From: Willy Tarreau <w@1wt.eu>
Date: Thu, 29 Feb 2024 10:57:28 +0000 (+0100)
Subject: OPTIM: ring: don't even try to update offset when failed to read
X-Git-Tag: v3.0-dev6~17
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=1f8b14b7be51efcfe3c57fe9aecdc88c7acf3f5c;p=thirdparty%2Fhaproxy.git

OPTIM: ring: don't even try to update offset when failed to read

If there's nothing to read, it's pointless for a reader to try to update
the offset pointer, that's two atomic ops to replace a value by itself
twice. Let's just stop this.
---

diff --git a/src/ring.c b/src/ring.c
index 826662aea3..31b75a13fe 100644
--- a/src/ring.c
+++ b/src/ring.c
@@ -538,17 +538,19 @@ int ring_dispatch_messages(struct ring *ring, void *ctx, size_t *ofs_ptr, size_t
 
 	vp_data_to_ring(v1, v2, (char *)ring_area, ring_size, &head_ofs, &tail_ofs);
 
-	/* inc readers count on new place */
-	do {
-		readers = _HA_ATOMIC_LOAD(ring_area + head_ofs);
-	} while ((readers > RING_MAX_READERS ||
-		  !_HA_ATOMIC_CAS(ring_area + head_ofs, &readers, readers + 1)) && __ha_cpu_relax());
+	if (head_ofs != prev_ofs) {
+		/* inc readers count on new place */
+		do {
+			readers = _HA_ATOMIC_LOAD(ring_area + head_ofs);
+		} while ((readers > RING_MAX_READERS ||
+			  !_HA_ATOMIC_CAS(ring_area + head_ofs, &readers, readers + 1)) && __ha_cpu_relax());
 
-	/* dec readers count on old place */
-	do {
-		readers = _HA_ATOMIC_LOAD(ring_area + prev_ofs);
-	} while ((readers > RING_MAX_READERS ||
-		  !_HA_ATOMIC_CAS(ring_area + prev_ofs, &readers, readers - 1)) && __ha_cpu_relax());
+		/* dec readers count on old place */
+		do {
+			readers = _HA_ATOMIC_LOAD(ring_area + prev_ofs);
+		} while ((readers > RING_MAX_READERS ||
+			  !_HA_ATOMIC_CAS(ring_area + prev_ofs, &readers, readers - 1)) && __ha_cpu_relax());
+	}
 
 	if (last_ofs_ptr)
 		*last_ofs_ptr = tail_ofs;