Commit d1b222c6 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu/nocb: Add bypass callback queueing 



Use of the rcu_data structure's segmented ->cblist for no-CBs CPUs
takes advantage of unrelated grace periods, thus reducing the memory
footprint in the face of floods of call_rcu() invocations.  However,
the ->cblist field is a more-complex rcu_segcblist structure which must
be protected via locking.  Even though there are only three entities
which can acquire this lock (the CPU invoking call_rcu(), the no-CBs
grace-period kthread, and the no-CBs callbacks kthread), the contention
on this lock is excessive under heavy stress.

This commit therefore greatly reduces contention by provisioning
an rcu_cblist structure field named ->nocb_bypass within the
rcu_data structure.  Each no-CBs CPU is permitted only a limited
number of enqueues onto the ->cblist per jiffy, controlled by a new
nocb_nobypass_lim_per_jiffy kernel boot parameter that defaults to
about 16 enqueues per millisecond (16 * 1000 / HZ).  When that limit is
exceeded, the CPU instead enqueues onto the new ->nocb_bypass.

The ->nocb_bypass is flushed into the ->cblist every jiffy or when
the number of callbacks on ->nocb_bypass exceeds qhimark, whichever
happens first.  During call_rcu() floods, this flushing is carried out
by the CPU during the course of its call_rcu() invocations.  However,
a CPU could simply stop invoking call_rcu() at any time.  The no-CBs
grace-period kthread therefore carries out less-aggressive flushing
(every few jiffies or when the number of callbacks on ->nocb_bypass
exceeds (2 * qhimark), whichever comes first).  This means that the
no-CBs grace-period kthread cannot be permitted to do unbounded waits
while there are callbacks on ->nocb_bypass.  A ->nocb_bypass_timer is
used to provide the needed wakeups.

[ paulmck: Apply Coverity feedback reported by Colin Ian King. ]
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.ibm.com>
parent eda669a6

kernel/rcu/rcu_segcblist.c

+30 −0
Original line number Diff line number Diff line
@@ -36,6 +36,36 @@ void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp) 
	WRITE_ONCE(rclp->len, rclp->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

 * element of the second rcu_cblist structure, but ensuring that the second

 * rcu_cblist structure, if initially non-empty, always appears non-empty

 * throughout the process.  If rdp is NULL, the second rcu_cblist structure

 * is instead initialized to empty.

 */

void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,

			      struct rcu_cblist *srclp,

			      struct rcu_head *rhp)

{

	drclp->head = srclp->head;

	if (drclp->head)

		drclp->tail = srclp->tail;

	else

		drclp->tail = &drclp->head;

	drclp->len = srclp->len;

	drclp->len_lazy = srclp->len_lazy;

	if (!rhp) {

		rcu_cblist_init(srclp);

	} else {

		rhp->next = NULL;

		srclp->head = rhp;

		srclp->tail = &rhp->next;

		WRITE_ONCE(srclp->len, 1);

		srclp->len_lazy = 0;

	}

}



/*

 * Dequeue the oldest rcu_head structure from the specified callback

 * list.  This function assumes that the callback is non-lazy, but

kernel/rcu/rcu_segcblist.h

+5 −0
Original line number Diff line number Diff line
@@ -25,6 +25,10 @@ static inline void rcu_cblist_dequeued_lazy(struct rcu_cblist *rclp) 
}



void rcu_cblist_init(struct rcu_cblist *rclp);

void rcu_cblist_enqueue(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);

struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp);



/*
@@ -92,6 +96,7 @@ static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg) 
	return !READ_ONCE(*READ_ONCE(rsclp->tails[seg]));

}



void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp);

void rcu_segcblist_init(struct rcu_segcblist *rsclp);

void rcu_segcblist_disable(struct rcu_segcblist *rsclp);

void rcu_segcblist_offload(struct rcu_segcblist *rsclp);

kernel/rcu/tree.c

+12 −4
Original line number Diff line number Diff line
@@ -1251,6 +1251,7 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp) 
	unsigned long gp_seq_req;

	bool ret = false;



	rcu_lockdep_assert_cblist_protected(rdp);

	raw_lockdep_assert_held_rcu_node(rnp);



