linux-kernel - [PATCH rcu 2/5] rcu: Refactor rcu

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Date:   Fri,  4 Feb 2022 15:20:33 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 2/5] 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 <paulmck@...nel.org>
---
 include/trace/events/rcu.h |  9 ++++-----
 kernel/rcu/tree.c          | 25 ++++++++-----------------
 2 files changed, 12 insertions(+), 22 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 670e41783edd8..90b2fb0292cb1 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -794,16 +794,15 @@ TRACE_EVENT_RCU(rcu_torture_read,
  * 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.
  */
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8f6cf86a76c2d..87ea516f87779 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4030,8 +4030,7 @@ void rcu_barrier(void)
 
 	/* 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;
@@ -4059,26 +4058,18 @@ void rcu_barrier(void)
 	 */
 	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();
-- 
2.31.1.189.g2e36527f23