writeback: speed up writeback of big dirty files

After making dirty a 100M file, the normal behavior is to start the
writeback for all data after 30s delays.  But sometimes the following
happens instead:

	- after 30s:    ~4M
	- after 5s:     ~4M
	- after 5s:     all remaining 92M

Some analyze shows that the internal io dispatch queues goes like this:

		s_io            s_more_io
		-------------------------
	1)	100M,1K         0
	2)	1K              96M
	3)	0               96M
1) initial state with a 100M file and a 1K file

2) 4M written, nr_to_write <= 0, so write more

3) 1K written, nr_to_write > 0, no more writes(BUG)

nr_to_write > 0 in (3) fools the upper layer to think that data have all
been written out.  The big dirty file is actually still sitting in
s_more_io.  We cannot simply splice s_more_io back to s_io as soon as s_io
becomes empty, and let the loop in generic_sync_sb_inodes() continue: this
may starve newly expired inodes in s_dirty.  It is also not an option to
draw inodes from both s_more_io and s_dirty, an let the loop go on: this
might lead to live locks, and might also starve other superblocks in sync
time(well kupdate may still starve some superblocks, that's another bug).

We have to return when a full scan of s_io completes.  So nr_to_write > 0
does not necessarily mean that "all data are written".  This patch
introduces a flag writeback_control.more_io to indicate that more io should
be done.  With it the big dirty file no longer has to wait for the next
kupdate invokation 5s later.

In sync_sb_inodes() we only set more_io on super_blocks we actually
visited.  This avoids the interaction between two pdflush deamons.

Also in __sync_single_inode() we don't blindly keep requeuing the io if the
filesystem cannot progress.  Failing to do so may lead to 100% iowait.

Tested-by: Mike Snitzer <snitzer@gmail.com>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Michael Rubin <mrubin@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 2 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 3fe782d70a71..0b3064079fa5 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -284,7 +284,17 @@ __sync_single_inode(struct inode *inode, struct writeback_control *wbc)
                                  * soon as the queue becomes uncongested.
                                  */
                                 inode->i_state |= I_DIRTY_PAGES;
-                                requeue_io(inode);
+                                if (wbc->nr_to_write <= 0) {
+                                        /*
+                                         * slice used up: queue for next turn
+                                         */
+                                        requeue_io(inode);
+                                } else {
+                                        /*
+                                         * somehow blocked: retry later
+                                         */
+                                        redirty_tail(inode);
+                                }
                         } else {
                                 /*
                                  * Otherwise fully redirty the inode so that
@@ -468,8 +478,12 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
                 iput(inode);
                 cond_resched();
                 spin_lock(&inode_lock);
-                if (wbc->nr_to_write <= 0)
+                if (wbc->nr_to_write <= 0) {
+                        wbc->more_io = 1;
                         break;
+                }
+                if (!list_empty(&sb->s_more_io))
+                        wbc->more_io = 1;
         }
         return;         /* Leave any unwritten inodes on s_io */
 }