	/* If no pending (not yet ready to invoke) callbacks, nothing to do. */
@@ -1292,7 +1293,7 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp, 
	unsigned long c;

	bool needwake;



	lockdep_assert_irqs_disabled();

	rcu_lockdep_assert_cblist_protected(rdp);

	c = rcu_seq_snap(&rcu_state.gp_seq);

	if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {

		/* Old request still live, so mark recent callbacks. */
@@ -1318,6 +1319,7 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp, 
 */

static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp)

{

	rcu_lockdep_assert_cblist_protected(rdp);

	raw_lockdep_assert_held_rcu_node(rnp);



	/* If no pending (not yet ready to invoke) callbacks, nothing to do. */
@@ -1341,6 +1343,7 @@ static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp) 
static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp,

						  struct rcu_data *rdp)

{

	rcu_lockdep_assert_cblist_protected(rdp);

	if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)) ||

	    !raw_spin_trylock_rcu_node(rnp))

		return;
@@ -2187,7 +2190,9 @@ static void rcu_do_batch(struct rcu_data *rdp) 
	 * The following usually indicates a double call_rcu().  To track

	 * this down, try building with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y.

	 */

	WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));

	WARN_ON_ONCE(count == 0 && !rcu_segcblist_empty(&rdp->cblist));

	WARN_ON_ONCE(!IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&

		     count != 0 && rcu_segcblist_empty(&rdp->cblist));



	rcu_nocb_unlock_irqrestore(rdp, flags);
@@ -2564,8 +2569,9 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func, bool lazy) 
		if (rcu_segcblist_empty(&rdp->cblist))

			rcu_segcblist_init(&rdp->cblist);

	}

	rcu_nocb_lock(rdp);

	was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);

	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))

		return; // Enqueued onto ->nocb_bypass, so just leave.

	/* If we get here, rcu_nocb_try_bypass() acquired ->nocb_lock. */

	rcu_segcblist_enqueue(&rdp->cblist, head, lazy);

	if (__is_kfree_rcu_offset((unsigned long)func))

		trace_rcu_kfree_callback(rcu_state.name, head,
@@ -2839,6 +2845,7 @@ static void rcu_barrier_func(void *unused) 
	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));

	if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head, 0)) {

		atomic_inc(&rcu_state.barrier_cpu_count);

	} else {
@@ -3192,6 +3199,7 @@ 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));

	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) ||

kernel/rcu/tree.h

+18 −10
Original line number Diff line number Diff line
@@ -200,18 +200,26 @@ struct rcu_data { 
	atomic_t nocb_lock_contended;	/* Contention experienced. */

	int nocb_defer_wakeup;		/* Defer wakeup of nocb_kthread. */

	struct timer_list nocb_timer;	/* Enforce finite deferral. */

	unsigned long nocb_gp_adv_time;	/* Last call_rcu() CB adv (jiffies). */



	/* The following fields are used by call_rcu, hence own cacheline. */

	raw_spinlock_t nocb_bypass_lock ____cacheline_internodealigned_in_smp;

	struct rcu_cblist nocb_bypass;	/* Lock-contention-bypass CB list. */

	unsigned long nocb_bypass_first; /* Time (jiffies) of first enqueue. */

	unsigned long nocb_nobypass_last; /* Last ->cblist enqueue (jiffies). */

	int nocb_nobypass_count;	/* # ->cblist enqueues at ^^^ time. */



	/* The following fields are used by GP kthread, hence own cacheline. */

	raw_spinlock_t nocb_gp_lock ____cacheline_internodealigned_in_smp;

	bool nocb_gp_sleep;

					/* Is the nocb GP thread asleep? */

	struct timer_list nocb_bypass_timer; /* Force nocb_bypass flush. */

	bool nocb_gp_sleep;		/* Is the nocb GP thread asleep? */

	struct swait_queue_head nocb_gp_wq; /* For nocb kthreads to sleep on. */

	bool nocb_cb_sleep;		/* Is the nocb CB thread asleep? */

	struct task_struct *nocb_cb_kthread;

	struct rcu_data *nocb_next_cb_rdp;

