Associate a simple count of waiters with each condition variable. The

count is protected by the mutex that protects the condition, so the count
does not require any extra locking or atomic operations.  It serves as an
optimization to avoid calling into the sleepqueue code at all if there are
no waiters.

Note that the count can get temporarily out of sync when threads sleeping
on a condition variable time out or are aborted.  However, it doesn't hurt
to call the sleepqueue code for either a signal or a broadcast when there
are no waiters, and the count is never out of sync in the opposite
direction unless we have more than INT_MAX sleeping threads.

1 files changed, 13 insertions, 2 deletions

diff --git a/sys/kern/kern_condvar.c b/sys/kern/kern_condvar.c
index 6fff7e0..2b7bd97 100644
--- a/sys/kern/kern_condvar.c
+++ b/sys/kern/kern_condvar.c
@@ -65,6 +65,7 @@ cv_init(struct cv *cvp, const char *desc)
 {
 
 	cvp->cv_description = desc;
+	cvp->cv_waiters = 0;
 }
 
 /*
@@ -119,6 +120,7 @@ cv_wait(struct cv *cvp, struct mtx *mp)
 
 	sq = sleepq_lookup(cvp);
 
+	cvp->cv_waiters++;
 	DROP_GIANT();
 	mtx_unlock(mp);
 
@@ -175,6 +177,7 @@ cv_wait_sig(struct cv *cvp, struct mtx *mp)
 
 	/* XXX: Missing the threading checks from msleep! */
 
+	cvp->cv_waiters++;
 	DROP_GIANT();
 	mtx_unlock(mp);
 
@@ -241,6 +244,7 @@ cv_timedwait(struct cv *cvp, struct mtx *mp, int timo)
 
 	sq = sleepq_lookup(cvp);
 
+	cvp->cv_waiters++;
 	DROP_GIANT();
 	mtx_unlock(mp);
 
@@ -299,6 +303,7 @@ cv_timedwait_sig(struct cv *cvp, struct mtx *mp, int timo)
 
 	sq = sleepq_lookup(cvp);
 
+	cvp->cv_waiters++;
 	DROP_GIANT();
 	mtx_unlock(mp);
 
@@ -341,7 +346,10 @@ void
 cv_signal(struct cv *cvp)
 {
 
-	sleepq_signal(cvp, SLEEPQ_CONDVAR, -1);
+	if (cvp->cv_waiters > 0) {
+		cvp->cv_waiters--;
+		sleepq_signal(cvp, SLEEPQ_CONDVAR, -1);
+	}
 }
 
 /*
@@ -352,5 +360,8 @@ void
 cv_broadcastpri(struct cv *cvp, int pri)
 {
 
-	sleepq_broadcast(cvp, SLEEPQ_CONDVAR, pri);
+	if (cvp->cv_waiters > 0) {
+		cvp->cv_waiters = 0;
+		sleepq_broadcast(cvp, SLEEPQ_CONDVAR, pri);
+	}
 }