rcu/nocb: Start moving nocb code to its own plugin file

The kernel/rcu/tree_plugin.h file contains not only the plugins for
preemptible RCU, but also many other features including rcu_nocbs
callback offloading.  This offloading has become large and complex,
so it is time to put it in its own file.

This commit starts that process.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
[ paulmck: Rename to tree_nocb.h, add Frederic as author. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 1496 insertions, 0 deletions

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
new file mode 100644
index 000000000000..8fdf44f8523f
--- /dev/null
+++ b/kernel/rcu/tree_nocb.h
@@ -0,0 +1,1496 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Read-Copy Update mechanism for mutual exclusion (tree-based version)
+ * Internal non-public definitions that provide either classic
+ * or preemptible semantics.
+ *
+ * Copyright Red Hat, 2009
+ * Copyright IBM Corporation, 2009
+ * Copyright SUSE, 2021
+ *
+ * Author: Ingo Molnar <mingo@elte.hu>
+ *	   Paul E. McKenney <paulmck@linux.ibm.com>
+ *	   Frederic Weisbecker <frederic@kernel.org>
+ */
+
+#ifdef CONFIG_RCU_NOCB_CPU
+static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
+static bool __read_mostly rcu_nocb_poll;    /* Offload kthread are to poll. */
+static inline int rcu_lockdep_is_held_nocb(struct rcu_data *rdp)
+{
+	return lockdep_is_held(&rdp->nocb_lock);
+}
+
+static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
+{
+	/* Race on early boot between thread creation and assignment */
+	if (!rdp->nocb_cb_kthread || !rdp->nocb_gp_kthread)
+		return true;
+
+	if (current == rdp->nocb_cb_kthread || current == rdp->nocb_gp_kthread)
+		if (in_task())
+			return true;
+	return false;
+}
+
+/*
+ * Offload callback processing from the boot-time-specified set of CPUs
+ * specified by rcu_nocb_mask.  For the CPUs in the set, there are kthreads
+ * created that pull the callbacks from the corresponding CPU, wait for
+ * a grace period to elapse, and invoke the callbacks.  These kthreads
+ * are organized into GP kthreads, which manage incoming callbacks, wait for
+ * grace periods, and awaken CB kthreads, and the CB kthreads, which only
+ * invoke callbacks.  Each GP kthread invokes its own CBs.  The no-CBs CPUs
+ * do a wake_up() on their GP kthread when they insert a callback into any
+ * empty list, unless the rcu_nocb_poll boot parameter has been specified,
+ * in which case each kthread actively polls its CPU.  (Which isn't so great
+ * for energy efficiency, but which does reduce RCU's overhead on that CPU.)
+ *
+ * This is intended to be used in conjunction with Frederic Weisbecker's
+ * adaptive-idle work, which would seriously reduce OS jitter on CPUs
+ * running CPU-bound user-mode computations.
+ *
+ * Offloading of callbacks can also be used as an energy-efficiency
+ * measure because CPUs with no RCU callbacks queued are more aggressive
+ * about entering dyntick-idle mode.
+ */
+
+
+/*
+ * Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.
+ * If the list is invalid, a warning is emitted and all CPUs are offloaded.
+ */
+static int __init rcu_nocb_setup(char *str)
+{
+	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
+	if (cpulist_parse(str, rcu_nocb_mask)) {
+		pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
+		cpumask_setall(rcu_nocb_mask);
+	}
+	return 1;
+}
+__setup("rcu_nocbs=", rcu_nocb_setup);
+
+static int __init parse_rcu_nocb_poll(char *arg)
+{
+	rcu_nocb_poll = true;
+	return 0;
+}
+early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
+
+/*
+ * Don't bother bypassing ->cblist if the call_rcu() rate is low.
+ * After all, the main point of bypassing is to avoid lock contention
+ * on ->nocb_lock, which only can happen at high call_rcu() rates.
+ */
+static int nocb_nobypass_lim_per_jiffy = 16 * 1000 / HZ;
+module_param(nocb_nobypass_lim_per_jiffy, int, 0);
+
+/*
+ * Acquire the specified rcu_data structure's ->nocb_bypass_lock.  If the
+ * lock isn't immediately available, increment ->nocb_lock_contended to
+ * flag the contention.
+ */
+static void rcu_nocb_bypass_lock(struct rcu_data *rdp)
+	__acquires(&rdp->nocb_bypass_lock)
+{
+	lockdep_assert_irqs_disabled();
+	if (raw_spin_trylock(&rdp->nocb_bypass_lock))
+		return;
+	atomic_inc(&rdp->nocb_lock_contended);
+	WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
+	smp_mb__after_atomic(); /* atomic_inc() before lock. */
+	raw_spin_lock(&rdp->nocb_bypass_lock);
+	smp_mb__before_atomic(); /* atomic_dec() after lock. */
+	atomic_dec(&rdp->nocb_lock_contended);
+}
+
+/*
+ * Spinwait until the specified rcu_data structure's ->nocb_lock is
+ * not contended.  Please note that this is extremely special-purpose,
+ * relying on the fact that at most two kthreads and one CPU contend for
+ * this lock, and also that the two kthreads are guaranteed to have frequent
+ * grace-period-duration time intervals between successive acquisitions
+ * of the lock.  This allows us to use an extremely simple throttling
+ * mechanism, and further to apply it only to the CPU doing floods of
+ * call_rcu() invocations.  Don't try this at home!
+ */
+static void rcu_nocb_wait_contended(struct rcu_data *rdp)
+{
+	WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
+	while (WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended)))
+		cpu_relax();
+}
+
+/*
+ * Conditionally acquire the specified rcu_data structure's
+ * ->nocb_bypass_lock.
+ */
+static bool rcu_nocb_bypass_trylock(struct rcu_data *rdp)
+{
+	lockdep_assert_irqs_disabled();
+	return raw_spin_trylock(&rdp->nocb_bypass_lock);
+}
+
+/*
+ * Release the specified rcu_data structure's ->nocb_bypass_lock.
+ */
+static void rcu_nocb_bypass_unlock(struct rcu_data *rdp)
+	__releases(&rdp->nocb_bypass_lock)
+{
+	lockdep_assert_irqs_disabled();
+	raw_spin_unlock(&rdp->nocb_bypass_lock);
+}
+
+/*
+ * Acquire the specified rcu_data structure's ->nocb_lock, but only
+ * if it corresponds to a no-CBs CPU.
+ */
+static void rcu_nocb_lock(struct rcu_data *rdp)
+{
+	lockdep_assert_irqs_disabled();
+	if (!rcu_rdp_is_offloaded(rdp))
+		return;
+	raw_spin_lock(&rdp->nocb_lock);
+}
+
+/*
+ * Release the specified rcu_data structure's ->nocb_lock, but only
+ * if it corresponds to a no-CBs CPU.
