rcu: Refactor rcu_barrier() empty-list handling

This commit saves a few lines by checking first for an empty callback
list. If the callback list is empty, then that CPU is taken care of,
regardless of its online or nocb state. Also simplify tracing accordingly
and fold a few lines together.

Signed-off-by: Paul E. McKenney <[e-mail]>

include/trace/events/rcu.h

···
  * Tracepoint for rcu_barrier() execution.  The string "s" describes
  * the rcu_barrier phase:
  *	"Begin": rcu_barrier() started.
+ *	"CB": An rcu_barrier_callback() invoked a callback, not the last.
  *	"EarlyExit": rcu_barrier() piggybacked, thus early exit.
  *	"Inc1": rcu_barrier() piggyback check counter incremented.
- *	"OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
- *	"OnlineQ": rcu_barrier() found online CPU with callbacks.
- *	"OnlineNQ": rcu_barrier() found online CPU, no callbacks.
+ *	"Inc2": rcu_barrier() piggyback check counter incremented.
  *	"IRQ": An rcu_barrier_callback() callback posted on remote CPU.
  *	"IRQNQ": An rcu_barrier_callback() callback found no callbacks.
- *	"CB": An rcu_barrier_callback() invoked a callback, not the last.
  *	"LastCB": An rcu_barrier_callback() invoked the last callback.
- *	"Inc2": rcu_barrier() piggyback check counter incremented.
+ *	"NQ": rcu_barrier() found a CPU with no callbacks.
+ *	"OnlineQ": rcu_barrier() found online CPU with callbacks.
  * The "cpu" argument is the CPU or -1 if meaningless, the "cnt" argument
  * is the count of remaining callbacks, and "done" is the piggybacking count.
  */

kernel/rcu/tree.c

···
 
 	/* Did someone else do our work for us? */
 	if (rcu_seq_done(&rcu_state.barrier_sequence, s)) {
-		rcu_barrier_trace(TPS("EarlyExit"), -1,
-				  rcu_state.barrier_sequence);
+		rcu_barrier_trace(TPS("EarlyExit"), -1, rcu_state.barrier_sequence);
 		smp_mb(); /* caller's subsequent code after above check. */
 		mutex_unlock(&rcu_state.barrier_mutex);
 		return;
···
 	 */
 	for_each_possible_cpu(cpu) {
 		rdp = per_cpu_ptr(&rcu_data, cpu);
-		if (cpu_is_offline(cpu) &&
-		    !rcu_rdp_is_offloaded(rdp))
+		if (!rcu_segcblist_n_cbs(&rdp->cblist)) {
+			rcu_barrier_trace(TPS("NQ"), cpu, rcu_state.barrier_sequence);
 			continue;
-		if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
-			rcu_barrier_trace(TPS("OnlineQ"), cpu,
-					  rcu_state.barrier_sequence);
+		}
+		if (cpu_online(cpu)) {
+			rcu_barrier_trace(TPS("OnlineQ"), cpu, rcu_state.barrier_sequence);
 			smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
-		} else if (rcu_segcblist_n_cbs(&rdp->cblist) &&
-			   cpu_is_offline(cpu)) {
-			rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
-					  rcu_state.barrier_sequence);
+		} else {
+			rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu, rcu_state.barrier_sequence);
 			local_irq_disable();
 			rcu_barrier_func((void *)cpu);
 			local_irq_enable();
-		} else if (cpu_is_offline(cpu)) {
-			rcu_barrier_trace(TPS("OfflineNoCBNoQ"), cpu,
-					  rcu_state.barrier_sequence);
-		} else {
-			rcu_barrier_trace(TPS("OnlineNQ"), cpu,
-					  rcu_state.barrier_sequence);
 		}
 	}
 	cpus_read_unlock();