					/* Next rcu_data in wakeup chain. */



	/* The following fields are used by CB kthread, hence new cachline. */

	/* The following fields are used by CB kthread, hence new cacheline. */

	struct rcu_data *nocb_gp_rdp ____cacheline_internodealigned_in_smp;

					/* GP rdp takes GP-end wakeups. */

#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
@@ -419,6 +427,10 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp); 
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);

static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,

				bool *was_alldone, unsigned long flags);

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);
@@ -430,19 +442,15 @@ static void rcu_nocb_lock(struct rcu_data *rdp); 
static void rcu_nocb_unlock(struct rcu_data *rdp);

static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,

				       unsigned long flags);

static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp);

#ifdef CONFIG_RCU_NOCB_CPU

static void __init rcu_organize_nocb_kthreads(void);

#define rcu_nocb_lock_irqsave(rdp, flags)				\

do {									\

	if (!rcu_segcblist_is_offloaded(&(rdp)->cblist)) {		\

	if (!rcu_segcblist_is_offloaded(&(rdp)->cblist))		\

		local_irq_save(flags);					\

	} else if (!raw_spin_trylock_irqsave(&(rdp)->nocb_lock, (flags))) {\

		atomic_inc(&(rdp)->nocb_lock_contended);		\

		smp_mb__after_atomic(); /* atomic_inc() before lock. */	\

	else								\

		raw_spin_lock_irqsave(&(rdp)->nocb_lock, (flags));	\

		smp_mb__before_atomic(); /* atomic_dec() after lock. */	\

		atomic_dec(&(rdp)->nocb_lock_contended);		\

	}								\

} while (0)

#else /* #ifdef CONFIG_RCU_NOCB_CPU */

#define rcu_nocb_lock_irqsave(rdp, flags) local_irq_save(flags)

kernel/rcu/tree_plugin.h

+330 −27
Original line number Diff line number Diff line
@@ -1497,19 +1497,26 @@ static int __init parse_rcu_nocb_poll(char *arg) 
early_param("rcu_nocb_poll", parse_rcu_nocb_poll);



/*

 * Acquire the specified rcu_data structure's ->nocb_lock, but only

 * if it corresponds to a no-CBs CPU.  If the lock isn't immediately

 * available, increment ->nocb_lock_contended to flag the contention.

 * 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 void rcu_nocb_lock(struct rcu_data *rdp)

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)

{

	lockdep_assert_irqs_disabled();

	if (!rcu_segcblist_is_offloaded(&rdp->cblist) ||

	    raw_spin_trylock(&rdp->nocb_lock))

	if (raw_spin_trylock(&rdp->nocb_bypass_lock))

		return;

	atomic_inc(&rdp->nocb_lock_contended);

	smp_mb__after_atomic(); /* atomic_inc() before lock. */

	raw_spin_lock(&rdp->nocb_lock);

	raw_spin_lock(&rdp->nocb_bypass_lock);

	smp_mb__before_atomic(); /* atomic_dec() after lock. */

	atomic_dec(&rdp->nocb_lock_contended);

}
@@ -1530,6 +1537,37 @@ static void rcu_nocb_wait_contended(struct rcu_data *rdp) 
		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)

{

	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_segcblist_is_offloaded(&rdp->cblist))

		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.
@@ -1557,6 +1595,15 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp, 
	}

}



/* 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_segcblist_is_offloaded(&rdp->cblist) &&

	    cpu_online(rdp->cpu))

		lockdep_assert_held(&rdp->nocb_lock);

}



/*

 * Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended

 * grace period.
@@ -1593,24 +1640,27 @@ static void wake_nocb_gp(struct rcu_data *rdp, bool force, 
			   unsigned long flags)

	__releases(rdp->nocb_lock)

{

	bool needwake = false;

	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;



	lockdep_assert_held(&rdp->nocb_lock);

	if (!READ_ONCE(rdp_gp->nocb_gp_kthread)) {

		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,

				    TPS("AlreadyAwake"));

		rcu_nocb_unlock_irqrestore(rdp, flags);

		return;

