rcu: Let non-offloaded idle CPUs with callbacks defer tick

When a CPU goes idle, rcu_needs_cpu() is invoked to determine whether or
not RCU needs the scheduler-clock tick to keep interrupting.  Right now,
RCU keeps the tick on for a given idle CPU if there are any non-offloaded
callbacks queued on that CPU.

But if all of these callbacks are waiting for a grace period to finish,
there is no point in scheduling a tick before that grace period has any
reasonable chance of completing.  This commit therefore delays the tick
in the case where all the callbacks are waiting for a specific grace
period to elapse.  In theory, this should result in a 50-70% reduction in
RCU-induced scheduling-clock ticks on mostly-idle CPUs.  In practice, TBD.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>

Author: paulmckrcu

include/linux/rcutiny.h

@@ -133,7 +133,7 @@ static inline void rcu_softirq_qs(void)
 		rcu_tasks_qs(current, (preempt)); \
 	} while (0)
 
-static inline int rcu_needs_cpu(void)
+static inline int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 {
 	return 0;
 }

include/linux/rcutree.h

@@ -19,7 +19,7 @@
 
 void rcu_softirq_qs(void);
 void rcu_note_context_switch(bool preempt);
-int rcu_needs_cpu(void);
+int rcu_needs_cpu(u64 basemono, u64 *nextevt);
 void rcu_cpu_stall_reset(void);
 
 /*

kernel/rcu/tree.c

@@ -676,12 +676,33 @@ void __rcu_irq_enter_check_tick(void)
  * scheduler-clock interrupt.
  *
  * Just check whether or not this CPU has non-offloaded RCU callbacks
- * queued.
+ * queued that need immediate attention.
  */
-int rcu_needs_cpu(void)
+int rcu_needs_cpu(u64 basemono, u64 *nextevt)
 {
-	return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
-		!rcu_rdp_is_offloaded(this_cpu_ptr(&rcu_data));
+	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
+	struct rcu_segcblist *rsclp = &rdp->cblist;
+
+	// Disabled, empty, or offloaded means nothing to do.
+	if (!rcu_segcblist_is_enabled(rsclp) ||
+	    rcu_segcblist_empty(rsclp) || rcu_rdp_is_offloaded(rdp)) {
+		*nextevt = KTIME_MAX;
+		return 0;
+	}
+
+	// Callbacks ready to invoke or that have not already been
+	// assigned a grace period need immediate attention.
+	if (!rcu_segcblist_segempty(rsclp, RCU_DONE_TAIL) ||
+	    !rcu_segcblist_segempty(rsclp, RCU_NEXT_TAIL))
+		return 1;
+
+	// There are callbacks waiting for some later grace period.
+	// Wait for about a grace period or two for the next tick, at which
+	// point there is high probability that this CPU will need to do some
+	// work for RCU.
+	*nextevt = basemono + TICK_NSEC * (READ_ONCE(jiffies_till_first_fqs) +
+					   READ_ONCE(jiffies_till_next_fqs) + 1);
+	return 0;
 }
 
 /*

kernel/time/tick-sched.c

@@ -784,7 +784,7 @@ static inline bool local_timer_softirq_pending(void)
 
 static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 {
-	u64 basemono, next_tick, delta, expires;
+	u64 basemono, next_tick, next_tmr, next_rcu, delta, expires;
 	unsigned long basejiff;
 	unsigned int seq;
 
@@ -807,7 +807,7 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 	 * minimal delta which brings us back to this place
 	 * immediately. Lather, rinse and repeat...
 	 */
-	if (rcu_needs_cpu() || arch_needs_cpu() ||
+	if (rcu_needs_cpu(basemono, &next_rcu) || arch_needs_cpu() ||
 	    irq_work_needs_cpu() || local_timer_softirq_pending()) {
 		next_tick = basemono + TICK_NSEC;
 	} else {
@@ -818,8 +818,10 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
 		 * disabled this also looks at the next expiring
 		 * hrtimer.
 		 */
-		next_tick = get_next_timer_interrupt(basejiff, basemono);
-		ts->next_timer = next_tick;
+		next_tmr = get_next_timer_interrupt(basejiff, basemono);
+		ts->next_timer = next_tmr;
+		/* Take the next rcu event into account */
+		next_tick = next_rcu < next_tmr ? next_rcu : next_tmr;
 	}
 
 	/*