commit 1335eb9867b76c8e4570ad4242dc728287af3d56
author Willy Tarreau <w@1wt.eu> Fri Jun 18 10:51:58 2021 +0200
committer Willy Tarreau <w@1wt.eu> Tue Jun 22 18:57:18 2021 +0200
tree eff5cca31b70e854d8f94b1d0a8eeb689682ce9a
parent 3eecdb65c5a6b933399ebb0ac4ef86a7b97cf85d

MINOR: queue: use atomic-ops to update the queue's index

Doing so allows to retrieve and update the pendconn's queue index outside
of the queue's lock and to save one more percent CPU on a highly-contented
backend.

src/queue.c

diff --git a/src/queue.c b/src/queue.c
index 9208911..20880a2 100644
--- a/src/queue.c
+++ b/src/queue.c
@@ -133,7 +133,7 @@
  */
 static void __pendconn_unlink_srv(struct pendconn *p)
 {
-	p->strm->logs.srv_queue_pos += p->srv->queue.idx - p->queue_idx;
+	p->strm->logs.srv_queue_pos += _HA_ATOMIC_LOAD(&p->srv->queue.idx) - p->queue_idx;
 	eb32_delete(&p->node);
 }
 
@@ -146,7 +146,7 @@
  */
 static void __pendconn_unlink_prx(struct pendconn *p)
 {
-	p->strm->logs.prx_queue_pos += p->px->queue.idx - p->queue_idx;
+	p->strm->logs.prx_queue_pos += _HA_ATOMIC_LOAD(&p->px->queue.idx) - p->queue_idx;
 	eb32_delete(&p->node);
 }
 
@@ -313,16 +313,16 @@
  use_pp:
 	/* Let's switch from the server pendconn to the proxy pendconn */
 	__pendconn_unlink_prx(pp);
+	_HA_ATOMIC_INC(&px->queue.idx);
 	_HA_ATOMIC_DEC(&px->queue.length);
 	_HA_ATOMIC_DEC(&px->totpend);
-	px->queue.idx++;
 	p = pp;
 	goto unlinked;
  use_p:
 	__pendconn_unlink_srv(p);
+	_HA_ATOMIC_INC(&srv->queue.idx);
 	_HA_ATOMIC_DEC(&srv->queue.length);
 	_HA_ATOMIC_DEC(&px->totpend);
-	srv->queue.idx++;
  unlinked:
 	p->strm_flags |= SF_ASSIGNED;
 	p->target = srv;
@@ -422,6 +422,7 @@
 		max_ptr = &px->be_counters.nbpend_max;
 	}
 
+	p->queue_idx  = _HA_ATOMIC_LOAD(&q->idx) - 1; // for logging only
 	new_max = _HA_ATOMIC_ADD_FETCH(&q->length, 1);
 	old_max = _HA_ATOMIC_LOAD(max_ptr);
 	while (new_max > old_max) {
@@ -431,7 +432,6 @@
 	__ha_barrier_atomic_store();
 
 	HA_SPIN_LOCK(QUEUE_LOCK, &q->lock);
-	p->queue_idx = q->idx - 1; // for increment
 	eb32_insert(&q->head, &p->node);
 	HA_SPIN_UNLOCK(QUEUE_LOCK, &q->lock);