+ */
+static void rcu_nocb_unlock(struct rcu_data *rdp)
+{
+	if (rcu_rdp_is_offloaded(rdp)) {
+		lockdep_assert_irqs_disabled();
+		raw_spin_unlock(&rdp->nocb_lock);
+	}
+}
+
+/*
+ * Release the specified rcu_data structure's ->nocb_lock and restore
+ * interrupts, but only if it corresponds to a no-CBs CPU.
+ */
+static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
+				       unsigned long flags)
+{
+	if (rcu_rdp_is_offloaded(rdp)) {
+		lockdep_assert_irqs_disabled();
+		raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+	} else {
+		local_irq_restore(flags);
+	}
+}
+
+/* Lockdep check that ->cblist may be safely accessed. */
+static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
+{
+	lockdep_assert_irqs_disabled();
+	if (rcu_rdp_is_offloaded(rdp))
+		lockdep_assert_held(&rdp->nocb_lock);
+}
+
+/*
+ * Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended
+ * grace period.
+ */
+static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
+{
+	swake_up_all(sq);
+}
+
+static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
+{
+	return &rnp->nocb_gp_wq[rcu_seq_ctr(rnp->gp_seq) & 0x1];
+}
+
+static void rcu_init_one_nocb(struct rcu_node *rnp)
+{
+	init_swait_queue_head(&rnp->nocb_gp_wq[0]);
+	init_swait_queue_head(&rnp->nocb_gp_wq[1]);
+}
+
+/* Is the specified CPU a no-CBs CPU? */
+bool rcu_is_nocb_cpu(int cpu)
+{
+	if (cpumask_available(rcu_nocb_mask))
+		return cpumask_test_cpu(cpu, rcu_nocb_mask);
+	return false;
+}
+
+static bool __wake_nocb_gp(struct rcu_data *rdp_gp,
+			   struct rcu_data *rdp,
+			   bool force, unsigned long flags)
+	__releases(rdp_gp->nocb_gp_lock)
+{
+	bool needwake = false;
+
+	if (!READ_ONCE(rdp_gp->nocb_gp_kthread)) {
+		raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
+		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
+				    TPS("AlreadyAwake"));
+		return false;
+	}
+
+	if (rdp_gp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
+		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
+		del_timer(&rdp_gp->nocb_timer);
+	}
+
+	if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) {
+		WRITE_ONCE(rdp_gp->nocb_gp_sleep, false);
+		needwake = true;
+	}
+	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
+	if (needwake) {
+		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DoWake"));
+		wake_up_process(rdp_gp->nocb_gp_kthread);
+	}
+
+	return needwake;
+}
+
+/*
+ * Kick the GP kthread for this NOCB group.
+ */
+static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
+{
+	unsigned long flags;
+	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
+
+	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
+	return __wake_nocb_gp(rdp_gp, rdp, force, flags);
+}
+
+/*
+ * Arrange to wake the GP kthread for this NOCB group at some future
+ * time when it is safe to do so.
+ */
+static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
+			       const char *reason)
+{
+	unsigned long flags;
+	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
+
+	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
+
+	/*
+	 * Bypass wakeup overrides previous deferments. In case
+	 * of callback storm, no need to wake up too early.
+	 */
+	if (waketype == RCU_NOCB_WAKE_BYPASS) {
+		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
+		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
+	} else {
+		if (rdp_gp->nocb_defer_wakeup < RCU_NOCB_WAKE)
+			mod_timer(&rdp_gp->nocb_timer, jiffies + 1);
+		if (rdp_gp->nocb_defer_wakeup < waketype)
+			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
+	}
+
+	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
+
+	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, reason);
+}
+
+/*
+ * Flush the ->nocb_bypass queue into ->cblist, enqueuing rhp if non-NULL.
+ * However, if there is a callback to be enqueued and if ->nocb_bypass
+ * proves to be initially empty, just return false because the no-CB GP
+ * kthread may need to be awakened in this case.
+ *
+ * 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)
+{
+	struct rcu_cblist rcl;
+
+	WARN_ON_ONCE(!rcu_rdp_is_offloaded(rdp));
+	rcu_lockdep_assert_cblist_protected(rdp);
+	lockdep_assert_held(&rdp->nocb_bypass_lock);
+	if (rhp && !rcu_cblist_n_cbs(&rdp->nocb_bypass)) {
+		raw_spin_unlock(&rdp->nocb_bypass_lock);
+		return false;
+	}
+	/* 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);
+	rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
+	WRITE_ONCE(rdp->nocb_bypass_first, j);
+	rcu_nocb_bypass_unlock(rdp);
+	return true;
+}
+
+/*
+ * Flush the ->nocb_bypass queue into ->cblist, enqueuing rhp if non-NULL.
+ * However, if there is a callback to be enqueued and if ->nocb_bypass
+ * proves to be initially empty, just return false because the no-CB GP
+ * kthread may need to be awakened in this case.
+ *
+ * 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)
+{
+	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);
+}
+
+/*
+ * If the ->nocb_bypass_lock is immediately available, flush the
+ * ->nocb_bypass queue into ->cblist.
+ */
+static void rcu_nocb_try_flush_bypass(struct rcu_data *rdp, unsigned long j)
+{
+	rcu_lockdep_assert_cblist_protected(rdp);
+	if (!rcu_rdp_is_offloaded(rdp) ||
+	    !rcu_nocb_bypass_trylock(rdp))
+		return;
+	WARN_ON_ONCE(!rcu_nocb_do_flush_bypass(rdp, NULL, j));
+}
+
+/*
+ * See whether it is appropriate to use the ->nocb_bypass list in order
+ * to control contention on ->nocb_lock.  A limited number of direct
+ * enqueues are permitted into ->cblist per jiffy.  If ->nocb_bypass
+ * is non-empty, further callbacks must be placed into ->nocb_bypass,
+ * otherwise rcu_barrier() breaks.  Use rcu_nocb_flush_bypass() to switch
+ * back to direct use of ->cblist.  However, ->nocb_bypass should not be
+ * used if ->cblist is empty, because otherwise callbacks can be stranded
+ * on ->nocb_bypass because we cannot count on the current CPU ever again
+ * invoking call_rcu().  The general rule is that if ->nocb_bypass is
+ * non-empty, the corresponding no-CBs grace-period kthread must not be
+ * in an indefinite sleep state.
