logging: Attempt a synchronous, non-blocking write first (file, stderr)

In the old days, we performed synchronous, blocking writes to the log
file or stderr.  This was replaced by code that turned all log
file/stderr writes into non-blocking writes behind a write_queue.

This patch now introduces a further optimization: If we currently
don't have any log messages pending in the write queue, we are not
back-logged and assume we have a fair chance of writing the log message
right now, synchronously.  So we try that first, and only enqueue
the log message if the write fails (no bytes or insufficient number
of bytes written).

This way we should get the best of both worlds: No delay/re-ordering
(and lower select syscall load) for the "normal" case (benefits of
the old synchronous writes) while at the same time never risking to
block on log output.

Related: OS#4311
Change-Id: I08469a7e4be9bc5bbd39140457bb582f4a0b1703

1 files changed, 30 insertions, 17 deletions

diff --git a/src/logging.c b/src/logging.c
index 9b2a589a..1ab31431 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -905,6 +905,24 @@ void log_set_category_filter(struct log_target *target, int category,
 }
 
 #if (!EMBEDDED)
+/* write-queue tells us we should write another msgb (log line) to the output fd */
+static int _file_wq_write_cb(struct osmo_fd *ofd, struct msgb *msg)
+{
+	int rc;
+
+	rc = write(ofd->fd, msgb_data(msg), msgb_length(msg));
+	if (rc < 0)
+		return rc;
+	if (rc != msgb_length(msg)) {
+		/* pull the number of bytes we have already written */
+		msgb_pull(msg, rc);
+		/* ask write_queue to re-insert the msgb at the head of the queue */
+		return -EAGAIN;
+	}
+	return 0;
+}
+
+/* output via buffered, blocking stdio streams */
 static void _file_output_stream(struct log_target *target, unsigned int level,
 			 const char *log)
 {
@@ -930,6 +948,18 @@ static void _file_raw_output(struct log_target *target, int subsys, unsigned int
 	 * and call _file_wq_write_cb() */
 	rc = _output_buf((char *)msgb_data(msg), msgb_tailroom(msg), target, subsys, level, file, line, cont, format, ap);
 	msgb_put(msg, rc);
+
+	/* attempt a synchronous, non-blocking write, if the write queue is empty */
+	if (target->tgt_file.wqueue->current_length == 0) {
+		rc = _file_wq_write_cb(&target->tgt_file.wqueue->bfd, msg);
+		if (rc == 0) {
+			/* the write was complete, we can exit early */
+			msgb_free(msg);
+			return;
+		}
+	}
+	/* if we reach here, either we already had elements in the write_queue, or the synchronous write
+	 * failed: enqueue the message to the write_queue (backlog) */
 	osmo_wqueue_enqueue_quiet(target->tgt_file.wqueue, msg);
 }
 #endif
@@ -1001,23 +1031,6 @@ struct log_target *log_target_create_stderr(void)
 }
 
 #if (!EMBEDDED)
-/* write-queue tells us we should write another msgb (log line) to the output fd */
-static int _file_wq_write_cb(struct osmo_fd *ofd, struct msgb *msg)
-{
-	int rc;
-
-	rc = write(ofd->fd, msgb_data(msg), msgb_length(msg));
-	if (rc < 0)
-		return rc;
-	if (rc != msgb_length(msg)) {
-		/* pull the number of bytes we have already written */
-		msgb_pull(msg, rc);
-		/* ask write_queue to re-insert the msgb at the head of the queue */
-		return -EAGAIN;
-	}
-	return 0;
-}
-
 /*! Create a new file-based log target using buffered, blocking stream output
  *  \param[in] fname File name of the new log file
  *  \returns Log target in case of success, NULL otherwise