Message-Id: <20220622225102.2112026-3-joel@joelfernandes.org>
Date:   Wed, 22 Jun 2022 22:50:55 +0000
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, rushikesh.s.kadam@...el.com,
        urezki@...il.com, neeraj.iitr10@...il.com, frederic@...nel.org,
        paulmck@...nel.org, rostedt@...dmis.org, vineeth@...byteword.org,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>
Subject: [PATCH v2 1/8] rcu: Introduce call_rcu_lazy() API implementation

Implement timer-based RCU lazy callback batching. The batch is flushed
whenever a certain amount of time has passed, or the batch on a
particular CPU grows too big. Also memory pressure will flush it in a
future patch.

To handle several corner cases automagically (such as rcu_barrier() and
hotplug), we re-use bypass lists to handle lazy CBs. The bypass list
length has the lazy CB length included in it. A separate lazy CB length
counter is also introduced to keep track of the number of lazy CBs.

Suggested-by: Paul McKenney <paulmck@...nel.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 include/linux/rcu_segcblist.h |   1 +
 include/linux/rcupdate.h      |   6 ++
 kernel/rcu/Kconfig            |   8 +++
 kernel/rcu/rcu_segcblist.c    |  19 ++++++
 kernel/rcu/rcu_segcblist.h    |  14 ++++
 kernel/rcu/tree.c             |  24 +++++--
 kernel/rcu/tree.h             |  10 +--
 kernel/rcu/tree_nocb.h        | 125 +++++++++++++++++++++++++---------
 8 files changed, 164 insertions(+), 43 deletions(-)

diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
index 659d13a7ddaa..9a992707917b 100644
--- a/include/linux/rcu_segcblist.h
+++ b/include/linux/rcu_segcblist.h
@@ -22,6 +22,7 @@ struct rcu_cblist {
 	struct rcu_head *head;
 	struct rcu_head **tail;
 	long len;
+	long lazy_len;
 };
 
 #define RCU_CBLIST_INITIALIZER(n) { .head = NULL, .tail = &n.head }
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 1a32036c918c..9191a3d88087 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -82,6 +82,12 @@ static inline int rcu_preempt_depth(void)
 
 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 
+#ifdef CONFIG_RCU_LAZY
+void call_rcu_lazy(struct rcu_head *head, rcu_callback_t func);
+#else
+#define call_rcu_lazy(head, func) call_rcu(head, func)
+#endif
+
 /* Internal to kernel */
 void rcu_init(void);
 extern int rcu_scheduler_active;
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 27aab870ae4c..0bffa992fdc4 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -293,4 +293,12 @@ config TASKS_TRACE_RCU_READ_MB
 	  Say N here if you hate read-side memory barriers.
 	  Take the default if you are unsure.
 
+config RCU_LAZY
+	bool "RCU callback lazy invocation functionality"
+	depends on RCU_NOCB_CPU
+	default n
+	help
+	  To save power, batch RCU callbacks and flush after delay, memory
+          pressure or callback list growing too big.
+
 endmenu # "RCU Subsystem"
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index c54ea2b6a36b..627a3218a372 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -20,6 +20,7 @@ void rcu_cblist_init(struct rcu_cblist *rclp)
 	rclp->head = NULL;
 	rclp->tail = &rclp->head;
 	rclp->len = 0;
+	rclp->lazy_len = 0;
 }
 
 /*
@@ -32,6 +33,15 @@ void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp)
 	WRITE_ONCE(rclp->len, rclp->len + 1);
 }
 
+/*
+ * Enqueue an rcu_head structure onto the specified callback list.
+ */
+void rcu_cblist_enqueue_lazy(struct rcu_cblist *rclp, struct rcu_head *rhp)
+{
+	rcu_cblist_enqueue(rclp, rhp);
+	WRITE_ONCE(rclp->lazy_len, rclp->lazy_len + 1);
+}
+
 /*
  * Flush the second rcu_cblist structure onto the first one, obliterating
  * any contents of the first.  If rhp is non-NULL, enqueue it as the sole
@@ -60,6 +70,15 @@ void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
 	}
 }
 
