rcu: Add tracing of normal (non-NOCB) grace-period requests

This commit adds tracing to the normal grace-period request points.
These are rcu_gp_cleanup(), which checks for the need for another
grace period at the end of the previous grace period, and
rcu_start_gp_advanced(), which restarts RCU's state machine after
an idle period.  These trace events are intended to help track down
bugs where RCU remains idle despite there being work for it to do.
Reported-by: Clark Williams <williams@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/trace/events/rcu.h

@@ -45,6 +45,7 @@ TRACE_EVENT(rcu_utilization,
  *
  *	"AccReadyCB": CPU acclerates new callbacks to RCU_NEXT_READY_TAIL.
  *	"AccWaitCB": CPU accelerates new callbacks to RCU_WAIT_TAIL.
+ *	"newreq": Request a new grace period.
  *	"start": Start a grace period.
  *	"cpustart": CPU first notices a grace-period start.
  *	"cpuqs": CPU passes through a quiescent state.

kernel/rcutree.c

@@ -1459,8 +1459,12 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	rsp->fqs_state = RCU_GP_IDLE;
 	rdp = this_cpu_ptr(rsp->rda);
 	rcu_advance_cbs(rsp, rnp, rdp);  /* Reduce false positives below. */
-	if (cpu_needs_another_gp(rsp, rdp))
+	if (cpu_needs_another_gp(rsp, rdp)) {
 		rsp->gp_flags = 1;
+		trace_rcu_grace_period(rsp->name,
+				       ACCESS_ONCE(rsp->gpnum),
+				       TPS("newreq"));
+	}
 	raw_spin_unlock_irq(&rnp->lock);
 }
 
@@ -1584,6 +1588,8 @@ rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
 		return;
 	}
 	rsp->gp_flags = RCU_GP_FLAG_INIT;
+	trace_rcu_grace_period(rsp->name, ACCESS_ONCE(rsp->gpnum),
+			       TPS("newreq"));
 
 	/*
 	 * We can't do wakeups while holding the rnp->lock, as that