[4.2.y-ckt stable] Patch "fs-writeback: unplug before cond_resched in writeback_sb_inodes" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    fs-writeback: unplug before cond_resched in writeback_sb_inodes

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt7.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 0ba0321886785c9c882087b11b25a78cbd151568 Mon Sep 17 00:00:00 2001
From: Chris Mason <clm at fb.com>
Date: Fri, 18 Sep 2015 13:35:08 -0400
Subject: fs-writeback: unplug before cond_resched in writeback_sb_inodes

commit 590dca3a71875461e8fea3013af74386945191b2 upstream.

Commit 505a666ee3fc ("writeback: plug writeback in wb_writeback() and
writeback_inodes_wb()") has us holding a plug during writeback_sb_inodes,
which increases the merge rate when relatively contiguous small files
are written by the filesystem.  It helps both on flash and spindles.

For an fs_mark workload creating 4K files in parallel across 8 drives,
this commit improves performance ~9% more by unplugging before calling
cond_resched().  cond_resched() doesn't trigger an implicit unplug, so
explicitly getting the IO down to the device before scheduling reduces
latencies for anyone waiting on clean pages.

It also cuts down on how often we use kblockd to unplug, which means
less work bouncing from one workqueue to another.

Many more details about how we got here:

  https://lkml.org/lkml/2015/9/11/570

Signed-off-by: Chris Mason <clm at fb.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/fs-writeback.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 5760ea9..f907b9d 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1520,6 +1520,21 @@ static long writeback_sb_inodes(struct super_block *sb,
 		wbc_detach_inode(&wbc);
 		work->nr_pages -= write_chunk - wbc.nr_to_write;
 		wrote += write_chunk - wbc.nr_to_write;
+
+		if (need_resched()) {
+			/*
+			 * We're trying to balance between building up a nice
+			 * long list of IOs to improve our merge rate, and
+			 * getting those IOs out quickly for anyone throttling
+			 * in balance_dirty_pages().  cond_resched() doesn't
+			 * unplug, so get our IOs out the door before we
+			 * give up the CPU.
+			 */
+			blk_flush_plug(current);
+			cond_resched();
+		}
+
+
 		spin_lock(&wb->list_lock);
 		spin_lock(&inode->i_lock);
 		if (!(inode->i_state & I_DIRTY_ALL))
@@ -1527,7 +1542,7 @@ static long writeback_sb_inodes(struct super_block *sb,
 		requeue_inode(inode, wb, &wbc);
 		inode_sync_complete(inode);
 		spin_unlock(&inode->i_lock);
-		cond_resched_lock(&wb->list_lock);
+
 		/*
 		 * bail out to wb_writeback() often enough to check
 		 * background threshold and other termination conditions.
--
2.7.4