+ *
+ * Finally, it is not permitted to use the bypass during early boot,
+ * as doing so would confuse the auto-initialization code.  Besides
+ * which, there is no point in worrying about lock contention while
+ * 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)
+{
+	unsigned long c;
+	unsigned long cur_gp_seq;
+	unsigned long j = jiffies;
+	long ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
+
+	lockdep_assert_irqs_disabled();
+
+	// Pure softirq/rcuc based processing: no bypassing, no
+	// locking.
+	if (!rcu_rdp_is_offloaded(rdp)) {
+		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
+		return false;
+	}
+
+	// In the process of (de-)offloading: no bypassing, but
+	// locking.
+	if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) {
+		rcu_nocb_lock(rdp);
+		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
+		return false; /* Not offloaded, no bypassing. */
+	}
+
+	// Don't use ->nocb_bypass during early boot.
+	if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING) {
+		rcu_nocb_lock(rdp);
+		WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
+		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
+		return false;
+	}
+
+	// If we have advanced to a new jiffy, reset counts to allow
+	// moving back from ->nocb_bypass to ->cblist.
+	if (j == rdp->nocb_nobypass_last) {
+		c = rdp->nocb_nobypass_count + 1;
+	} else {
+		WRITE_ONCE(rdp->nocb_nobypass_last, j);
+		c = rdp->nocb_nobypass_count - nocb_nobypass_lim_per_jiffy;
+		if (ULONG_CMP_LT(rdp->nocb_nobypass_count,
+				 nocb_nobypass_lim_per_jiffy))
+			c = 0;
+		else if (c > nocb_nobypass_lim_per_jiffy)
+			c = nocb_nobypass_lim_per_jiffy;
+	}
+	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) {
+		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));
+		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) {
+		rcu_nocb_lock(rdp);
+		if (!rcu_nocb_flush_bypass(rdp, rhp, j)) {
+			*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_cblist_n_cbs(&rdp->nocb_bypass));
+			return false; // Caller must enqueue the callback.
+		}
+		if (j != rdp->nocb_gp_adv_time &&
+		    rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
+		    rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
+			rcu_advance_cbs_nowake(rdp->mynode, rdp);
+			rdp->nocb_gp_adv_time = j;
+		}
+		rcu_nocb_unlock_irqrestore(rdp, flags);
+		return true; // Callback already enqueued.
+	}
+
+	// We need to use the bypass.
+	rcu_nocb_wait_contended(rdp);
+	rcu_nocb_bypass_lock(rdp);
+	ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
+	rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
+	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"));
+	}
+	rcu_nocb_bypass_unlock(rdp);
+	smp_mb(); /* Order enqueue before wake. */
+	if (ncbs) {
+		local_irq_restore(flags);
+	} else {
+		// No-CBs GP kthread might be indefinitely asleep, if so, wake.
+		rcu_nocb_lock(rdp); // Rare during call_rcu() flood.
+		if (!rcu_segcblist_pend_cbs(&rdp->cblist)) {
+			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
+					    TPS("FirstBQwake"));
+			__call_rcu_nocb_wake(rdp, true, flags);
+		} else {
+			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
+					    TPS("FirstBQnoWake"));
+			rcu_nocb_unlock_irqrestore(rdp, flags);
+		}
+	}
+	return true; // Callback already enqueued.
+}
+
+/*
+ * Awaken the no-CBs grace-period kthread if needed, either due to it
+ * legitimately being asleep or due to overload conditions.
+ *
+ * If warranted, also wake up the kthread servicing this CPUs queues.
+ */
+static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
+				 unsigned long flags)
+				 __releases(rdp->nocb_lock)
+{
+	unsigned long cur_gp_seq;
+	unsigned long j;
+	long len;
+	struct task_struct *t;
+
+	// If we are being polled or there is no kthread, just leave.
+	t = READ_ONCE(rdp->nocb_gp_kthread);
+	if (rcu_nocb_poll || !t) {
+		rcu_nocb_unlock_irqrestore(rdp, flags);
+		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
+				    TPS("WakeNotPoll"));
+		return;
+	}
+	// Need to actually to a wakeup.
+	len = rcu_segcblist_n_cbs(&rdp->cblist);
+	if (was_alldone) {
+		rdp->qlen_last_fqs_check = len;
+		if (!irqs_disabled_flags(flags)) {
+			/* ... if queue was empty ... */
+			rcu_nocb_unlock_irqrestore(rdp, flags);
+			wake_nocb_gp(rdp, false);
+			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
+					    TPS("WakeEmpty"));
+		} else {
+			rcu_nocb_unlock_irqrestore(rdp, flags);
+			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE,
+					   TPS("WakeEmptyIsDeferred"));
+		}
+	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
+		/* ... or if many callbacks queued. */
+		rdp->qlen_last_fqs_check = len;
+		j = jiffies;
+		if (j != rdp->nocb_gp_adv_time &&
+		    rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
+		    rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
+			rcu_advance_cbs_nowake(rdp->mynode, rdp);
+			rdp->nocb_gp_adv_time = j;
+		}
+		smp_mb(); /* Enqueue before timer_pending(). */
+		if ((rdp->nocb_cb_sleep ||
+		     !rcu_segcblist_ready_cbs(&rdp->cblist)) &&
+		    !timer_pending(&rdp->nocb_timer)) {
+			rcu_nocb_unlock_irqrestore(rdp, flags);
+			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
+					   TPS("WakeOvfIsDeferred"));
+		} else {
+			rcu_nocb_unlock_irqrestore(rdp, flags);
+			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
+		}
+	} else {
+		rcu_nocb_unlock_irqrestore(rdp, flags);
+		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
+	}
+	return;
+}
+
+/*
+ * Check if we ignore this rdp.
+ *
+ * We check that without holding the nocb lock but
+ * we make sure not to miss a freshly offloaded rdp
+ * with the current ordering:
+ *
+ *  rdp_offload_toggle()        nocb_gp_enabled_cb()
+ * -------------------------   ----------------------------
+ *    WRITE flags                 LOCK nocb_gp_lock
+ *    LOCK nocb_gp_lock           READ/WRITE nocb_gp_sleep
+ *    READ/WRITE nocb_gp_sleep    UNLOCK nocb_gp_lock
+ *    UNLOCK nocb_gp_lock         READ flags
+ */
+static inline bool nocb_gp_enabled_cb(struct rcu_data *rdp)
+{
+	u8 flags = SEGCBLIST_OFFLOADED | SEGCBLIST_KTHREAD_GP;
+
+	return rcu_segcblist_test_flags(&rdp->cblist, flags);
+}
+
+static inline bool nocb_gp_update_state_deoffloading(struct rcu_data *rdp,
+						     bool *needwake_state)
+{
+	struct rcu_segcblist *cblist = &rdp->cblist;
+
+	if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
+		if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP)) {
+			rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_GP);
+			if (rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB))
+				*needwake_state = true;
+		}
+		return false;
+	}
+
+	/*
+	 * De-offloading. Clear our flag and notify the de-offload worker.
