channelz: use atomic instead of mutex (#2218)

rpc_util.go

@@ -750,6 +750,19 @@ func parseDialTarget(target string) (net string, addr string) {
 	return net, target
 }
 
+// channelzData is used to store channelz related data for ClientConn, addrConn and Server.
+// These fields cannot be embedded in the original structs (e.g. ClientConn), since to do atomic
+// operation on int64 variable on 32-bit machine, user is responsible to enforce memory alignment.
+// Here, by grouping those int64 fields inside a struct, we are enforcing the alignment.
+type channelzData struct {
+	callsStarted   int64
+	callsFailed    int64
+	callsSucceeded int64
+	// lastCallStartedTime stores the timestamp that last call starts. It is of int64 type instead of
+	// time.Time since it's more costly to atomically update time.Time variable than int64 variable.
+	lastCallStartedTime int64
+}
+
 // The SupportPackageIsVersion variables are referenced from generated protocol
 // buffer files to ensure compatibility with the gRPC version used.  The latest
 // support package version is 5.