+void rcu_cblist_flush_enqueue_lazy(struct rcu_cblist *drclp,
+			      struct rcu_cblist *srclp,
+			      struct rcu_head *rhp)
+{
+	rcu_cblist_flush_enqueue(drclp, srclp, rhp);
+	if (rhp)
+		WRITE_ONCE(srclp->lazy_len, 1);
+}
+
 /*
  * Dequeue the oldest rcu_head structure from the specified callback
  * list.
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 431cee212467..c3d7de65b689 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -15,14 +15,28 @@ static inline long rcu_cblist_n_cbs(struct rcu_cblist *rclp)
 	return READ_ONCE(rclp->len);
 }
 
+/* Return number of callbacks in the specified callback list. */
+static inline long rcu_cblist_n_lazy_cbs(struct rcu_cblist *rclp)
+{
+#ifdef CONFIG_RCU_LAZY
+	return READ_ONCE(rclp->lazy_len);
+#else
+	return 0;
+#endif
+}
+
 /* Return number of callbacks in segmented callback list by summing seglen. */
 long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp);
 
 void rcu_cblist_init(struct rcu_cblist *rclp);
 void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp);
+void rcu_cblist_enqueue_lazy(struct rcu_cblist *rclp, struct rcu_head *rhp);
 void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
 			      struct rcu_cblist *srclp,
 			      struct rcu_head *rhp);
+void rcu_cblist_flush_enqueue_lazy(struct rcu_cblist *drclp,
+			      struct rcu_cblist *srclp,
+			      struct rcu_head *rhp);
 struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp);
 
 /*
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c25ba442044a..d2e3d6e176d2 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3098,7 +3098,8 @@ static void check_cb_ovld(struct rcu_data *rdp)
  * Implementation of these memory-ordering guarantees is described here:
  * Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst.
  */
-void call_rcu(struct rcu_head *head, rcu_callback_t func)
+static void
+__call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy)
 {
 	static atomic_t doublefrees;
 	unsigned long flags;
@@ -3139,7 +3140,7 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
 	}
 
 	check_cb_ovld(rdp);
-	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
+	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy))
 		return; // Enqueued onto ->nocb_bypass, so just leave.
 	// If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
 	rcu_segcblist_enqueue(&rdp->cblist, head);
@@ -3161,8 +3162,21 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
 		local_irq_restore(flags);
 	}
 }
-EXPORT_SYMBOL_GPL(call_rcu);
 
+#ifdef CONFIG_RCU_LAZY
+void call_rcu_lazy(struct rcu_head *head, rcu_callback_t func)
+{
+	return __call_rcu_common(head, func, true);
+}
+EXPORT_SYMBOL_GPL(call_rcu_lazy);
+#endif
+
+void call_rcu(struct rcu_head *head, rcu_callback_t func)
+{
+	return __call_rcu_common(head, func, false);
+
+}
+EXPORT_SYMBOL_GPL(call_rcu);
 
 /* Maximum number of jiffies to wait before draining a batch. */
 #define KFREE_DRAIN_JIFFIES (HZ / 50)
@@ -4056,7 +4070,7 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
 	rdp->barrier_head.func = rcu_barrier_callback;
 	debug_rcu_head_queue(&rdp->barrier_head);
 	rcu_nocb_lock(rdp);
-	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
+	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
 	if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
 		atomic_inc(&rcu_state.barrier_cpu_count);
 	} else {
@@ -4476,7 +4490,7 @@ void rcutree_migrate_callbacks(int cpu)
 	my_rdp = this_cpu_ptr(&rcu_data);
 	my_rnp = my_rdp->mynode;
 	rcu_nocb_lock(my_rdp); /* irqs already disabled. */
-	WARN_ON_ONCE(!rcu_nocb_flush_bypass(my_rdp, NULL, jiffies));
+	WARN_ON_ONCE(!rcu_nocb_flush_bypass(my_rdp, NULL, jiffies, false));
 	raw_spin_lock_rcu_node(my_rnp); /* irqs already disabled. */
 	/* Leverage recent GPs and set GP for new callbacks. */
 	needwake = rcu_advance_cbs(my_rnp, rdp) ||
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 2ccf5845957d..fec4fad6654b 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -267,8 +267,9 @@ struct rcu_data {
 /* Values for nocb_defer_wakeup field in struct rcu_data. */
 #define RCU_NOCB_WAKE_NOT	0
 #define RCU_NOCB_WAKE_BYPASS	1
-#define RCU_NOCB_WAKE		2
-#define RCU_NOCB_WAKE_FORCE	3
+#define RCU_NOCB_WAKE_LAZY	2
+#define RCU_NOCB_WAKE		3
+#define RCU_NOCB_WAKE_FORCE	4
 
