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>

include/trace/events/rcu.h

@@ -44,8 +44,10 @@ TRACE_EVENT(rcu_utilization,
  * of a new grace period or the end of an old grace period ("cpustart"
  * and "cpuend", respectively), a CPU passing through a quiescent
  * state ("cpuqs"), a CPU coming online or going offline ("cpuonl"
- * and "cpuofl", respectively), and a CPU being kicked for being too
- * long in dyntick-idle mode ("kick").
+ * and "cpuofl", respectively), a CPU being kicked for being too
+ * long in dyntick-idle mode ("kick"), a CPU accelerating its new
+ * callbacks to RCU_NEXT_READY_TAIL ("AccReadyCB"), and a CPU
+ * accelerating its new callbacks to RCU_WAIT_TAIL ("AccWaitCB").
  */
 TRACE_EVENT(rcu_grace_period,
 

kernel/rcutree.c

@@ -1168,6 +1168,12 @@ static void rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
 		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");
 }
 
 /*