xfs: avoid cil push lock if possible

Because now it hurts when the CIL fills up.

  - 37.20% __xfs_trans_commit
      - 35.84% xfs_log_commit_cil
         - 19.34% _raw_spin_lock
            - do_raw_spin_lock
                 19.01% __pv_queued_spin_lock_slowpath
         - 4.20% xfs_log_ticket_ungrant
              0.90% xfs_log_space_wake
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_log_cil.c

@@ -1422,10 +1422,18 @@ xlog_cil_push_background(
 	ASSERT(!test_bit(XLOG_CIL_EMPTY, &cil->xc_flags));
 
 	/*
-	 * Don't do a background push if we haven't used up all the
-	 * space available yet.
+	 * We are done if:
+	 * - we haven't used up all the space available yet; or
+	 * - we've already queued up a push; and
+	 * - we're not over the hard limit; and
+	 * - nothing has been over the hard limit.
+	 *
+	 * If so, we don't need to take the push lock as there's nothing to do.
 	 */
-	if (space_used < XLOG_CIL_SPACE_LIMIT(log)) {
+	if (space_used < XLOG_CIL_SPACE_LIMIT(log) ||
+	    (cil->xc_push_seq == cil->xc_current_sequence &&
+	     space_used < XLOG_CIL_BLOCKING_SPACE_LIMIT(log) &&
+	     !waitqueue_active(&cil->xc_push_wait))) {
 		up_read(&cil->xc_ctx_lock);
 		return;
 	}