sched: Optimize hrtimer handling

schedule() provides several mechanisms to update the hrtick timer:

  1) When the next task is picked

  2) When the balance callbacks are invoked before rq::lock is released

Each of them can result in a first expiring timer and cause a reprogram of
the clock event device.

Solve this by deferring the rearm to the end of schedule() right before
releasing rq::lock by setting a flag on entry which tells hrtick_start() to
cache the runtime constraint in rq::hrtick_delay without touching the timer
itself.

Right before releasing rq::lock evaluate the flags and either rearm or
cancel the hrtick timer.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260224163429.273068659@kernel.org

kernel/sched/core.c

@@ -872,6 +872,12 @@ void update_rq_clock(struct rq *rq)
  * Use HR-timers to deliver accurate preemption points.
  */
 
+enum {
+	HRTICK_SCHED_NONE		= 0,
+	HRTICK_SCHED_DEFER		= BIT(1),
+	HRTICK_SCHED_START		= BIT(2),
+};
+
 static void hrtick_clear(struct rq *rq)
 {
 	if (hrtimer_active(&rq->hrtick_timer))
@@ -932,6 +938,17 @@ void hrtick_start(struct rq *rq, u64 delay)
 	 * doesn't make sense and can cause timer DoS.
 	 */
 	delta = max_t(s64, delay, 10000LL);
+
+	/*
+	 * If this is in the middle of schedule() only note the delay
+	 * and let hrtick_schedule_exit() deal with it.
+	 */
+	if (rq->hrtick_sched) {
+		rq->hrtick_sched |= HRTICK_SCHED_START;
+		rq->hrtick_delay = delta;
+		return;
+	}
+
 	rq->hrtick_time = ktime_add_ns(ktime_get(), delta);
 
 	if (rq == this_rq())
@@ -940,19 +957,40 @@ void hrtick_start(struct rq *rq, u64 delay)
 		smp_call_function_single_async(cpu_of(rq), &rq->hrtick_csd);
 }
 
+static inline void hrtick_schedule_enter(struct rq *rq)
+{
+	rq->hrtick_sched = HRTICK_SCHED_DEFER;
+}
+
+static inline void hrtick_schedule_exit(struct rq *rq)
+{
+	if (rq->hrtick_sched & HRTICK_SCHED_START) {
+		rq->hrtick_time = ktime_add_ns(ktime_get(), rq->hrtick_delay);
+		__hrtick_restart(rq);
+	} else if (idle_rq(rq)) {
+		/*
+		 * No need for using hrtimer_is_active(). The timer is CPU local
+		 * and interrupts are disabled, so the callback cannot be
+		 * running and the queued state is valid.
+		 */
+		if (hrtimer_is_queued(&rq->hrtick_timer))
+			hrtimer_cancel(&rq->hrtick_timer);
+	}
+
+	rq->hrtick_sched = HRTICK_SCHED_NONE;
+}
+
 static void hrtick_rq_init(struct rq *rq)
 {
 	INIT_CSD(&rq->hrtick_csd, __hrtick_start, rq);
+	rq->hrtick_sched = HRTICK_SCHED_NONE;
 	hrtimer_setup(&rq->hrtick_timer, hrtick, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 }
 #else /* !CONFIG_SCHED_HRTICK: */
-static inline void hrtick_clear(struct rq *rq)
-{
-}
-
-static inline void hrtick_rq_init(struct rq *rq)
-{
-}
+static inline void hrtick_clear(struct rq *rq) { }
+static inline void hrtick_rq_init(struct rq *rq) { }
+static inline void hrtick_schedule_enter(struct rq *rq) { }
+static inline void hrtick_schedule_exit(struct rq *rq) { }
 #endif /* !CONFIG_SCHED_HRTICK */
 
 /*
@@ -5028,6 +5066,7 @@ static inline void finish_lock_switch(struct rq *rq)
 	 */
 	spin_acquire(&__rq_lockp(rq)->dep_map, 0, 0, _THIS_IP_);
 	__balance_callbacks(rq, NULL);
+	hrtick_schedule_exit(rq);
 	raw_spin_rq_unlock_irq(rq);
 }
 
@@ -6781,9 +6820,6 @@ static void __sched notrace __schedule(int sched_mode)
 
 	schedule_debug(prev, preempt);
 
-	if (sched_feat(HRTICK) || sched_feat(HRTICK_DL))
-		hrtick_clear(rq);
-
 	klp_sched_try_switch(prev);
 
 	local_irq_disable();
@@ -6810,6 +6846,8 @@ static void __sched notrace __schedule(int sched_mode)
 	rq_lock(rq, &rf);
 	smp_mb__after_spinlock();
 
+	hrtick_schedule_enter(rq);
+
 	/* Promote REQ to ACT */
 	rq->clock_update_flags <<= 1;
 	update_rq_clock(rq);
@@ -6911,6 +6949,7 @@ static void __sched notrace __schedule(int sched_mode)
 
 		rq_unpin_lock(rq, &rf);
 		__balance_callbacks(rq, NULL);
+		hrtick_schedule_exit(rq);
 		raw_spin_rq_unlock_irq(rq);
 	}
 	trace_sched_exit_tp(is_switch);

kernel/sched/sched.h

@@ -1285,6 +1285,8 @@ struct rq {
 	call_single_data_t	hrtick_csd;
 	struct hrtimer		hrtick_timer;
 	ktime_t			hrtick_time;
+	ktime_t			hrtick_delay;
+	unsigned int		hrtick_sched;
 #endif
 
 #ifdef CONFIG_SCHEDSTATS