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

[Paolo Bonzini]

Optimization posted to upstream mailing list.

Signed-off-by: Paolo Bonzini <pbonz...@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