 #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500))
 					/* For jiffies_till_first_fqs and */
@@ -436,9 +437,10 @@ static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp);
 static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq);
 static void rcu_init_one_nocb(struct rcu_node *rnp);
 static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
-				  unsigned long j);
+				  unsigned long j, bool lazy);
 static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
-				bool *was_alldone, unsigned long flags);
+				bool *was_alldone, unsigned long flags,
+				bool lazy);
 static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
 				 unsigned long flags);
 static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level);
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index e369efe94fda..b9244f22e102 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -256,6 +256,8 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
 	return __wake_nocb_gp(rdp_gp, rdp, force, flags);
 }
 
+#define LAZY_FLUSH_JIFFIES (10 * HZ)
+
 /*
  * Arrange to wake the GP kthread for this NOCB group at some future
  * time when it is safe to do so.
@@ -272,7 +274,10 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
 	 * Bypass wakeup overrides previous deferments. In case
 	 * of callback storm, no need to wake up too early.
 	 */
-	if (waketype == RCU_NOCB_WAKE_BYPASS) {
+	if (waketype == RCU_NOCB_WAKE_LAZY) {
+		mod_timer(&rdp_gp->nocb_timer, jiffies + LAZY_FLUSH_JIFFIES);
+		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
+	} else if (waketype == RCU_NOCB_WAKE_BYPASS) {
 		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
 		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
 	} else {
@@ -296,7 +301,7 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
  * Note that this function always returns true if rhp is NULL.
  */
 static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
-				     unsigned long j)
+				     unsigned long j, bool lazy)
 {
 	struct rcu_cblist rcl;
 
@@ -310,7 +315,13 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 	/* Note: ->cblist.len already accounts for ->nocb_bypass contents. */
 	if (rhp)
 		rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
-	rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
+
+	trace_printk("call_rcu_lazy callbacks = %ld\n", READ_ONCE(rdp->nocb_bypass.lazy_len));
+	/* The lazy CBs are being flushed, but a new one might be enqueued. */
+	if (lazy)
+		rcu_cblist_flush_enqueue_lazy(&rcl, &rdp->nocb_bypass, rhp);
+	else
+		rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
 	rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
 	WRITE_ONCE(rdp->nocb_bypass_first, j);
 	rcu_nocb_bypass_unlock(rdp);
@@ -326,13 +337,13 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
  * Note that this function always returns true if rhp is NULL.
  */
 static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
-				  unsigned long j)
+				  unsigned long j, bool lazy)
 {
 	if (!rcu_rdp_is_offloaded(rdp))
 		return true;
 	rcu_lockdep_assert_cblist_protected(rdp);
 	rcu_nocb_bypass_lock(rdp);
-	return rcu_nocb_do_flush_bypass(rdp, rhp, j);
+	return rcu_nocb_do_flush_bypass(rdp, rhp, j, lazy);
 }
 
 /*
@@ -345,7 +356,7 @@ static void rcu_nocb_try_flush_bypass(struct rcu_data *rdp, unsigned long j)
 	if (!rcu_rdp_is_offloaded(rdp) ||
 	    !rcu_nocb_bypass_trylock(rdp))
 		return;
-	WARN_ON_ONCE(!rcu_nocb_do_flush_bypass(rdp, NULL, j));
+	WARN_ON_ONCE(!rcu_nocb_do_flush_bypass(rdp, NULL, j, false));
 }
 
 /*
@@ -367,12 +378,14 @@ static void rcu_nocb_try_flush_bypass(struct rcu_data *rdp, unsigned long j)
  * there is only one CPU in operation.
  */
 static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
-				bool *was_alldone, unsigned long flags)
+				bool *was_alldone, unsigned long flags,
+				bool lazy)
 {
 	unsigned long c;
 	unsigned long cur_gp_seq;
 	unsigned long j = jiffies;
 	long ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
+	long n_lazy_cbs = rcu_cblist_n_lazy_cbs(&rdp->nocb_bypass);
 
 	lockdep_assert_irqs_disabled();
 
