rcu: Add functions to test for trivial grace periods

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>

2 files changed, 35 insertions, 0 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 991c62f2f1ca..50cb208c64fe 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3418,6 +3418,18 @@ 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
@@ -3445,6 +3457,18 @@ 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 98cd997b078e..69c6eb27c37f 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_barrier - Wait until all in-flight call_rcu() callbacks complete.
  *
  * Note that this primitive does not necessarily wait for an RCU grace period