timers: Introduce add_timer() variants which modify timer flags

A timer might be used as a pinned timer (using add_timer_on()) and later on
as non-pinned timer using add_timer(). When the "NOHZ timer pull at expiry
model" is in place, the TIMER_PINNED flag is required to be used whenever a
timer needs to expire on a dedicated CPU. Otherwise the flag must not be
set if expiration on a dedicated CPU is not required.

add_timer_on()'s behavior will be changed during the preparation patches
for the "NOHZ timer pull at expiry model" to unconditionally set the
TIMER_PINNED flag. To be able to clear/ set the flag when queueing a
timer, two variants of add_timer() are introduced.

This is a preparatory step and has 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/20240221090548.36600-6-anna-maria@linutronix.de

include/linux/timer.h

@@ -165,6 +165,8 @@ extern int timer_reduce(struct timer_list *timer, unsigned long expires);
 #define NEXT_TIMER_MAX_DELTA	((1UL << 30) - 1)
 
 extern void add_timer(struct timer_list *timer);
+extern void add_timer_local(struct timer_list *timer);
+extern void add_timer_global(struct timer_list *timer);
 
 extern int try_to_del_timer_sync(struct timer_list *timer);
 extern int timer_delete_sync(struct timer_list *timer);

kernel/time/timer.c

@@ -1290,6 +1290,40 @@ void add_timer(struct timer_list *timer)
 }
 EXPORT_SYMBOL(add_timer);
 
+/**
+ * add_timer_local() - Start a timer on the local CPU
+ * @timer:	The timer to be started
+ *
+ * Same as add_timer() except that the timer flag TIMER_PINNED is set.
+ *
+ * See add_timer() for further details.
+ */
+void add_timer_local(struct timer_list *timer)
+{
+	if (WARN_ON_ONCE(timer_pending(timer)))
+		return;
+	timer->flags |= TIMER_PINNED;
+	__mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING);
+}
+EXPORT_SYMBOL(add_timer_local);
+
+/**
+ * add_timer_global() - Start a timer without TIMER_PINNED flag set
+ * @timer:	The timer to be started
+ *
+ * Same as add_timer() except that the timer flag TIMER_PINNED is unset.
+ *
+ * See add_timer() for further details.
+ */
+void add_timer_global(struct timer_list *timer)
+{
+	if (WARN_ON_ONCE(timer_pending(timer)))
+		return;
+	timer->flags &= ~TIMER_PINNED;
+	__mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING);
+}
+EXPORT_SYMBOL(add_timer_global);
+
 /**
  * add_timer_on - Start a timer on a particular CPU
  * @timer:	The timer to be started