@@ -414,30 +427,37 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 	}
 	WRITE_ONCE(rdp->nocb_nobypass_count, c);
 
-	// If there hasn't yet been all that many ->cblist enqueues
-	// this jiffy, tell the caller to enqueue onto ->cblist.  But flush
-	// ->nocb_bypass first.
-	if (rdp->nocb_nobypass_count < nocb_nobypass_lim_per_jiffy) {
+	// If caller passed a non-lazy CB and there hasn't yet been all that
+	// many ->cblist enqueues this jiffy, tell the caller to enqueue it
+	// onto ->cblist.  But flush ->nocb_bypass first. Also do so, if total
+	// number of CBs (lazy + non-lazy) grows too much.
+	//
+	// Note that if the bypass list has lazy CBs, and the main list is
+	// empty, and rhp happens to be non-lazy, then we end up flushing all
+	// the lazy CBs to the main list as well. That's the right thing to do,
+	// since we are kick-starting RCU GP processing anyway for the non-lazy
+	// one, we can just reuse that GP for the already queued-up lazy ones.
+	if ((rdp->nocb_nobypass_count < nocb_nobypass_lim_per_jiffy && !lazy) ||
+	    (lazy && n_lazy_cbs >= qhimark)) {
 		rcu_nocb_lock(rdp);
 		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
 		if (*was_alldone)
 			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
-					    TPS("FirstQ"));
-		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j));
+					    lazy ? TPS("FirstLazyQ") : TPS("FirstQ"));
+		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j, false));
 		WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
 		return false; // Caller must enqueue the callback.
 	}
 
 	// If ->nocb_bypass has been used too long or is too full,
 	// flush ->nocb_bypass to ->cblist.
-	if ((ncbs && j != READ_ONCE(rdp->nocb_bypass_first)) ||
-	    ncbs >= qhimark) {
+	if ((ncbs && j != READ_ONCE(rdp->nocb_bypass_first)) || ncbs >= qhimark) {
 		rcu_nocb_lock(rdp);
-		if (!rcu_nocb_flush_bypass(rdp, rhp, j)) {
+		if (!rcu_nocb_flush_bypass(rdp, rhp, j, true)) {
 			*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
 			if (*was_alldone)
 				trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
-						    TPS("FirstQ"));
+						    lazy ? TPS("FirstLazyQ") : TPS("FirstQ"));
 			WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
 			return false; // Caller must enqueue the callback.
 		}
@@ -455,12 +475,20 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
 	rcu_nocb_wait_contended(rdp);
 	rcu_nocb_bypass_lock(rdp);
 	ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
+	n_lazy_cbs = rcu_cblist_n_lazy_cbs(&rdp->nocb_bypass);
 	rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
-	rcu_cblist_enqueue(&rdp->nocb_bypass, rhp);
+	if (lazy)
+		rcu_cblist_enqueue_lazy(&rdp->nocb_bypass, rhp);
+	else
+		rcu_cblist_enqueue(&rdp->nocb_bypass, rhp);
 	if (!ncbs) {
 		WRITE_ONCE(rdp->nocb_bypass_first, j);
-		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstBQ"));
+		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
+				    lazy ? TPS("FirstLazyBQ") : TPS("FirstBQ"));
+	} else if (!n_lazy_cbs && lazy) {
+		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstLazyBQ"));
 	}
+
 	rcu_nocb_bypass_unlock(rdp);
 	smp_mb(); /* Order enqueue before wake. */
 	if (ncbs) {
@@ -493,7 +521,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 {
 	unsigned long cur_gp_seq;
 	unsigned long j;
-	long len;
+	long len, lazy_len, bypass_len;
 	struct task_struct *t;
 