+	 * We will ignore this rdp until it ever gets re-offloaded.
+	 */
+	WARN_ON_ONCE(!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));
+	rcu_segcblist_clear_flags(cblist, SEGCBLIST_KTHREAD_GP);
+	if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB))
+		*needwake_state = true;
+	return true;
+}
+
+
+/*
+ * No-CBs GP kthreads come here to wait for additional callbacks to show up
+ * or for grace periods to end.
+ */
+static void nocb_gp_wait(struct rcu_data *my_rdp)
+{
+	bool bypass = false;
+	long bypass_ncbs;
+	int __maybe_unused cpu = my_rdp->cpu;
+	unsigned long cur_gp_seq;
+	unsigned long flags;
+	bool gotcbs = false;
+	unsigned long j = jiffies;
+	bool needwait_gp = false; // This prevents actual uninitialized use.
+	bool needwake;
+	bool needwake_gp;
+	struct rcu_data *rdp;
+	struct rcu_node *rnp;
+	unsigned long wait_gp_seq = 0; // Suppress "use uninitialized" warning.
+	bool wasempty = false;
+
+	/*
+	 * Each pass through the following loop checks for CBs and for the
+	 * nearest grace period (if any) to wait for next.  The CB kthreads
+	 * and the global grace-period kthread are awakened if needed.
+	 */
+	WARN_ON_ONCE(my_rdp->nocb_gp_rdp != my_rdp);
+	for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) {
+		bool needwake_state = false;
+
+		if (!nocb_gp_enabled_cb(rdp))
+			continue;
+		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));
+		rcu_nocb_lock_irqsave(rdp, flags);
+		if (nocb_gp_update_state_deoffloading(rdp, &needwake_state)) {
+			rcu_nocb_unlock_irqrestore(rdp, flags);
+			if (needwake_state)
+				swake_up_one(&rdp->nocb_state_wq);
+			continue;
+		}
+		bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
+		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);
+		} else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
+			rcu_nocb_unlock_irqrestore(rdp, flags);
+			if (needwake_state)
+				swake_up_one(&rdp->nocb_state_wq);
+			continue; /* No callbacks here, try next. */
+		}
+		if (bypass_ncbs) {
+			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
+					    TPS("Bypass"));
+			bypass = true;
+		}
+		rnp = rdp->mynode;
+
+		// Advance callbacks if helpful and low contention.
+		needwake_gp = false;
+		if (!rcu_segcblist_restempty(&rdp->cblist,
+					     RCU_NEXT_READY_TAIL) ||
+		    (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
+		     rcu_seq_done(&rnp->gp_seq, cur_gp_seq))) {
+			raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
+			needwake_gp = rcu_advance_cbs(rnp, rdp);
+			wasempty = rcu_segcblist_restempty(&rdp->cblist,
+							   RCU_NEXT_READY_TAIL);
+			raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */
+		}
+		// Need to wait on some grace period?
+		WARN_ON_ONCE(wasempty &&
+			     !rcu_segcblist_restempty(&rdp->cblist,
+						      RCU_NEXT_READY_TAIL));
+		if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq)) {
+			if (!needwait_gp ||
+			    ULONG_CMP_LT(cur_gp_seq, wait_gp_seq))
+				wait_gp_seq = cur_gp_seq;
+			needwait_gp = true;
+			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
+					    TPS("NeedWaitGP"));
+		}
+		if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
+			needwake = rdp->nocb_cb_sleep;
+			WRITE_ONCE(rdp->nocb_cb_sleep, false);
+			smp_mb(); /* CB invocation -after- GP end. */
+		} else {
+			needwake = false;
+		}
+		rcu_nocb_unlock_irqrestore(rdp, flags);
+		if (needwake) {
+			swake_up_one(&rdp->nocb_cb_wq);
+			gotcbs = true;
+		}
+		if (needwake_gp)
+			rcu_gp_kthread_wake();
+		if (needwake_state)
+			swake_up_one(&rdp->nocb_state_wq);
+	}
+
+	my_rdp->nocb_gp_bypass = bypass;
+	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"));
+	}
+	if (rcu_nocb_poll) {
+		/* Polling, so trace if first poll in the series. */
+		if (gotcbs)
+			trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
+		schedule_timeout_idle(1);
+	} else if (!needwait_gp) {
+		/* Wait for callbacks to appear. */
+		trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
+		swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq,
+				!READ_ONCE(my_rdp->nocb_gp_sleep));
+		trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep"));
+	} else {
+		rnp = my_rdp->mynode;
+		trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("StartWait"));
+		swait_event_interruptible_exclusive(
+			rnp->nocb_gp_wq[rcu_seq_ctr(wait_gp_seq) & 0x1],
+			rcu_seq_done(&rnp->gp_seq, wait_gp_seq) ||
+			!READ_ONCE(my_rdp->nocb_gp_sleep));
+		trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("EndWait"));
+	}
+	if (!rcu_nocb_poll) {
+		raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
+		if (my_rdp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
+			WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
+			del_timer(&my_rdp->nocb_timer);
+		}
+		WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
+		raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
+	}
+	my_rdp->nocb_gp_seq = -1;
+	WARN_ON(signal_pending(current));
+}
+
+/*
+ * No-CBs grace-period-wait kthread.  There is one of these per group
+ * of CPUs, but only once at least one CPU in that group has come online
+ * at least once since boot.  This kthread checks for newly posted
+ * callbacks from any of the CPUs it is responsible for, waits for a
+ * grace period, then awakens all of the rcu_nocb_cb_kthread() instances
+ * that then have callback-invocation work to do.
+ */
+static int rcu_nocb_gp_kthread(void *arg)
+{
+	struct rcu_data *rdp = arg;
+
+	for (;;) {
+		WRITE_ONCE(rdp->nocb_gp_loops, rdp->nocb_gp_loops + 1);
+		nocb_gp_wait(rdp);
+		cond_resched_tasks_rcu_qs();
+	}
+	return 0;
+}
+
+static inline bool nocb_cb_can_run(struct rcu_data *rdp)
+{
+	u8 flags = SEGCBLIST_OFFLOADED | SEGCBLIST_KTHREAD_CB;
+	return rcu_segcblist_test_flags(&rdp->cblist, flags);
+}
+
+static inline bool nocb_cb_wait_cond(struct rcu_data *rdp)
+{
+	return nocb_cb_can_run(rdp) && !READ_ONCE(rdp->nocb_cb_sleep);
+}
+
+/*
+ * Invoke any ready callbacks from the corresponding no-CBs CPU,
+ * then, if there are no more, wait for more to appear.
