rcu: Add more tracing of expedited grace periods

This commit adds more tracing of expedited grace periods to enable
improved debugging of slowdowns.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/trace/events/rcu.h

@@ -179,6 +179,9 @@ TRACE_EVENT(rcu_grace_period_init,
  *
  *	"snap": Captured snapshot of expedited grace period sequence number.
  *	"start": Started a real expedited grace period.
+ *	"reset": Started resetting the tree
+ *	"select": Started selecting the CPUs to wait on.
+ *	"startwait": Started waiting on selected CPUs.
  *	"end": Ended a real expedited grace period.
  *	"endwake": Woke piggybackers up.
  *	"done": Someone else did the expedited grace period for us.

kernel/rcu/rcu.h

@@ -77,12 +77,18 @@ static inline void rcu_seq_start(unsigned long *sp)
 	WARN_ON_ONCE(rcu_seq_state(*sp) != 1);
 }
 
+/* Compute the end-of-grace-period value for the specified sequence number. */
+static inline unsigned long rcu_seq_endval(unsigned long *sp)
+{
+	return (*sp | RCU_SEQ_STATE_MASK) + 1;
+}
+
 /* Adjust sequence number for end of update-side operation. */
 static inline void rcu_seq_end(unsigned long *sp)
 {
 	smp_mb(); /* Ensure update-side operation before counter increment. */
 	WARN_ON_ONCE(!rcu_seq_state(*sp));
-	WRITE_ONCE(*sp, (*sp | RCU_SEQ_STATE_MASK) + 1);
+	WRITE_ONCE(*sp, rcu_seq_endval(sp));
 }
 
 /* Take a snapshot of the update side's sequence number. */

kernel/rcu/tree_exp.h

@@ -28,6 +28,15 @@ static void rcu_exp_gp_seq_start(struct rcu_state *rsp)
 	rcu_seq_start(&rsp->expedited_sequence);
 }
 
+/*
+ * Return then value that expedited-grace-period counter will have
+ * at the end of the current grace period.
+ */
+static unsigned long rcu_exp_gp_seq_endval(struct rcu_state *rsp)
+{
+	return rcu_seq_endval(&rsp->expedited_sequence);
+}
+
 /*
  * Record the end of an expedited grace period.
  */
@@ -366,7 +375,9 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
 	int ret;
 	struct rcu_node *rnp;
 
+	trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("reset"));
 	sync_exp_reset_tree(rsp);
+	trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("select"));
 	rcu_for_each_leaf_node(rsp, rnp) {
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 
@@ -443,6 +454,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 	struct rcu_node *rnp_root = rcu_get_root(rsp);
 	int ret;
 
+	trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("startwait"));
 	jiffies_stall = rcu_jiffies_till_stall_check();
 	jiffies_start = jiffies;