fs-writeback: unplug before cond_resched in writeback_sb_inodes

[ Upstream commit 590dca3a71875461e8fea3013af74386945191b2 ]

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@fb.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 2d609a5fbfea96e6ebe8dc65ebf64cd64dca0c8d..a07634599cd74a2249c15f019d30424280ee94f0 100644 (file)
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -705,6 +705,21 @@ static long writeback_sb_inodes(struct super_block *sb,
 
                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))
@@ -712,7 +727,7 @@ 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.