rcu: Trace callback acceleration

This commit adds event tracing for callback acceleration to allow better
tracking of callbacks through the system.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index ac6a75d..e9dce4f 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1168,6 +1168,12 @@
 		rdp->nxttail[i] = rdp->nxttail[RCU_NEXT_TAIL];
 		rdp->nxtcompleted[i] = c;
 	}
+
+	/* Trace depending on how much we were able to accelerate. */
+	if (!*rdp->nxttail[RCU_WAIT_TAIL])
+		trace_rcu_grace_period(rsp->name, rdp->gpnum, "AccWaitCB");
+	else
+		trace_rcu_grace_period(rsp->name, rdp->gpnum, "AccReadyCB");
 }
 
 /*