+ */
+static void nocb_cb_wait(struct rcu_data *rdp)
+{
+	struct rcu_segcblist *cblist = &rdp->cblist;
+	unsigned long cur_gp_seq;
+	unsigned long flags;
+	bool needwake_state = false;
+	bool needwake_gp = false;
+	bool can_sleep = true;
+	struct rcu_node *rnp = rdp->mynode;
+
+	local_irq_save(flags);
+	rcu_momentary_dyntick_idle();
+	local_irq_restore(flags);
+	/*
+	 * Disable BH to provide the expected environment.  Also, when
+	 * transitioning to/from NOCB mode, a self-requeuing callback might
+	 * be invoked from softirq.  A short grace period could cause both
+	 * instances of this callback would execute concurrently.
+	 */
+	local_bh_disable();
+	rcu_do_batch(rdp);
+	local_bh_enable();
+	lockdep_assert_irqs_enabled();
+	rcu_nocb_lock_irqsave(rdp, flags);
+	if (rcu_segcblist_nextgp(cblist, &cur_gp_seq) &&
+	    rcu_seq_done(&rnp->gp_seq, cur_gp_seq) &&
+	    raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */
+		needwake_gp = rcu_advance_cbs(rdp->mynode, rdp);
+		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
+	}
+
+	if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
+		if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB)) {
+			rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_CB);
+			if (rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP))
+				needwake_state = true;
+		}
+		if (rcu_segcblist_ready_cbs(cblist))
+			can_sleep = false;
+	} else {
+		/*
+		 * De-offloading. Clear our flag and notify the de-offload worker.
+		 * We won't touch the callbacks and keep sleeping until we ever
+		 * get re-offloaded.
+		 */
+		WARN_ON_ONCE(!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB));
+		rcu_segcblist_clear_flags(cblist, SEGCBLIST_KTHREAD_CB);
+		if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP))
+			needwake_state = true;
+	}
+
+	WRITE_ONCE(rdp->nocb_cb_sleep, can_sleep);
+
+	if (rdp->nocb_cb_sleep)
+		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));
+
+	rcu_nocb_unlock_irqrestore(rdp, flags);
+	if (needwake_gp)
+		rcu_gp_kthread_wake();
+
+	if (needwake_state)
+		swake_up_one(&rdp->nocb_state_wq);
+
+	do {
+		swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
+						    nocb_cb_wait_cond(rdp));
+
+		// VVV Ensure CB invocation follows _sleep test.
+		if (smp_load_acquire(&rdp->nocb_cb_sleep)) { // ^^^
+			WARN_ON(signal_pending(current));
+			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
+		}
+	} while (!nocb_cb_can_run(rdp));
+}
+
+/*
+ * Per-rcu_data kthread, but only for no-CBs CPUs.  Repeatedly invoke
+ * nocb_cb_wait() to do the dirty work.
+ */
+static int rcu_nocb_cb_kthread(void *arg)
+{
+	struct rcu_data *rdp = arg;
+
+	// Each pass through this loop does one callback batch, and,
+	// if there are no more ready callbacks, waits for them.
+	for (;;) {
+		nocb_cb_wait(rdp);
+		cond_resched_tasks_rcu_qs();
+	}
+	return 0;
+}
+
+/* Is a deferred wakeup of rcu_nocb_kthread() required? */
+static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level)
+{
+	return READ_ONCE(rdp->nocb_defer_wakeup) >= level;
+}
+
+/* Do a deferred wakeup of rcu_nocb_kthread(). */
+static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp_gp,
+					   struct rcu_data *rdp, int level,
+					   unsigned long flags)
+	__releases(rdp_gp->nocb_gp_lock)
+{
+	int ndw;
+	int ret;
+
+	if (!rcu_nocb_need_deferred_wakeup(rdp_gp, level)) {
+		raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
+		return false;
+	}
+
+	ndw = rdp_gp->nocb_defer_wakeup;
+	ret = __wake_nocb_gp(rdp_gp, rdp, ndw == RCU_NOCB_WAKE_FORCE, flags);
+	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DeferredWake"));
+
+	return ret;
+}
+
+/* Do a deferred wakeup of rcu_nocb_kthread() from a timer handler. */
+static void do_nocb_deferred_wakeup_timer(struct timer_list *t)
+{
+	unsigned long flags;
+	struct rcu_data *rdp = from_timer(rdp, t, nocb_timer);
+
+	WARN_ON_ONCE(rdp->nocb_gp_rdp != rdp);
+	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));
+
+	raw_spin_lock_irqsave(&rdp->nocb_gp_lock, flags);
+	smp_mb__after_spinlock(); /* Timer expire before wakeup. */
+	do_nocb_deferred_wakeup_common(rdp, rdp, RCU_NOCB_WAKE_BYPASS, flags);
+}
+
+/*
+ * Do a deferred wakeup of rcu_nocb_kthread() from fastpath.
+ * This means we do an inexact common-case check.  Note that if
+ * we miss, ->nocb_timer will eventually clean things up.
+ */
+static bool do_nocb_deferred_wakeup(struct rcu_data *rdp)
+{
+	unsigned long flags;
+	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
+
+	if (!rdp_gp || !rcu_nocb_need_deferred_wakeup(rdp_gp, RCU_NOCB_WAKE))
+		return false;
+
+	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
+	return do_nocb_deferred_wakeup_common(rdp_gp, rdp, RCU_NOCB_WAKE, flags);
+}
+
+void rcu_nocb_flush_deferred_wakeup(void)
+{
+	do_nocb_deferred_wakeup(this_cpu_ptr(&rcu_data));
+}
+EXPORT_SYMBOL_GPL(rcu_nocb_flush_deferred_wakeup);
+
+static int rdp_offload_toggle(struct rcu_data *rdp,
+			       bool offload, unsigned long flags)
+	__releases(rdp->nocb_lock)
+{
+	struct rcu_segcblist *cblist = &rdp->cblist;
+	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
+	bool wake_gp = false;
+
+	rcu_segcblist_offload(cblist, offload);
+
+	if (rdp->nocb_cb_sleep)
+		rdp->nocb_cb_sleep = false;
+	rcu_nocb_unlock_irqrestore(rdp, flags);
+
+	/*
+	 * Ignore former value of nocb_cb_sleep and force wake up as it could
+	 * have been spuriously set to false already.
