tick/sched: Split out jiffies update helper function

The logic to get the time of the last jiffies update will be needed by
the timer pull model as well.

Move the code into a global function in anticipation of the new caller.

No functional change.

Signed-off-by: Richard Cochran (linutronix GmbH) <richardcochran@gmail.com>
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/20240221090548.36600-17-anna-maria@linutronix.de

kernel/time/tick-internal.h

@@ -157,6 +157,7 @@ static inline void tick_nohz_init(void) { }
 #ifdef CONFIG_NO_HZ_COMMON
 extern unsigned long tick_nohz_active;
 extern void timers_update_nohz(void);
+extern u64 get_jiffies_update(unsigned long *basej);
 # ifdef CONFIG_SMP
 extern struct static_key_false timers_migration_enabled;
 extern void fetch_next_timer_interrupt_remote(unsigned long basej, u64 basem,

kernel/time/tick-sched.c

@@ -799,6 +799,24 @@ static inline bool local_timer_softirq_pending(void)
 	return local_softirq_pending() & BIT(TIMER_SOFTIRQ);
 }
 
+/*
+ * Read jiffies and the time when jiffies were updated last
+ */
+u64 get_jiffies_update(unsigned long *basej)
+{
+	unsigned long basejiff;
+	unsigned int seq;
+	u64 basemono;
+
+	do {
+		seq = read_seqcount_begin(&jiffies_seq);
+		basemono = last_jiffies_update;
+		basejiff = jiffies;
+	} while (read_seqcount_retry(&jiffies_seq, seq));
+	*basej = basejiff;
+	return basemono;
+}
+
 /**
  * tick_nohz_next_event() - return the clock monotonic based next event
  * @ts:		pointer to tick_sched struct
@@ -813,14 +831,8 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 {
 	u64 basemono, next_tick, delta, expires;
 	unsigned long basejiff;
-	unsigned int seq;
 
-	/* Read jiffies and the time when jiffies were updated last */
-	do {
-		seq = read_seqcount_begin(&jiffies_seq);
-		basemono = last_jiffies_update;
-		basejiff = jiffies;
-	} while (read_seqcount_retry(&jiffies_seq, seq));
+	basemono = get_jiffies_update(&basejiff);
 	ts->last_jiffies = basejiff;
 	ts->timer_expires_base = basemono;