[ltt-dev] [PATCH] urcu-qsbr: avoid useless futex wakeups and burning CPU for long grace periods

[Paolo Bonzini]

I noticed that urcu makes exactly _one_ attempt at using futexes
to avoid busy looping on synchronize_rcu.  The attached patch instead
switches from busy waiting to futexes after RCU_QS_ACTIVE_ATTEMPTS.
To limit the amount of system calls, reading threads remember whether
they already had a quiescent state in this grace period; if so they were
already removed from the list, and can avoid signaling the futex.

Performance measured with rcutorture (nreaders: 10, nupdaters: 1,
duration: 10, median of nine runs):

     RCU_QS_ACTIVE_ATTEMPTS == 100, no patch         n_updates = 302
     RCU_QS_ACTIVE_ATTEMPTS == 1, no patch           n_updates = 290
     RCU_QS_ACTIVE_ATTEMPTS == 100, with patch       n_updates = 408
     RCU_QS_ACTIVE_ATTEMPTS == 1, with patch         n_updates = 404

The first two cases are obviously the same; the only change is
when the futex is used, but over many calls there is no difference.

Signed-off-by: Paolo Bonzini  <pbonzini at redhat.com>
---
 urcu-qsbr.c             |   12 +++++++-----
 urcu/static/urcu-qsbr.h |    6 +++++-
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index a239be0..64a95e7 100644
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -150,26 +150,28 @@ static void update_counter_and_wait(void)
 	 */
 	for (;;) {
 		wait_loops++;
-		if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
-			uatomic_dec(&gp_futex);
+		if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
+			uatomic_set(&gp_futex, -1);
+			cds_list_for_each_entry(index, &registry, node) {
+				_CMM_STORE_SHARED(index->waiting, 1);
+			}
 			/* Write futex before read reader_gp */
 			cmm_smp_mb();
 		}
-
 		cds_list_for_each_entry_safe(index, tmp, &registry, node) {
 			if (!rcu_gp_ongoing(&index->ctr))
 				cds_list_move(&index->node, &qsreaders);
 		}
 
 		if (cds_list_empty(&registry)) {
-			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+			if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 				/* Read reader_gp before write futex */
 				cmm_smp_mb();
 				uatomic_set(&gp_futex, 0);
 			}
 			break;
 		} else {
-			if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+			if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
 				wait_gp();
 			} else {
 #ifndef HAS_INCOHERENT_CACHES
diff --git a/urcu/static/urcu-qsbr.h b/urcu/static/urcu-qsbr.h
index 5b7adac..b2c5fbf 100644
--- a/urcu/static/urcu-qsbr.h
+++ b/urcu/static/urcu-qsbr.h
@@ -124,6 +124,7 @@ struct rcu_reader {
 	unsigned long ctr;
 	/* Data used for registry */
 	struct cds_list_head node __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
+	int waiting;
 	pthread_t tid;
 };
 
@@ -136,7 +137,10 @@ extern int32_t gp_futex;
  */
 static inline void wake_up_gp(void)
 {
-	if (unlikely(uatomic_read(&gp_futex) == -1)) {
+	if (unlikely(_CMM_LOAD_SHARED(rcu_reader.waiting))) {
+		_CMM_STORE_SHARED(rcu_reader.waiting, 0);
+		if (uatomic_read(&gp_futex) != -1)
+			return;
 		uatomic_set(&gp_futex, 0);
 		futex_noasync(&gp_futex, FUTEX_WAKE, 1,
 		      NULL, NULL, 0);
-- 
1.7.6