timers: Clarify check in forward_timer_base()

JIRA: https://issues.redhat.com/browse/RHEL-33787

commit 8a2c9c7
Author: Anna-Maria Behnsen <anna-maria@linutronix.de>
Date:   Fri Dec 1 10:26:30 2023 +0100

    timers: Clarify check in forward_timer_base()

    The current check whether a forward of the timer base is required can be
    simplified by using an already existing comparison function which is easier
    to read. The related comment is outdated and was not updated when the check
    changed in commit 36cd28a ("timers: Lower base clock forwarding
    threshold").

    Use time_before_eq() for the check and replace the comment by copying the
    comment from the same check inside get_next_timer_interrupt(). Move the
    precious information of the outdated comment to the proper place in
    __run_timers().

    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/20231201092654.34614-9-anna-maria@linutronix.de

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

kernel/time/timer.c

@@ -891,11 +891,10 @@ static inline void forward_timer_base(struct timer_base *base)
 	unsigned long jnow = READ_ONCE(jiffies);
 
 	/*
-	 * No need to forward if we are close enough below jiffies.
-	 * Also while executing timers, base->clk is 1 offset ahead
-	 * of jiffies to avoid endless requeuing to current jiffies.
+	 * Check whether we can forward the base. We can only do that when
+	 * @basej is past base->clk otherwise we might rewind base->clk.
 	 */
-	if ((long)(jnow - base->clk) < 1)
+	if (time_before_eq(jnow, base->clk))
 		return;
 
 	/*
@@ -1968,6 +1967,10 @@ static inline void __run_timers(struct timer_base *base)
 		 */
 		WARN_ON_ONCE(!levels && !base->next_expiry_recalc
 			     && base->timers_pending);
+		/*
+		 * While executing timers, base->clk is set 1 offset ahead of
+		 * jiffies to avoid endless requeuing to current jiffies.
+		 */
 		base->clk++;
 		next_expiry_recalc(base);