[PATCH 3/3] writeback: avoid extra sync work at enqueue time

[Wu Fengguang]

This removes writeback_control.wb_start and does more straightforward
sync livelock prevention by setting .older_than_this to prevent extra
inodes from being enqueued in the first place.

CC: Jan Kara <jack@xxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 fs/fs-writeback.c         |   17 ++++-------------
 include/linux/writeback.h |    3 ---
 2 files changed, 4 insertions(+), 16 deletions(-)

--- linux-next.orig/fs/fs-writeback.c	2011-05-02 11:17:24.000000000 +0800
+++ linux-next/fs/fs-writeback.c	2011-05-02 11:17:27.000000000 +0800
@@ -544,15 +544,6 @@ static int writeback_sb_inodes(struct su
 			continue;
 		}
 
-		/*
-		 * Was this inode dirtied after sync_sb_inodes was called?
-		 * This keeps sync from extra jobs and livelock.
-		 */
-		if (inode_dirtied_after(inode, wbc->wb_start)) {
-			spin_unlock(&inode->i_lock);
-			return 1;
-		}
-
 		__iget(inode);
 
 		pages_skipped = wbc->pages_skipped;
@@ -585,8 +576,6 @@ static void __writeback_inodes_wb(struct
 {
 	int ret = 0;
 
-	if (!wbc->wb_start)
-		wbc->wb_start = jiffies; /* livelock avoidance */
 
 	while (!list_empty(&wb->b_io)) {
 		struct inode *inode = wb_inode(wb->b_io.prev);
@@ -683,10 +672,12 @@ static long wb_writeback(struct bdi_writ
 	 *                   (quickly) tag currently dirty pages
 	 *                   (maybe slowly) sync all tagged pages
 	 */
-	if (wbc.sync_mode == WB_SYNC_ALL || wbc.tagged_sync)
+	if (wbc.sync_mode == WB_SYNC_ALL || wbc.tagged_sync) {
 		write_chunk = LONG_MAX;
+		oldest_jif = jiffies;
+		wbc.older_than_this = &oldest_jif;
+	}
 
-	wbc.wb_start = jiffies; /* livelock avoidance */
 	for (;;) {
 		/*
 		 * Stop writeback when nr_pages has been consumed
--- linux-next.orig/include/linux/writeback.h	2011-05-02 11:16:12.000000000 +0800
+++ linux-next/include/linux/writeback.h	2011-05-02 11:17:27.000000000 +0800
@@ -26,9 +26,6 @@ struct writeback_control {
 	enum writeback_sync_modes sync_mode;
 	unsigned long *older_than_this;	/* If !NULL, only write back inodes
 					   older than this */
-	unsigned long wb_start;         /* Time writeback_inodes_wb was
-					   called. This is needed to avoid
-					   extra jobs and livelock */
 	long nr_to_write;		/* Write this many pages, and decrement
 					   this for each page written */
 	long pages_skipped;		/* Pages which were not written */


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