commit 98c8910be748fa3a3f3ff6bff77b89177f5fd350
author Willy Tarreau <w@1wt.eu> Fri Jun 18 10:51:58 2021 +0200
committer Willy Tarreau <w@1wt.eu> Thu Jun 24 10:52:31 2021 +0200
tree 53a269a40f22508a95b8cc1c028d181e62645af7
parent 12529c0ed31cbdf1c83f36db3a52d5ff84cab3b7

MINOR: queue: use atomic-ops to update the queue's index (v2)

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 d9cae31..1aa3fb9 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);
 }
 
@@ -305,13 +305,13 @@
 	/* Let's switch from the server pendconn to the proxy pendconn */
 	__pendconn_unlink_prx(pp);
 	_HA_ATOMIC_DEC(&px->queue.length);
-	px->queue.idx++;
+	_HA_ATOMIC_INC(&px->queue.idx);
 	p = pp;
 	goto unlinked;
  use_p:
 	__pendconn_unlink_srv(p);
 	_HA_ATOMIC_DEC(&srv->queue.length);
-	srv->queue.idx++;
+	_HA_ATOMIC_INC(&srv->queue.idx);
  unlinked:
 	p->strm_flags |= SF_ASSIGNED;
 	p->target = srv;
@@ -419,6 +419,7 @@
 		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) {
@@ -428,7 +429,6 @@
 	__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);