+	 */
+	swake_up_one(&rdp->nocb_cb_wq);
+
+	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
+	if (rdp_gp->nocb_gp_sleep) {
+		rdp_gp->nocb_gp_sleep = false;
+		wake_gp = true;
+	}
+	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
+
+	if (wake_gp)
+		wake_up_process(rdp_gp->nocb_gp_kthread);
+
+	return 0;
+}
+
+static long rcu_nocb_rdp_deoffload(void *arg)
+{
+	struct rcu_data *rdp = arg;
+	struct rcu_segcblist *cblist = &rdp->cblist;
+	unsigned long flags;
+	int ret;
+
+	WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
+
+	pr_info("De-offloading %d\n", rdp->cpu);
+
+	rcu_nocb_lock_irqsave(rdp, flags);
+	/*
+	 * Flush once and for all now. This suffices because we are
+	 * running on the target CPU holding ->nocb_lock (thus having
+	 * interrupts disabled), and because rdp_offload_toggle()
+	 * invokes rcu_segcblist_offload(), which clears SEGCBLIST_OFFLOADED.
+	 * Thus future calls to rcu_segcblist_completely_offloaded() will
+	 * 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));
+	ret = rdp_offload_toggle(rdp, false, flags);
+	swait_event_exclusive(rdp->nocb_state_wq,
+			      !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB |
+							SEGCBLIST_KTHREAD_GP));
+	/*
+	 * Lock one last time to acquire latest callback updates from kthreads
+	 * so we can later handle callbacks locally without locking.
+	 */
+	rcu_nocb_lock_irqsave(rdp, flags);
+	/*
+	 * Theoretically we could set SEGCBLIST_SOFTIRQ_ONLY after the nocb
+	 * lock is released but how about being paranoid for once?
+	 */
+	rcu_segcblist_set_flags(cblist, SEGCBLIST_SOFTIRQ_ONLY);
+	/*
+	 * With SEGCBLIST_SOFTIRQ_ONLY, we can't use
+	 * rcu_nocb_unlock_irqrestore() anymore.
+	 */
+	raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+
+	/* Sanity check */
+	WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
+
+
+	return ret;
+}
+
+int rcu_nocb_cpu_deoffload(int cpu)
+{
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+	int ret = 0;
+
+	mutex_lock(&rcu_state.barrier_mutex);
+	cpus_read_lock();
+	if (rcu_rdp_is_offloaded(rdp)) {
+		if (cpu_online(cpu)) {
+			ret = work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
+			if (!ret)
+				cpumask_clear_cpu(cpu, rcu_nocb_mask);
+		} else {
+			pr_info("NOCB: Can't CB-deoffload an offline CPU\n");
+			ret = -EINVAL;
+		}
+	}
+	cpus_read_unlock();
+	mutex_unlock(&rcu_state.barrier_mutex);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);
+
+static long rcu_nocb_rdp_offload(void *arg)
+{
+	struct rcu_data *rdp = arg;
+	struct rcu_segcblist *cblist = &rdp->cblist;
+	unsigned long flags;
+	int ret;
+
+	WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
+	/*
+	 * For now we only support re-offload, ie: the rdp must have been
+	 * offloaded on boot first.
+	 */
+	if (!rdp->nocb_gp_rdp)
+		return -EINVAL;
+
+	pr_info("Offloading %d\n", rdp->cpu);
+	/*
+	 * Can't use rcu_nocb_lock_irqsave() while we are in
+	 * SEGCBLIST_SOFTIRQ_ONLY mode.
+	 */
+	raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
+
+	/*
+	 * We didn't take the nocb lock while working on the
+	 * rdp->cblist in SEGCBLIST_SOFTIRQ_ONLY mode.
+	 * Every modifications that have been done previously on
+	 * rdp->cblist must be visible remotely by the nocb kthreads
+	 * upon wake up after reading the cblist flags.
+	 *
+	 * The layout against nocb_lock enforces that ordering:
+	 *
+	 *  __rcu_nocb_rdp_offload()   nocb_cb_wait()/nocb_gp_wait()
+	 * -------------------------   ----------------------------
+	 *      WRITE callbacks           rcu_nocb_lock()
+	 *      rcu_nocb_lock()           READ flags
+	 *      WRITE flags               READ callbacks
+	 *      rcu_nocb_unlock()         rcu_nocb_unlock()
+	 */
+	ret = rdp_offload_toggle(rdp, true, flags);
+	swait_event_exclusive(rdp->nocb_state_wq,
+			      rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB) &&
+			      rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));
+
+	return ret;
+}
+
+int rcu_nocb_cpu_offload(int cpu)
+{
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+	int ret = 0;
+
+	mutex_lock(&rcu_state.barrier_mutex);
+	cpus_read_lock();
+	if (!rcu_rdp_is_offloaded(rdp)) {
+		if (cpu_online(cpu)) {
+			ret = work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
+			if (!ret)
+				cpumask_set_cpu(cpu, rcu_nocb_mask);
+		} else {
+			pr_info("NOCB: Can't CB-offload an offline CPU\n");
+			ret = -EINVAL;
+		}
+	}
+	cpus_read_unlock();
+	mutex_unlock(&rcu_state.barrier_mutex);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
+
+void __init rcu_init_nohz(void)
+{
+	int cpu;
+	bool need_rcu_nocb_mask = false;
+	struct rcu_data *rdp;
+
+#if defined(CONFIG_NO_HZ_FULL)
+	if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
+		need_rcu_nocb_mask = true;
+#endif /* #if defined(CONFIG_NO_HZ_FULL) */
+
+	if (!cpumask_available(rcu_nocb_mask) && need_rcu_nocb_mask) {
+		if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
+			pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
+			return;
+		}
+	}
+	if (!cpumask_available(rcu_nocb_mask))
+		return;
+
+#if defined(CONFIG_NO_HZ_FULL)
+	if (tick_nohz_full_running)
+		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
+#endif /* #if defined(CONFIG_NO_HZ_FULL) */
+
+	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
+		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
+		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
+			    rcu_nocb_mask);
+	}
+	if (cpumask_empty(rcu_nocb_mask))
+		pr_info("\tOffload RCU callbacks from CPUs: (none).\n");
+	else
+		pr_info("\tOffload RCU callbacks from CPUs: %*pbl.\n",
+			cpumask_pr_args(rcu_nocb_mask));
+	if (rcu_nocb_poll)
+		pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
+
+	for_each_cpu(cpu, rcu_nocb_mask) {
+		rdp = per_cpu_ptr(&rcu_data, cpu);
+		if (rcu_segcblist_empty(&rdp->cblist))
+			rcu_segcblist_init(&rdp->cblist);
+		rcu_segcblist_offload(&rdp->cblist, true);
+		rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_CB);
+		rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_GP);
+	}
+	rcu_organize_nocb_kthreads();
+}
+
+/* Initialize per-rcu_data variables for no-CBs CPUs. */
+static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
+{
+	init_swait_queue_head(&rdp->nocb_cb_wq);
+	init_swait_queue_head(&rdp->nocb_gp_wq);
+	init_swait_queue_head(&rdp->nocb_state_wq);
+	raw_spin_lock_init(&rdp->nocb_lock);
+	raw_spin_lock_init(&rdp->nocb_bypass_lock);
+	raw_spin_lock_init(&rdp->nocb_gp_lock);
+	timer_setup(&rdp->nocb_timer, do_nocb_deferred_wakeup_timer, 0);
+	rcu_cblist_init(&rdp->nocb_bypass);
+}
+
+/*
+ * If the specified CPU is a no-CBs CPU that does not already have its
+ * rcuo CB kthread, spawn it.  Additionally, if the rcuo GP kthread
+ * for this CPU's group has not yet been created, spawn it as well.
