[PATCH v4 01/14] rcu: Introduce call_rcu_lazy() API implementation

From: Joel Fernandes (Google)
Date: Fri Aug 19 2022 - 16:49:18 EST

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@xxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
include/linux/rcu_segcblist.h | 1 +
include/linux/rcupdate.h | 6 +
kernel/rcu/Kconfig | 8 ++
kernel/rcu/rcu.h | 11 ++
kernel/rcu/rcu_segcblist.c | 15 ++-
kernel/rcu/rcu_segcblist.h | 20 +++-
kernel/rcu/tree.c | 130 ++++++++++++++--------
kernel/rcu/tree.h | 10 +-
kernel/rcu/tree_nocb.h | 199 ++++++++++++++++++++++++++--------
9 files changed, 301 insertions(+), 99 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..779b6e84006b 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.h b/kernel/rcu/rcu.h
index 4916077119f3..608f6ab76c7f 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -463,6 +463,14 @@ enum rcutorture_type {
INVALID_RCU_FLAVOR
};

+#if defined(CONFIG_RCU_LAZY)
+unsigned long rcu_lazy_get_jiffies_till_flush(void);
+void rcu_lazy_set_jiffies_till_flush(unsigned long j);
+#else
+static inline unsigned long rcu_lazy_get_jiffies_till_flush(void) { return 0; }
+static inline void rcu_lazy_set_jiffies_till_flush(unsigned long j) { }
+#endif
+
#if defined(CONFIG_TREE_RCU)
void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,
unsigned long *gp_seq);
@@ -472,6 +480,8 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
unsigned long c_old,
unsigned long c);
void rcu_gp_set_torture_wait(int duration);
+void rcu_force_call_rcu_to_lazy(bool force);
+
#else
static inline void rcutorture_get_gp_data(enum rcutorture_type test_type,
int *flags, unsigned long *gp_seq)
@@ -490,6 +500,7 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
do { } while (0)
#endif
static inline void rcu_gp_set_torture_wait(int duration) { }
+static inline void rcu_force_call_rcu_to_lazy(bool force) { }
#endif

#if IS_ENABLED(CONFIG_RCU_TORTURE_TEST) || IS_MODULE(CONFIG_RCU_TORTURE_TEST)
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index c54ea2b6a36b..776647cd2d6c 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -20,16 +20,21 @@ void rcu_cblist_init(struct rcu_cblist *rclp)
rclp->head = NULL;
rclp->tail = &rclp->head;
rclp->len = 0;
+ rclp->lazy_len = 0;
}

/*
* Enqueue an rcu_head structure onto the specified callback list.
*/
-void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp)
+void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp,
+ bool lazy)
{
*rclp->tail = rhp;
rclp->tail = &rhp->next;
WRITE_ONCE(rclp->len, rclp->len + 1);
+
+ if (IS_ENABLED(CONFIG_RCU_LAZY) && lazy)
+ WRITE_ONCE(rclp->lazy_len, rclp->lazy_len + 1);
}

/*
@@ -38,11 +43,12 @@ void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp)
* 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.
+ * is instead initialized to empty. Also account for lazy_len for lazy CBs.
*/
void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
struct rcu_cblist *srclp,
- struct rcu_head *rhp)
+ struct rcu_head *rhp,
+ bool lazy)
{
drclp->head = srclp->head;
if (drclp->head)
@@ -58,6 +64,9 @@ void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
srclp->tail = &rhp->next;
WRITE_ONCE(srclp->len, 1);
}
+
+ if (IS_ENABLED(CONFIG_RCU_LAZY) && rhp && lazy)
+ WRITE_ONCE(srclp->lazy_len, 1);
}

/*
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 431cee212467..8e90b34adb00 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -15,14 +15,30 @@ 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)
+{
+ if (IS_ENABLED(CONFIG_RCU_LAZY))
+ return READ_ONCE(rclp->lazy_len);
+ return 0;
+}
+
+static inline void rcu_cblist_reset_lazy_len(struct rcu_cblist *rclp)
+{
+ if (IS_ENABLED(CONFIG_RCU_LAZY))
+ WRITE_ONCE(rclp->lazy_len, 0);
+}
+
/* 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(struct rcu_cblist *rclp, struct rcu_head *rhp,
+ bool lazy);
void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
struct rcu_cblist *srclp,
- struct rcu_head *rhp);
+ struct rcu_head *rhp,
+ bool lazy);
struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp);

/*
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c25ba442044a..e76fef8031be 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3058,47 +3058,8 @@ static void check_cb_ovld(struct rcu_data *rdp)
raw_spin_unlock_rcu_node(rnp);
}