	}

	if (READ_ONCE(rdp_gp->nocb_gp_sleep) || force) {

	del_timer(&rdp->nocb_timer);

	rcu_nocb_unlock_irqrestore(rdp, flags);

		smp_mb(); /* enqueue before ->nocb_gp_sleep. */

	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);

	if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) {

		WRITE_ONCE(rdp_gp->nocb_gp_sleep, false);

		needwake = true;

		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DoWake"));

	}

	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);

	if (needwake)

		wake_up_process(rdp_gp->nocb_gp_kthread);

	} else {

		rcu_nocb_unlock_irqrestore(rdp, flags);

	}

}



/*
@@ -1627,6 +1677,189 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype, 
	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_segcblist_is_offloaded(&rdp->cblist));

	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_segcblist_is_offloaded(&rdp->cblist))

		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_segcblist_is_offloaded(&rdp->cblist) ||

	    !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);



	if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {

		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);

		return false; /* Not offloaded, no bypassing. */

	}

	lockdep_assert_irqs_disabled();



	// 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 kthead if needed, either due to it

 * legitimately being asleep or due to overload conditions.
@@ -1685,23 +1918,33 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, 
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));

		rcu_nocb_unlock_irqrestore(rdp, flags);

	}

	if (!irqs_disabled_flags(flags)) {

		lockdep_assert_irqs_enabled();

		rcu_nocb_wait_contended(rdp);

	}

	return;

}



/* Wake up the no-CBs GP kthread to flush ->nocb_bypass. */

static void do_nocb_bypass_wakeup_timer(struct timer_list *t)

{

	unsigned long flags;

	struct rcu_data *rdp = from_timer(rdp, t, nocb_bypass_timer);



	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));

	rcu_nocb_lock_irqsave(rdp, flags);

	__call_rcu_nocb_wake(rdp, true, flags);

}



/*

 * 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;

	unsigned long j = jiffies;

	bool needwait_gp = false; // This prevents actual uninitialized use.

	bool needwake;

	bool needwake_gp;
@@ -1715,21 +1958,50 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) 
	 * and the global grace-period kthread are awakened if needed.

	 */

	for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) {

		if (rcu_segcblist_empty(&rdp->cblist))

		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));

		rcu_nocb_lock_irqsave(rdp, flags);

		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);

			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;

		rcu_nocb_lock_irqsave(rdp, flags);

		WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);

		if (bypass) {  // Avoid race with first bypass CB.

			WRITE_ONCE(my_rdp->nocb_defer_wakeup,

				   RCU_NOCB_WAKE_NOT);

			del_timer(&my_rdp->nocb_timer);

		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */

		}

		// 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);

		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */

			raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */

		}

		// Need to wait on some grace period?

		WARN_ON_ONCE(!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;
@@ -1747,6 +2019,13 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) 
			rcu_gp_kthread_wake();

	}



	if (bypass && !rcu_nocb_poll) {

		// At least one child with non-empty ->nocb_bypass, so set

		// timer in order to avoid stranding its callbacks.

		raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);

		mod_timer(&my_rdp->nocb_bypass_timer, j + 2);

		raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);

	}

	if (rcu_nocb_poll) {

		/* Polling, so trace if first poll in the series. */

		if (gotcbs)
@@ -1757,6 +2036,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) 
		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"));
@@ -1768,6 +2048,8 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) 
	}

	if (!rcu_nocb_poll) {

		raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);

		if (bypass)

			del_timer(&my_rdp->nocb_bypass_timer);

		WRITE_ONCE(my_rdp->nocb_gp_sleep, true);

		raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);

	}
@@ -1949,8 +2231,11 @@ 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);

	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);

	timer_setup(&rdp->nocb_bypass_timer, do_nocb_bypass_wakeup_timer, 0);

	rcu_cblist_init(&rdp->nocb_bypass);

}



/*
@@ -2094,6 +2379,12 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp, 
	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)

{

}
@@ -2107,6 +2398,18 @@ 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)

{

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