jbd2: don't wake kjournald unnecessarily

Don't send an extra wakeup to kjournald in the case where we
already have the proper target in j_commit_request, i.e. that
transaction has already been requested for commit.

commit deeeaf13 "jbd2: fix fsync() tid wraparound bug" changed
the logic leading to a wakeup, but it caused some extra wakeups
which were found to lead to a measurable performance regression.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[tytso@mit.edu: reworked check to make it clearer]
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/jbd2/journal.c

@@ -513,6 +513,10 @@ int __jbd2_log_space_left(journal_t *journal)
  */
 int __jbd2_log_start_commit(journal_t *journal, tid_t target)
 {
+	/* Return if the txn has already requested to be committed */
+	if (journal->j_commit_request == target)
+		return 0;
+
 	/*
 	 * The only transaction we can possibly wait upon is the
 	 * currently running transaction (if it exists).  Otherwise,