+ */
+static void rcu_spawn_one_nocb_kthread(int cpu)
+{
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+	struct rcu_data *rdp_gp;
+	struct task_struct *t;
+
+	/*
+	 * If this isn't a no-CBs CPU or if it already has an rcuo kthread,
+	 * then nothing to do.
+	 */
+	if (!rcu_is_nocb_cpu(cpu) || rdp->nocb_cb_kthread)
+		return;
+
+	/* If we didn't spawn the GP kthread first, reorganize! */
+	rdp_gp = rdp->nocb_gp_rdp;
+	if (!rdp_gp->nocb_gp_kthread) {
+		t = kthread_run(rcu_nocb_gp_kthread, rdp_gp,
+				"rcuog/%d", rdp_gp->cpu);
+		if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo GP kthread, OOM is now expected behavior\n", __func__))
+			return;
+		WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
+	}
+
+	/* Spawn the kthread for this CPU. */
+	t = kthread_run(rcu_nocb_cb_kthread, rdp,
+			"rcuo%c/%d", rcu_state.abbr, cpu);
+	if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo CB kthread, OOM is now expected behavior\n", __func__))
+		return;
+	WRITE_ONCE(rdp->nocb_cb_kthread, t);
+	WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
+}
+
+/*
+ * If the specified CPU is a no-CBs CPU that does not already have its
+ * rcuo kthread, spawn it.
+ */
+static void rcu_spawn_cpu_nocb_kthread(int cpu)
+{
+	if (rcu_scheduler_fully_active)
+		rcu_spawn_one_nocb_kthread(cpu);
+}
+
+/*
+ * Once the scheduler is running, spawn rcuo kthreads for all online
+ * no-CBs CPUs.  This assumes that the early_initcall()s happen before
+ * non-boot CPUs come online -- if this changes, we will need to add
+ * some mutual exclusion.
+ */
+static void __init rcu_spawn_nocb_kthreads(void)
+{
+	int cpu;
+
+	for_each_online_cpu(cpu)
+		rcu_spawn_cpu_nocb_kthread(cpu);
+}
+
+/* How many CB CPU IDs per GP kthread?  Default of -1 for sqrt(nr_cpu_ids). */
+static int rcu_nocb_gp_stride = -1;
+module_param(rcu_nocb_gp_stride, int, 0444);
+
+/*
+ * Initialize GP-CB relationships for all no-CBs CPU.
+ */
+static void __init rcu_organize_nocb_kthreads(void)
+{
+	int cpu;
+	bool firsttime = true;
+	bool gotnocbs = false;
+	bool gotnocbscbs = true;
+	int ls = rcu_nocb_gp_stride;
+	int nl = 0;  /* Next GP kthread. */
+	struct rcu_data *rdp;
+	struct rcu_data *rdp_gp = NULL;  /* Suppress misguided gcc warn. */
+	struct rcu_data *rdp_prev = NULL;
+
+	if (!cpumask_available(rcu_nocb_mask))
+		return;
+	if (ls == -1) {
+		ls = nr_cpu_ids / int_sqrt(nr_cpu_ids);
+		rcu_nocb_gp_stride = ls;
+	}
+
+	/*
+	 * Each pass through this loop sets up one rcu_data structure.
+	 * Should the corresponding CPU come online in the future, then
+	 * we will spawn the needed set of rcu_nocb_kthread() kthreads.
+	 */
+	for_each_cpu(cpu, rcu_nocb_mask) {
+		rdp = per_cpu_ptr(&rcu_data, cpu);
+		if (rdp->cpu >= nl) {
+			/* New GP kthread, set up for CBs & next GP. */
+			gotnocbs = true;
+			nl = DIV_ROUND_UP(rdp->cpu + 1, ls) * ls;
+			rdp->nocb_gp_rdp = rdp;
+			rdp_gp = rdp;
+			if (dump_tree) {
+				if (!firsttime)
+					pr_cont("%s\n", gotnocbscbs
+							? "" : " (self only)");
+				gotnocbscbs = false;
+				firsttime = false;
+				pr_alert("%s: No-CB GP kthread CPU %d:",
+					 __func__, cpu);
+			}
+		} else {
+			/* Another CB kthread, link to previous GP kthread. */
+			gotnocbscbs = true;
+			rdp->nocb_gp_rdp = rdp_gp;
+			rdp_prev->nocb_next_cb_rdp = rdp;
+			if (dump_tree)
+				pr_cont(" %d", cpu);
+		}
+		rdp_prev = rdp;
+	}
+	if (gotnocbs && dump_tree)
+		pr_cont("%s\n", gotnocbscbs ? "" : " (self only)");
+}
+
+/*
+ * Bind the current task to the offloaded CPUs.  If there are no offloaded
+ * CPUs, leave the task unbound.  Splat if the bind attempt fails.
+ */
+void rcu_bind_current_to_nocb(void)
+{
+	if (cpumask_available(rcu_nocb_mask) && cpumask_weight(rcu_nocb_mask))
+		WARN_ON(sched_setaffinity(current->pid, rcu_nocb_mask));
+}
+EXPORT_SYMBOL_GPL(rcu_bind_current_to_nocb);
+
+// The ->on_cpu field is available only in CONFIG_SMP=y, so...
+#ifdef CONFIG_SMP
+static char *show_rcu_should_be_on_cpu(struct task_struct *tsp)
+{
+	return tsp && task_is_running(tsp) && !tsp->on_cpu ? "!" : "";
+}
+#else // #ifdef CONFIG_SMP
+static char *show_rcu_should_be_on_cpu(struct task_struct *tsp)
+{
+	return "";
+}
+#endif // #else #ifdef CONFIG_SMP
+
+/*
+ * Dump out nocb grace-period kthread state for the specified rcu_data
+ * structure.
