[PATCH mm-unstable RFC 1/5] writeback: move wb_over_bg_thresh() call outside lock section

[Yosry Ahmed]

wb_over_bg_thresh() calls mem_cgroup_wb_stats() which invokes an rstat
flush, which can be expensive on large systems. Currently,
wb_writeback() calls wb_over_bg_thresh() within a lock section, so we
have to make the rstat flush atomically. On systems with a lot of
cpus/cgroups, this can cause us to disable irqs for a long time,
potentially causing problems.

Move the call to wb_over_bg_thresh() outside the lock section in
preparation to make the rstat flush in mem_cgroup_wb_stats() non-atomic.
The list_empty(&wb->work_list) should be okay outside the lock section
of wb->list_lock as it is protected by a separate lock (wb->work_lock),
and wb_over_bg_thresh() doesn't seem like it is modifying any of the b_*
lists the wb->list_lock is protecting. Also, the loop seems to be
already releasing and reacquring the lock, so this refactoring looks
safe.

Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
 fs/fs-writeback.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 195dc23e0d831..012357bc8daa3 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -2021,7 +2021,6 @@ static long wb_writeback(struct bdi_writeback *wb,
 	struct blk_plug plug;
 
 	blk_start_plug(&plug);
-	spin_lock(&wb->list_lock);
 	for (;;) {
 		/*
 		 * Stop writeback when nr_pages has been consumed
@@ -2046,6 +2045,9 @@ static long wb_writeback(struct bdi_writeback *wb,
 		if (work->for_background && !wb_over_bg_thresh(wb))
 			break;
 
+
+		spin_lock(&wb->list_lock);
+
 		/*
 		 * Kupdate and background works are special and we want to
 		 * include all inodes that need writing. Livelock avoidance is
@@ -2075,13 +2077,19 @@ static long wb_writeback(struct bdi_writeback *wb,
 		 * mean the overall work is done. So we keep looping as long
 		 * as made some progress on cleaning pages or inodes.
 		 */
-		if (progress)
+		if (progress) {
+			spin_unlock(&wb->list_lock);
 			continue;
+		}
+
 		/*
 		 * No more inodes for IO, bail
 		 */
-		if (list_empty(&wb->b_more_io))
+		if (list_empty(&wb->b_more_io)) {
+			spin_unlock(&wb->list_lock);
 			break;
+		}
+
 		/*
 		 * Nothing written. Wait for some inode to
 		 * become available for writeback. Otherwise
@@ -2093,9 +2101,7 @@ static long wb_writeback(struct bdi_writeback *wb,
 		spin_unlock(&wb->list_lock);
 		/* This function drops i_lock... */
 		inode_sleep_on_writeback(inode);
-		spin_lock(&wb->list_lock);
 	}
-	spin_unlock(&wb->list_lock);
 	blk_finish_plug(&plug);
 
 	return nr_pages - work->nr_pages;
-- 
2.40.0.348.gf938b09366-goog