[PATCH tip/core/rcu 10/20] rcu: Add functions to test for trivial grace periods

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Under some circumstances, RCU grace periods are zero cost.  For
RCU-preempt, this is the case during boot, and for RCU-bh and RCU-sched,
this is the case if there is only one CPU.  This means that RCU users
might wish to dispense with grace-period-avoidance strategies when
grace periods are zero cost, so this commit adds rcu_trivial_gp(),
rcu_bh_trivial_gp(), and rcu_sched_trivial_gp() to test for these
conditions.  Because the conditions leading to zero-cost grace periods
can change at any time (for example, when a second CPU is onlined), these
functions should be used as performance hints, and must not be relied
on for correctness.  For example, even if rcu_trivial_gp() returns true,
you are required to invoke synchronize_rcu().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rcupdate.h |  8 ++++++++
 include/linux/rcutiny.h  | 10 ++++++++++
 include/linux/rcutree.h  |  2 ++
 kernel/rcu/tree.c        | 24 ++++++++++++++++++++++++
 kernel/rcu/tree_plugin.h | 11 +++++++++++
 5 files changed, 55 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 01f71e1d2e94..a6222478b87d 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -293,6 +293,7 @@ void __rcu_read_lock(void);
 void __rcu_read_unlock(void);
 void rcu_read_unlock_special(struct task_struct *t);
 void synchronize_rcu(void);
+bool rcu_trivial_gp(void);
 
 /*
  * Defined as a macro as it is a very low level header included from
@@ -448,6 +449,13 @@ bool __rcu_is_watching(void);
 #define RCU_SCHEDULER_INIT	1
 #define RCU_SCHEDULER_RUNNING	2
 
+#ifndef CONFIG_PREEMPT_RCU
+static inline bool rcu_trivial_gp(void)
+{
+	return rcu_sched_trivial_gp();
+}
+#endif /* #ifndef CONFIG_PREEMPT_RCU */
+
 /*
  * init_rcu_head_on_stack()/destroy_rcu_head_on_stack() are needed for dynamic
  * initialization and destruction of rcu_head on the stack. rcu_head structures
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index ac81e4063b40..a77dafe79813 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -82,6 +82,16 @@ static inline void synchronize_sched_expedited(void)
 	synchronize_sched();
 }
 
+static inline bool rcu_sched_trivial_gp(void)
+{
+	return true;
+}
+
+static inline bool rcu_bh_trivial_gp(void)
+{
+	return true;
+}
+
 static inline void kfree_call_rcu(struct rcu_head *head,
 				  rcu_callback_t func)
 {
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 63a4e4cf40a5..fcd61cb08851 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -47,6 +47,8 @@ static inline void rcu_virt_note_context_switch(int cpu)
 void synchronize_rcu_bh(void);
 void synchronize_sched_expedited(void);
 void synchronize_rcu_expedited(void);
+bool rcu_sched_trivial_gp(void);
+bool rcu_bh_trivial_gp(void);
 
 void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
 
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d7b63b88434b..ed5a17aca281 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3293,6 +3293,18 @@ void synchronize_sched(void)
 EXPORT_SYMBOL_GPL(synchronize_sched);
 
 /**
+ * rcu_sched_trivial_gp - Are RCU-sched grace periods trivially zero cost?
+ *
+ * Returns true if RCU-sched grace periods are currently zero cost, which
+ * they are if there is only one CPU.  Note that unless you take steps to
+ * prevent it, the number of CPUs might change at any time.
+ */
+bool rcu_sched_trivial_gp(void)
+{
+	return rcu_blocking_is_gp();
+}
+
+/**
  * synchronize_rcu_bh - wait until an rcu_bh grace period has elapsed.
  *
  * Control will return to the caller some time after a full rcu_bh grace
@@ -3320,6 +3332,18 @@ void synchronize_rcu_bh(void)
 EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
 
 /**
+ * rcu_bh_trivial_gp - Are RCU-bh grace periods trivially zero cost?
+ *
+ * Returns true if RCU-bh grace periods are currently zero cost, which
+ * they are if there is only one CPU.  Note that unless you take steps to
+ * prevent it, the number of CPUs might change at any time.
+ */
+bool rcu_bh_trivial_gp(void)
+{
+	return rcu_blocking_is_gp();
+}
+
+/**
  * get_state_synchronize_rcu - Snapshot current RCU state
  *
  * Returns a cookie that is used by a later call to cond_synchronize_rcu()
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 56583e764ebf..e92d67a3fad2 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -680,6 +680,17 @@ void synchronize_rcu(void)
 EXPORT_SYMBOL_GPL(synchronize_rcu);
 
 /**
+ * rcu_trivial_gp - Are RCU grace periods trivially zero cost?
+ *
+ * Returns true if RCU grace periods are currently zero cost, which
+ * they are during boot.
+ */
+bool rcu_trivial_gp(void)
+{
+	return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
+}
+
+/**
  * rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
  *
  * Note that this primitive does not necessarily wait for an RCU grace period
-- 
2.5.2