From 885bceca7ff12021c9c17f58d12e12ec6e8e59a6 Mon Sep 17 00:00:00 2001
From: Steven Whitehouse <swhiteho@redhat.com>
Date: Mon, 3 Feb 2014 09:57:29 +0000
Subject: GFS2: Plug on AIL flush

When we do a flush of the AIL list, we are writing out what is
likely to be a lot of small I/Os, which are possibly in an order
which is not ideal performance-wise. Since this is done by calling
filemap_fdatatwrite for each individual inode's address space there
is no overall plugging going on.

In addition to that, we do not always wait for AIL i/o when we flush
it, so that it is possible for things to get left behind on the queue.
By adding explicit plugging here, we reduce the chances of this
being an issues. A quick test using the AIL flush tracepoint shows a
small, but measurable improvement.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

diff --git a/fs/gfs2/log.c b/fs/gfs2/log.c
index 9dcb977..1e1bda0 100644
--- a/fs/gfs2/log.c
+++ b/fs/gfs2/log.c
@@ -18,6 +18,7 @@
 #include <linux/kthread.h>
 #include <linux/freezer.h>
 #include <linux/bio.h>
+#include <linux/blkdev.h>
 #include <linux/writeback.h>
 #include <linux/list_sort.h>
 
@@ -145,8 +146,10 @@ void gfs2_ail1_flush(struct gfs2_sbd *sdp, struct writeback_control *wbc)
 {
 	struct list_head *head = &sdp->sd_ail1_list;
 	struct gfs2_trans *tr;
+	struct blk_plug plug;
 
 	trace_gfs2_ail_flush(sdp, wbc, 1);
+	blk_start_plug(&plug);
 	spin_lock(&sdp->sd_ail_lock);
 restart:
 	list_for_each_entry_reverse(tr, head, tr_list) {
@@ -156,6 +159,7 @@ restart:
 			goto restart;
 	}
 	spin_unlock(&sdp->sd_ail_lock);
+	blk_finish_plug(&plug);
 	trace_gfs2_ail_flush(sdp, wbc, 0);
 }
 
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