-/**
- * call_rcu() - Queue an RCU callback for invocation after a grace period.
- * @head: structure to be used for queueing the RCU updates.
- * @func: actual callback function to be invoked after the grace period
- *
- * The callback function will be invoked some time after a full grace
- * period elapses, in other words after all pre-existing RCU read-side
- * critical sections have completed. However, the callback function
- * might well execute concurrently with RCU read-side critical sections
- * that started after call_rcu() was invoked.
- *
- * RCU read-side critical sections are delimited by rcu_read_lock()
- * and rcu_read_unlock(), and may be nested. In addition, but only in
- * v5.0 and later, regions of code across which interrupts, preemption,
- * or softirqs have been disabled also serve as RCU read-side critical
- * sections. This includes hardware interrupt handlers, softirq handlers,
- * and NMI handlers.
- *
- * Note that all CPUs must agree that the grace period extended beyond
- * all pre-existing RCU read-side critical section. On systems with more
- * than one CPU, this means that when "func()" is invoked, each CPU is
- * guaranteed to have executed a full memory barrier since the end of its
- * last RCU read-side critical section whose beginning preceded the call
- * to call_rcu(). It also means that each CPU executing an RCU read-side
- * critical section that continues beyond the start of "func()" must have
- * executed a memory barrier after the call_rcu() but before the beginning
- * of that RCU read-side critical section. Note that these guarantees
- * include CPUs that are offline, idle, or executing in user mode, as
- * well as CPUs that are executing in the kernel.
- *
- * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
- * resulting RCU callback function "func()", then both CPU A and CPU B are
- * guaranteed to execute a full memory barrier during the time interval
- * between the call to call_rcu() and the invocation of "func()" -- even
- * if CPU A and CPU B are the same CPU (but again only if the system has
- * more than one CPU).
- *
- * 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 +3100,7 @@ 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 +3122,86 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
local_irq_restore(flags);
}
}
-EXPORT_SYMBOL_GPL(call_rcu);

+#ifdef CONFIG_RCU_LAZY
+/**
+ * call_rcu_lazy() - Lazily queue RCU callback for invocation after grace period.
+ * @head: structure to be used for queueing the RCU updates.
+ * @func: actual callback function to be invoked after the grace period
+ *
+ * The callback function will be invoked some time after a full grace
+ * period elapses, in other words after all pre-existing RCU read-side
+ * critical sections have completed.
+ *
+ * Use this API instead of call_rcu() if you don't mind the callback being
+ * invoked after very long periods of time on systems without memory pressure
+ * and on systems which are lightly loaded or mostly idle.
+ *
+ * Other than the extra delay in callbacks being invoked, this function is
+ * identical to, and reuses call_rcu()'s logic. Refer to call_rcu() for more
+ * details about memory ordering and other functionality.
+ */
+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
+
+static bool force_call_rcu_to_lazy;
+
+void rcu_force_call_rcu_to_lazy(bool force)
+{
+ if (IS_ENABLED(CONFIG_RCU_SCALE_TEST))
+ WRITE_ONCE(force_call_rcu_to_lazy, force);
+}
+EXPORT_SYMBOL_GPL(rcu_force_call_rcu_to_lazy);
+
+/**
+ * call_rcu() - Queue an RCU callback for invocation after a grace period.
+ * @head: structure to be used for queueing the RCU updates.
+ * @func: actual callback function to be invoked after the grace period
+ *
+ * The callback function will be invoked some time after a full grace
+ * period elapses, in other words after all pre-existing RCU read-side
+ * critical sections have completed. However, the callback function
+ * might well execute concurrently with RCU read-side critical sections
+ * that started after call_rcu() was invoked.
+ *
+ * RCU read-side critical sections are delimited by rcu_read_lock()
+ * and rcu_read_unlock(), and may be nested. In addition, but only in
+ * v5.0 and later, regions of code across which interrupts, preemption,
+ * or softirqs have been disabled also serve as RCU read-side critical
+ * sections. This includes hardware interrupt handlers, softirq handlers,
+ * and NMI handlers.
+ *
+ * Note that all CPUs must agree that the grace period extended beyond
+ * all pre-existing RCU read-side critical section. On systems with more
+ * than one CPU, this means that when "func()" is invoked, each CPU is
+ * guaranteed to have executed a full memory barrier since the end of its
+ * last RCU read-side critical section whose beginning preceded the call
+ * to call_rcu(). It also means that each CPU executing an RCU read-side
+ * critical section that continues beyond the start of "func()" must have
+ * executed a memory barrier after the call_rcu() but before the beginning
+ * of that RCU read-side critical section. Note that these guarantees
+ * include CPUs that are offline, idle, or executing in user mode, as
+ * well as CPUs that are executing in the kernel.
+ *
+ * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
+ * resulting RCU callback function "func()", then both CPU A and CPU B are
+ * guaranteed to execute a full memory barrier during the time interval
+ * between the call to call_rcu() and the invocation of "func()" -- even
+ * if CPU A and CPU B are the same CPU (but again only if the system has
+ * more than one CPU).
+ *
+ * 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)
+{
+ return __call_rcu_common(head, func, force_call_rcu_to_lazy);
+}
+EXPORT_SYMBOL_GPL(call_rcu);

