rcu/tree: nocb: Avoid raising softirq when there are ready to execute CBs

During testing, I see it is possible that rcu_pending() returns 1 when
offloaded callbacks are ready to execute thus raising the RCU softirq.

However, softirq does not execute offloaded callbacks. They are executed in a
kthread which is awakened independent of the softirq.

This commit therefore avoids raising the softirq in the first place. That's
probably a good thing considering that the purpose of callback offloading is to
reduce softirq activity.

Passed 30 minute tests of TREE01 through TREE09 each.

On TREE08, I notice that there is atmost 150us from when the softirq was
NOT raised when ready cbs were present, to when the ready callbacks were
invoked by the rcuop thread. This also further confirms that there is no
need to raise the softirq for ready cbs in the first place.

Cc: neeraju@codeaurora.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
option-prefix PATCH v2
option-start

diff-note-start

v1->v2: Also cleaned up another test of the nocb configuration macro.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 948d266..6a731d3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3707,13 +3707,13 @@
 		return 1;
 
 	/* Does this CPU have callbacks ready to invoke? */
-	if (rcu_segcblist_ready_cbs(&rdp->cblist))
+	if (!rcu_segcblist_is_offloaded(&rdp->cblist) &&
+	    rcu_segcblist_ready_cbs(&rdp->cblist))
 		return 1;
 
 	/* Has RCU gone idle with this CPU needing another grace period? */
 	if (!gp_in_progress && rcu_segcblist_is_enabled(&rdp->cblist) &&
-	    (!IS_ENABLED(CONFIG_RCU_NOCB_CPU) ||
-	     !rcu_segcblist_is_offloaded(&rdp->cblist)) &&
+	    (!rcu_segcblist_is_offloaded(&rdp->cblist)) &&
 	    !rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
 		return 1;