[Qemu-devel] [RFC PATCH 09/13] rcu: avoid repeated system calls

[Paolo Bonzini <pbonzini@redhat.com>]

Optimization posted to upstream mailing list.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rcu.c |    5 +++++
 rcu.h |   22 ++++++++++++----------
 2 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/rcu.c b/rcu.c
index e2f347a..aaf53ad 100644
--- a/rcu.c
+++ b/rcu.c
@@ -91,6 +91,11 @@ static void update_counter_and_wait(void)
 		 * while we walk the list.
 		 */
 		qemu_event_reset(&rcu_gp_event);
+		QLIST_FOREACH(index, &registry, node) {
+			ACCESS_ONCE(index->waiting) = true;
+		}
+		__sync_synchronize();
+
 		QLIST_FOREACH_SAFE(index, &registry, node, tmp) {
 			if (!rcu_gp_ongoing(&index->ctr)) {
 				QLIST_REMOVE(index, node);
diff --git a/rcu.h b/rcu.h
index 86a1fea..96ce9d3 100644
--- a/rcu.h
+++ b/rcu.h
@@ -74,6 +74,8 @@ extern QemuEvent rcu_gp_event;
 struct rcu_reader {
 	/* Data used by both reader and synchronize_rcu() */
 	unsigned long ctr;
+	bool waiting;
+
 	/* Data used for registry */
 	QLIST_ENTRY(rcu_reader) node;
 };
@@ -93,24 +95,24 @@ static inline void rcu_quiescent_state(void)
 {
 	smp_mb();
 	ACCESS_ONCE(rcu_reader.ctr) = ACCESS_ONCE(rcu_gp_ctr);
-#if 0
-	/* write rcu_reader.ctr before read futex.  Included in
-	 * qemu_event_set. */
+	/* write rcu_reader.ctr before read waiting.  */
 	smp_mb();
-#endif
-	qemu_event_set(&rcu_gp_event);
+	if (ACCESS_ONCE(rcu_reader.waiting)) {
+		ACCESS_ONCE(rcu_reader.waiting) = false;
+		qemu_event_set(&rcu_gp_event);
+	}
 }
 
 static inline void rcu_thread_offline(void)
 {
 	smp_mb();
 	ACCESS_ONCE(rcu_reader.ctr) = 0;
-#if 0
-	/* write rcu_reader.ctr before read futex.  Included in
-	 * qemu_event_set. */
+	/* write rcu_reader.ctr before read waiting.  */
 	smp_mb();
-#endif
-	qemu_event_set(&rcu_gp_event);
+	if (ACCESS_ONCE(rcu_reader.waiting)) {
+		ACCESS_ONCE(rcu_reader.waiting) = false;
+		qemu_event_set(&rcu_gp_event);
+	}
 }
 
 static inline void rcu_thread_online(void)
-- 
1.7.6