rcu/nocb: Avoid needless wakeups of no-CBs grace-period kthread

Currently, the code provides an extra wakeup for the no-CBs grace-period
kthread if one of its CPUs is generating excessive numbers of callbacks.
But satisfying though it is to wake something up when things are going
south, unless the thing being awakened can actually help solve the
problem, that extra wakeup does nothing but consume additional CPU time,
which is exactly what you don't want during a call_rcu() flood.

This commit therefore avoids doing anything if the corresponding
no-CBs callback kthread is going full tilt.  Otherwise, if advancing
callbacks immediately might help and if the leaf rcu_node structure's
lock is immediately available, this commit invokes a new variant of
rcu_advance_cbs() that advances callbacks only if doing so won't require
awakening the grace-period kthread (not to be confused with any of the
no-CBs grace-period kthreads).
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree.c

@@ -1334,6 +1334,19 @@ static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
 	return rcu_accelerate_cbs(rnp, rdp);
 }
 
+/*
+ * Move and classify callbacks, but only if doing so won't require
+ * that the RCU grace-period kthread be awakened.
+ */
+static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp,
+						  struct rcu_data *rdp)
+{
+	raw_lockdep_assert_held_rcu_node(rnp);
+	if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
+		return;
+	WARN_ON_ONCE(rcu_advance_cbs(rnp, rdp));
+}
+
 /*
  * Update CPU-local rcu_data state to record the beginnings and ends of
  * grace periods.  The caller must hold the ->lock of the leaf rcu_node
@@ -2118,6 +2131,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
 			      rcu_segcblist_n_lazy_cbs(&rdp->cblist),
 			      rcu_segcblist_n_cbs(&rdp->cblist), bl);
 	rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);
+	if (offloaded)
+		rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
 	rcu_nocb_unlock_irqrestore(rdp, flags);
 
 	/* Invoke callbacks. */

kernel/rcu/tree_plugin.h

@@ -1643,10 +1643,15 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
 	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
 		/* ... or if many callbacks queued. */
 		rdp->qlen_last_fqs_check = len;
-		if (!irqs_disabled_flags(flags)) {
-			wake_nocb_gp(rdp, true, flags);
-			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
-					    TPS("WakeOvf"));
+		if (!rdp->nocb_cb_sleep &&
+		    rcu_segcblist_ready_cbs(&rdp->cblist)) {
+			// Already going full tilt, so don't try to rewake.
+			rcu_nocb_unlock_irqrestore(rdp, flags);
+		} else if (rcu_segcblist_pend_cbs(&rdp->cblist) &&
+			   raw_spin_trylock_rcu_node(rdp->mynode)) {
+			rcu_advance_cbs_nowake(rdp->mynode, rdp);
+			raw_spin_unlock_rcu_node(rdp->mynode);
+			rcu_nocb_unlock_irqrestore(rdp, flags);
 		} else {
 			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
 					   TPS("WakeOvfIsDeferred"));