch_next_timer_interrupt(basej, basem, base_local,
+					     base_global, &tevt);
 
-forward:
 	/*
 	 * We have a fresh next event. Check whether we can forward the
 	 * base.