GFS2: Wait properly when flushing the ail list

The ail flush code has always relied upon log flushing to prevent
it from spinning needlessly. This fixes it to wait on the last
I/O request submitted (we don't need to wait for all of it)
instead of either spinning with io_schedule or sleeping.

As a result cpu usage of gfs2_logd is much reduced with certain
workloads.
Reported-by: Abhijith Das <adas@redhat.com>
Tested-by: Abhijith Das <adas@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/log.c

@@ -228,6 +228,27 @@ static int gfs2_ail1_empty(struct gfs2_sbd *sdp)
 	return ret;
 }
 
+static void gfs2_ail1_wait(struct gfs2_sbd *sdp)
+{
+	struct gfs2_ail *ai;
+	struct gfs2_bufdata *bd;
+	struct buffer_head *bh;
+
+	spin_lock(&sdp->sd_ail_lock);
+	list_for_each_entry_reverse(ai, &sdp->sd_ail1_list, ai_list) {
+		list_for_each_entry(bd, &ai->ai_ail1_list, bd_ail_st_list) {
+			bh = bd->bd_bh;
+			if (!buffer_locked(bh))
+				continue;
+			get_bh(bh);
+			spin_unlock(&sdp->sd_ail_lock);
+			wait_on_buffer(bh);
+			brelse(bh);
+			return;
+		}
+	}
+	spin_unlock(&sdp->sd_ail_lock);
+}
 
 /**
  * gfs2_ail2_empty_one - Check whether or not a trans in the AIL has been synced
@@ -878,9 +899,9 @@ void gfs2_meta_syncfs(struct gfs2_sbd *sdp)
 	gfs2_log_flush(sdp, NULL);
 	for (;;) {
 		gfs2_ail1_start(sdp);
+		gfs2_ail1_wait(sdp);
 		if (gfs2_ail1_empty(sdp))
 			break;
-		msleep(10);
 	}
 }
 
@@ -920,12 +941,14 @@ int gfs2_logd(void *data)
 
 		if (gfs2_ail_flush_reqd(sdp)) {
 			gfs2_ail1_start(sdp);
-			io_schedule();
+			gfs2_ail1_wait(sdp);
 			gfs2_ail1_empty(sdp);
 			gfs2_log_flush(sdp, NULL);
 		}
 
-		wake_up(&sdp->sd_log_waitq);
+		if (!gfs2_ail_flush_reqd(sdp))
+			wake_up(&sdp->sd_log_waitq);
+
 		t = gfs2_tune_get(sdp, gt_logd_secs) * HZ;
 		if (freezing(current))
 			refrigerator();