+ */
+static void show_rcu_nocb_gp_state(struct rcu_data *rdp)
+{
+	struct rcu_node *rnp = rdp->mynode;
+
+	pr_info("nocb GP %d %c%c%c%c%c %c[%c%c] %c%c:%ld rnp %d:%d %lu %c CPU %d%s\n",
+		rdp->cpu,
+		"kK"[!!rdp->nocb_gp_kthread],
+		"lL"[raw_spin_is_locked(&rdp->nocb_gp_lock)],
+		"dD"[!!rdp->nocb_defer_wakeup],
+		"tT"[timer_pending(&rdp->nocb_timer)],
+		"sS"[!!rdp->nocb_gp_sleep],
+		".W"[swait_active(&rdp->nocb_gp_wq)],
+		".W"[swait_active(&rnp->nocb_gp_wq[0])],
+		".W"[swait_active(&rnp->nocb_gp_wq[1])],
+		".B"[!!rdp->nocb_gp_bypass],
+		".G"[!!rdp->nocb_gp_gp],
+		(long)rdp->nocb_gp_seq,
+		rnp->grplo, rnp->grphi, READ_ONCE(rdp->nocb_gp_loops),
+		rdp->nocb_gp_kthread ? task_state_to_char(rdp->nocb_gp_kthread) : '.',
+		rdp->nocb_cb_kthread ? (int)task_cpu(rdp->nocb_gp_kthread) : -1,
+		show_rcu_should_be_on_cpu(rdp->nocb_cb_kthread));
+}
+
+/* Dump out nocb kthread state for the specified rcu_data structure. */
+static void show_rcu_nocb_state(struct rcu_data *rdp)
+{
+	char bufw[20];
+	char bufr[20];
+	struct rcu_segcblist *rsclp = &rdp->cblist;
+	bool waslocked;
+	bool wassleep;
+
+	if (rdp->nocb_gp_rdp == rdp)
+		show_rcu_nocb_gp_state(rdp);
+
+	sprintf(bufw, "%ld", rsclp->gp_seq[RCU_WAIT_TAIL]);
+	sprintf(bufr, "%ld", rsclp->gp_seq[RCU_NEXT_READY_TAIL]);
+	pr_info("   CB %d^%d->%d %c%c%c%c%c%c F%ld L%ld C%d %c%c%s%c%s%c%c q%ld %c CPU %d%s\n",
+		rdp->cpu, rdp->nocb_gp_rdp->cpu,
+		rdp->nocb_next_cb_rdp ? rdp->nocb_next_cb_rdp->cpu : -1,
+		"kK"[!!rdp->nocb_cb_kthread],
+		"bB"[raw_spin_is_locked(&rdp->nocb_bypass_lock)],
+		"cC"[!!atomic_read(&rdp->nocb_lock_contended)],
+		"lL"[raw_spin_is_locked(&rdp->nocb_lock)],
+		"sS"[!!rdp->nocb_cb_sleep],
+		".W"[swait_active(&rdp->nocb_cb_wq)],
+		jiffies - rdp->nocb_bypass_first,
+		jiffies - rdp->nocb_nobypass_last,
+		rdp->nocb_nobypass_count,
+		".D"[rcu_segcblist_ready_cbs(rsclp)],
+		".W"[!rcu_segcblist_segempty(rsclp, RCU_WAIT_TAIL)],
+		rcu_segcblist_segempty(rsclp, RCU_WAIT_TAIL) ? "" : bufw,
+		".R"[!rcu_segcblist_segempty(rsclp, RCU_NEXT_READY_TAIL)],
+		rcu_segcblist_segempty(rsclp, RCU_NEXT_READY_TAIL) ? "" : bufr,
+		".N"[!rcu_segcblist_segempty(rsclp, RCU_NEXT_TAIL)],
+		".B"[!!rcu_cblist_n_cbs(&rdp->nocb_bypass)],
+		rcu_segcblist_n_cbs(&rdp->cblist),
+		rdp->nocb_cb_kthread ? task_state_to_char(rdp->nocb_cb_kthread) : '.',
+		rdp->nocb_cb_kthread ? (int)task_cpu(rdp->nocb_gp_kthread) : -1,
+		show_rcu_should_be_on_cpu(rdp->nocb_cb_kthread));
+
+	/* It is OK for GP kthreads to have GP state. */
+	if (rdp->nocb_gp_rdp == rdp)
+		return;
+
+	waslocked = raw_spin_is_locked(&rdp->nocb_gp_lock);
+	wassleep = swait_active(&rdp->nocb_gp_wq);
+	if (!rdp->nocb_gp_sleep && !waslocked && !wassleep)
+		return;  /* Nothing untoward. */
+
+	pr_info("   nocb GP activity on CB-only CPU!!! %c%c%c %c\n",
+		"lL"[waslocked],
+		"dD"[!!rdp->nocb_defer_wakeup],
+		"sS"[!!rdp->nocb_gp_sleep],
+		".W"[wassleep]);
+}
+
+#else /* #ifdef CONFIG_RCU_NOCB_CPU */
+
+static inline int rcu_lockdep_is_held_nocb(struct rcu_data *rdp)
+{
+	return 0;
+}
+
+static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
+{
+	return false;
+}
+
+/* No ->nocb_lock to acquire.  */
+static void rcu_nocb_lock(struct rcu_data *rdp)
+{
+}
+
+/* No ->nocb_lock to release.  */
+static void rcu_nocb_unlock(struct rcu_data *rdp)
+{
+}
+
+/* No ->nocb_lock to release.  */
+static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
+				       unsigned long flags)
+{
+	local_irq_restore(flags);
+}
+
+/* Lockdep check that ->cblist may be safely accessed. */
+static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
+{
+	lockdep_assert_irqs_disabled();
+}
+
+static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
+{
+}
+
+static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
+{
+	return NULL;
+}
+
+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)
+{
+	return true;
+}
+
+static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
+				bool *was_alldone, unsigned long flags)
+{
+	return false;
+}
+
+static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
+				 unsigned long flags)
+{
+	WARN_ON_ONCE(1);  /* Should be dead code! */
+}
+
+static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
+{
+}
+
+static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level)
+{
+	return false;
+}
+
+static bool do_nocb_deferred_wakeup(struct rcu_data *rdp)
+{
+	return false;
+}
+
+static void rcu_spawn_cpu_nocb_kthread(int cpu)
+{
+}
+
+static void __init rcu_spawn_nocb_kthreads(void)
+{
+}
+
+static void show_rcu_nocb_state(struct rcu_data *rdp)
+{
+}
+
+#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */