[PATCHv4 17/17] writeback: lessen sync_supers wakeup count

[Artem Bityutskiy <dedekind1@gmail.com>]

From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

The 'sync_supers' thread wakes up every 5 seconds (by default) and
writes back all super blocks. It keeps waking up even if there
are no dirty super-blocks. For many file-systems the superblock
becomes dirty very rarely, if ever, so 'sync_supers' does not do
anything most of the time.

This patch improves 'sync_supers' and makes sleep if all superblocks
are clean and there is nothing to do. This helps saving the power.
This optimization is important for small battery-powered devices.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 include/linux/fs.h |    5 +----
 mm/backing-dev.c   |   36 +++++++++++++++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c2ddeee..2d2560b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1786,10 +1786,7 @@ extern void simple_set_mnt(struct vfsmount *mnt, struct super_block *sb);
  * Note, VFS does not provide any serialization for the super block clean/dirty
  * state changes, file-systems should take care of this.
  */
-static inline void mark_sb_dirty(struct super_block *sb)
-{
-	sb->s_dirty = 1;
-}
+void mark_sb_dirty(struct super_block *sb);
 static inline void mark_sb_clean(struct super_block *sb)
 {
 	sb->s_dirty = 0;
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 660a87a..14f3eb7 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -45,6 +45,8 @@ LIST_HEAD(bdi_pending_list);
 
 static struct task_struct *sync_supers_tsk;
 static struct timer_list sync_supers_timer;
+static int supers_timer_armed;
+static DEFINE_SPINLOCK(supers_timer_lock);
 
 static int bdi_sync_supers(void *);
 static void sync_supers_timer_fn(unsigned long);
@@ -355,6 +357,11 @@ static void bdi_flush_io(struct backing_dev_info *bdi)
  * or we risk deadlocking on ->s_umount. The longer term solution would be
  * to implement sync_supers_bdi() or similar and simply do it from the
  * bdi writeback tasks individually.
+ *
+ * Historically this thread woke up periodically, regardless of whether
+ * there was any dirty superblock. However, nowadays it is optimized to
+ * wake up only when there is something to synchronize - this helps to save
+ * power.
  */
 static int bdi_sync_supers(void *unused)
 {
@@ -364,10 +371,24 @@ static int bdi_sync_supers(void *unused)
 		set_current_state(TASK_INTERRUPTIBLE);
 		schedule();
 
+		spin_lock(&supers_timer_lock);
+		/* Indicate that 'sync_supers' is in progress */
+		supers_timer_armed = -1;
+		spin_unlock(&supers_timer_lock);
+
 		/*
 		 * Do this periodically, like kupdated() did before.
 		 */
 		sync_supers();
+
+		spin_lock(&supers_timer_lock);
+		if (supers_timer_armed == 1)
+			/* A super block was marked as dirty meanwhile */
+			bdi_arm_supers_timer();
+		else
+			/* No more dirty superblocks - we've synced'em all */
+			supers_timer_armed = 0;
+		spin_unlock(&supers_timer_lock);
 	}
 
 	return 0;
@@ -387,9 +408,22 @@ void bdi_arm_supers_timer(void)
 static void sync_supers_timer_fn(unsigned long unused)
 {
 	wake_up_process(sync_supers_tsk);
-	bdi_arm_supers_timer();
 }
 
+void mark_sb_dirty(struct super_block *sb)
+{
+	sb->s_dirty = 1;
+
+	spin_lock(&supers_timer_lock);
+	if (!supers_timer_armed) {
+		bdi_arm_supers_timer();
+		supers_timer_armed = 1;
+	} else if (supers_timer_armed == -1)
+		supers_timer_armed = 1;
+	spin_unlock(&supers_timer_lock);
+}
+EXPORT_SYMBOL(mark_sb_dirty);
+
 static int bdi_forker_task(void *ptr)
 {
 	struct bdi_writeback *me = ptr;
-- 
1.6.6.1