[patch 40/48] hrtimer: Keep track of first expiring timer per clock base

[Thomas Gleixner]

Evaluating the next expiry time of all clock bases is cache line expensive
as the expiry time of the first expiring timer is not cached in the base
and requires to access the timer itself, which is definitely in a different
cache line.

It's way more efficient to keep track of the expiry time on enqueue and
dequeue operations as the relevant data is already in the cache at that
point.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
---
 include/linux/hrtimer_defs.h |    2 ++
 kernel/time/hrtimer.c        |   37 ++++++++++++++++++++++++++++++++++---
 2 files changed, 36 insertions(+), 3 deletions(-)

--- a/include/linux/hrtimer_defs.h
+++ b/include/linux/hrtimer_defs.h
@@ -19,6 +19,7 @@
  *			timer to a base on another cpu.
  * @clockid:		clock id for per_cpu support
  * @seq:		seqcount around __run_hrtimer
+ * @expires_next:	Absolute time of the next event in this clock base
  * @running:		pointer to the currently running hrtimer
  * @active:		red black tree root node for the active timers
  * @offset:		offset of this clock to the monotonic base
@@ -28,6 +29,7 @@ struct hrtimer_clock_base {
 	unsigned int		index;
 	clockid_t		clockid;
 	seqcount_raw_spinlock_t	seq;
+	ktime_t			expires_next;
 	struct hrtimer		*running;
 	struct timerqueue_head	active;
 	ktime_t			offset;
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1107,7 +1107,18 @@ static bool enqueue_hrtimer(struct hrtim
 	/* Pairs with the lockless read in hrtimer_is_queued() */
 	WRITE_ONCE(timer->is_queued, HRTIMER_STATE_ENQUEUED);
 
-	return timerqueue_add(&base->active, &timer->node);
+	if (!timerqueue_add(&base->active, &timer->node))
+		return false;
+
+	base->expires_next = hrtimer_get_expires(timer);
+	return true;
+}
+
+static inline void base_update_next_timer(struct hrtimer_clock_base *base)
+{
+	struct timerqueue_node *next = timerqueue_getnext(&base->active);
+
+	base->expires_next = next ? next->expires : KTIME_MAX;
 }
 
 /*
@@ -1122,6 +1133,7 @@ static void __remove_hrtimer(struct hrti
 			     bool newstate, bool reprogram)
 {
 	struct hrtimer_cpu_base *cpu_base = base->cpu_base;
+	bool was_first;
 
 	lockdep_assert_held(&cpu_base->lock);
 
@@ -1131,9 +1143,17 @@ static void __remove_hrtimer(struct hrti
 	/* Pairs with the lockless read in hrtimer_is_queued() */
 	WRITE_ONCE(timer->is_queued, newstate);
 
+	was_first = &timer->node == timerqueue_getnext(&base->active);
+
 	if (!timerqueue_del(&base->active, &timer->node))
 		cpu_base->active_bases &= ~(1 << base->index);
 
+	/* Nothing to update if this was not the first timer in the base */
+	if (!was_first)
+		return;
+
+	base_update_next_timer(base);
+
 	/*
 	 * If reprogram is false don't update cpu_base->next_timer and do not
 	 * touch the clock event device.
@@ -1182,9 +1202,12 @@ static inline bool
 remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *base,
 			     const enum hrtimer_mode mode, ktime_t expires, u64 delta_ns)
 {
+	bool was_first = false;
+
 	/* Remove it from the timer queue if active */
 	if (timer->is_queued) {
 		debug_hrtimer_deactivate(timer);
+		was_first = &timer->node == timerqueue_getnext(&base->active);
 		timerqueue_del(&base->active, &timer->node);
 	}
 
@@ -1197,8 +1220,16 @@ remove_and_enqueue_same_base(struct hrti
 	/* Pairs with the lockless read in hrtimer_is_queued() */
 	WRITE_ONCE(timer->is_queued, HRTIMER_STATE_ENQUEUED);
 
-	/* Returns true if this is the first expiring timer */
-	return timerqueue_add(&base->active, &timer->node);
+	/* If it's the first expiring timer now or again, update base */
+	if (timerqueue_add(&base->active, &timer->node)) {
+		base->expires_next = expires;
+		return true;
+	}
+
+	if (was_first)
+		base_update_next_timer(base);
+
+	return false;
 }
 
 static inline ktime_t hrtimer_update_lowres(struct hrtimer *timer, ktime_t tim,