 	// If we are being polled or there is no kthread, just leave.
@@ -506,9 +534,16 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 	}
 	// Need to actually to a wakeup.
 	len = rcu_segcblist_n_cbs(&rdp->cblist);
+	bypass_len = rcu_cblist_n_cbs(&rdp->nocb_bypass);
+	lazy_len = rcu_cblist_n_lazy_cbs(&rdp->nocb_bypass);
 	if (was_alldone) {
 		rdp->qlen_last_fqs_check = len;
-		if (!irqs_disabled_flags(flags)) {
+		// Only lazy CBs in bypass list
+		if (lazy_len && bypass_len == lazy_len) {
+			rcu_nocb_unlock_irqrestore(rdp, flags);
+			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY,
+					   TPS("WakeLazy"));
+		} else if (!irqs_disabled_flags(flags)) {
 			/* ... if queue was empty ... */
 			rcu_nocb_unlock_irqrestore(rdp, flags);
 			wake_nocb_gp(rdp, false);
@@ -599,8 +634,8 @@ static inline bool nocb_gp_update_state_deoffloading(struct rcu_data *rdp,
  */
 static void nocb_gp_wait(struct rcu_data *my_rdp)
 {
-	bool bypass = false;
-	long bypass_ncbs;
+	bool bypass = false, lazy = false;
+	long bypass_ncbs, lazy_ncbs;
 	int __maybe_unused cpu = my_rdp->cpu;
 	unsigned long cur_gp_seq;
 	unsigned long flags;
@@ -648,12 +683,21 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 			continue;
 		}
 		bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
-		if (bypass_ncbs &&
+		lazy_ncbs = rcu_cblist_n_lazy_cbs(&rdp->nocb_bypass);
+		if (lazy_ncbs &&
+		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + LAZY_FLUSH_JIFFIES) ||
+		     bypass_ncbs > qhimark)) {
+			// Bypass full or old, so flush it.
+			(void)rcu_nocb_try_flush_bypass(rdp, j);
+			bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
+			lazy_ncbs = rcu_cblist_n_lazy_cbs(&rdp->nocb_bypass);
+		} else if (bypass_ncbs &&
 		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + 1) ||
 		     bypass_ncbs > 2 * qhimark)) {
 			// Bypass full or old, so flush it.
 			(void)rcu_nocb_try_flush_bypass(rdp, j);
 			bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
+			lazy_ncbs = rcu_cblist_n_lazy_cbs(&rdp->nocb_bypass);
 		} else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
 			rcu_nocb_unlock_irqrestore(rdp, flags);
 			if (needwake_state)
@@ -662,8 +706,11 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 		}
 		if (bypass_ncbs) {
 			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
-					    TPS("Bypass"));
-			bypass = true;
+				    bypass_ncbs == lazy_ncbs ? TPS("Lazy") : TPS("Bypass"));
+			if (bypass_ncbs == lazy_ncbs)
+				lazy = true;
+			else
+				bypass = true;
 		}
 		rnp = rdp->mynode;
 
@@ -713,12 +760,21 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 	my_rdp->nocb_gp_gp = needwait_gp;
 	my_rdp->nocb_gp_seq = needwait_gp ? wait_gp_seq : 0;
 
-	if (bypass && !rcu_nocb_poll) {
-		// At least one child with non-empty ->nocb_bypass, so set
-		// timer in order to avoid stranding its callbacks.
-		wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_BYPASS,
-				   TPS("WakeBypassIsDeferred"));
+	// At least one child with non-empty ->nocb_bypass, so set
+	// timer in order to avoid stranding its callbacks.
+	if (!rcu_nocb_poll) {
+		// If bypass list only has lazy CBs. Add a deferred
+		// lazy wake up.
+		if (lazy && !bypass) {
+			wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_LAZY,
+					TPS("WakeLazyIsDeferred"));
+		// Otherwise add a deferred bypass wake up.
+		} else if (bypass) {
+			wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_BYPASS,
+					TPS("WakeBypassIsDeferred"));
+		}
 	}
+
 	if (rcu_nocb_poll) {
 		/* Polling, so trace if first poll in the series. */
 		if (gotcbs)
@@ -999,7 +1055,7 @@ static long rcu_nocb_rdp_deoffload(void *arg)
 	 * return false, which means that future calls to rcu_nocb_try_bypass()
 	 * will refuse to put anything into the bypass.
 	 */
-	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
+	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
 	/*
 	 * Start with invoking rcu_core() early. This way if the current thread
 	 * happens to preempt an ongoing call to rcu_core() in the middle,
@@ -1500,13 +1556,14 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
 }
 
 static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
-				  unsigned long j)
+				  unsigned long j, bool lazy)
 {
 	return true;
 }
 
 static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
-				bool *was_alldone, unsigned long flags)
+				bool *was_alldone, unsigned long flags,
+				bool lazy)
 {
 	return false;
 }
-- 
2.37.0.rc0.104.g0611611a94-goog

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