/* Maximum number of jiffies to wait before draining a batch. */
#define KFREE_DRAIN_JIFFIES (HZ / 50)
@@ -4056,7 +4095,8 @@ 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,
+ /* wake gp thread */ true));
if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
atomic_inc(&rcu_state.barrier_cpu_count);
} else {
@@ -4476,7 +4516,7 @@ 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, 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..7b1ddee6a159 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, bool wakegp);
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..55636da76bc2 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -256,6 +256,31 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
return __wake_nocb_gp(rdp_gp, rdp, force, flags);
}

+/*
+ * LAZY_FLUSH_JIFFIES decides the maximum amount of time that
+ * can elapse before lazy callbacks are flushed. Lazy callbacks
+ * could be flushed much earlier for a number of other reasons
+ * however, LAZY_FLUSH_JIFFIES will ensure no lazy callbacks are
+ * left unsubmitted to RCU after those many jiffies.
+ */
+#define LAZY_FLUSH_JIFFIES (10 * HZ)
+unsigned long jiffies_till_flush = LAZY_FLUSH_JIFFIES;
+
+#ifdef CONFIG_RCU_LAZY
+// To be called only from test code.
+void rcu_lazy_set_jiffies_till_flush(unsigned long jif)
+{
+ jiffies_till_flush = jif;
+}
+EXPORT_SYMBOL(rcu_lazy_set_jiffies_till_flush);
+
+unsigned long rcu_lazy_get_jiffies_till_flush(void)
+{
+ return jiffies_till_flush;
+}
+EXPORT_SYMBOL(rcu_lazy_get_jiffies_till_flush);
+#endif
+
/*
* Arrange to wake the GP kthread for this NOCB group at some future
* time when it is safe to do so.
@@ -265,6 +290,7 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
{
unsigned long flags;
struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
+ unsigned long mod_jif = 0;

raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);

@@ -272,16 +298,32 @@ 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) {
- mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
- WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
- } else {
+ switch (waketype) {
+ case RCU_NOCB_WAKE_LAZY:
+ if (rdp->nocb_defer_wakeup != RCU_NOCB_WAKE_LAZY)
+ mod_jif = jiffies_till_flush;
+ break;
+
+ case RCU_NOCB_WAKE_BYPASS:
+ mod_jif = 2;
+ break;
+
+ case RCU_NOCB_WAKE:
+ case RCU_NOCB_WAKE_FORCE:
+ // If the type of deferred wake is "stronger"
+ // than it was before, make it wake up the soonest.
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);
+ mod_jif = 1;
+ break;
}

+ if (mod_jif)
+ mod_timer(&rdp_gp->nocb_timer, jiffies + mod_jif);
+
+ // If new type of wake up is stronger than before, promote.
+ 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);
@@ -296,7 +338,7 @@ 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 +352,9 @@ 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);
+
+ /* The lazy CBs are being flushed, but a new one might be enqueued. */
+ rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp, lazy);
rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
WRITE_ONCE(rdp->nocb_bypass_first, j);
rcu_nocb_bypass_unlock(rdp);
@@ -326,13 +370,20 @@ 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, bool wake_gp)
{
+ bool ret;
+
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);
+ ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, lazy);
+
+ if (wake_gp)
+ wake_nocb_gp(rdp, true);
+
+ return ret;
}

/*
@@ -345,7 +396,7 @@ 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 +418,14 @@ 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 +467,47 @@ 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, or there were lazy
+ // CBs previously queued and the current one is non-lazy.
+ //
+ // 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) ||
+ (lazy && n_lazy_cbs >= qhimark)) {
rcu_nocb_lock(rdp);
- *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
+
+ // This variable helps decide if a wakeup of the rcuog thread
+ // is needed. It is passed to __call_rcu_nocb_wake() by the
+ // caller. If only lazy CBs were previously queued and this one
+ // is non-lazy, make sure the caller does a wake up.
+ *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist) ||
+ (!lazy && n_lazy_cbs);
+
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, lazy, 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)) {
- *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
+ if (!rcu_nocb_flush_bypass(rdp, rhp, j, lazy, false)) {
+ *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist) ||
+ (!lazy && n_lazy_cbs);
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 +525,18 @@ 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);
+ rcu_cblist_enqueue(&rdp->nocb_bypass, rhp, lazy);
+
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 +569,7 @@ 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 +582,16 @@ 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 +682,8 @@ 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;
@@ -636,6 +719,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
*/
list_for_each_entry_rcu(rdp, &my_rdp->nocb_head_rdp, nocb_entry_rdp, 1) {
bool needwake_state = false;
+ bool flush_bypass = false;

if (!nocb_gp_enabled_cb(rdp))
continue;
@@ -648,22 +732,37 @@ 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) + jiffies_till_flush) ||
+ bypass_ncbs > 2 * qhimark)) {
+ flush_bypass = true;
+ } else if (bypass_ncbs && (lazy_ncbs != 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);
+ flush_bypass = true;
} 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 (flush_bypass) {
+ // 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);
+ }
+
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 +812,21 @@ 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 +1107,7 @@ 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, 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 +1608,14 @@ 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, bool wakegp)
{
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;
}
--
2.37.2.609.g9ff673ca1a-goog