writeback: move last_active to bdi

Currently bdi threads use local variable 'last_active' which stores last time
when the bdi thread did some useful work. Move this local variable to 'struct
bdi_writeback'. This is just a preparation for the further patches which will
make the forker thread decide when bdi threads should be killed.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/fs-writeback.c

@@ -800,12 +800,12 @@ int bdi_writeback_thread(void *data)
 {
 	struct bdi_writeback *wb = data;
 	struct backing_dev_info *bdi = wb->bdi;
-	unsigned long last_active = jiffies;
 	unsigned long wait_jiffies = -1UL;
 	long pages_written;
 
 	current->flags |= PF_FLUSHER | PF_SWAPWRITE;
 	set_freezable();
+	wb->last_active = jiffies;
 
 	/*
 	 * Our parent may run at a different priority, just set us to normal
@@ -827,7 +827,7 @@ int bdi_writeback_thread(void *data)
 		trace_writeback_pages_written(pages_written);
 
 		if (pages_written)
-			last_active = jiffies;
+			wb->last_active = jiffies;
 		else if (wait_jiffies != -1UL) {
 			unsigned long max_idle;
 
@@ -837,7 +837,7 @@ int bdi_writeback_thread(void *data)
 			 * recreated automatically.
 			 */
 			max_idle = max(5UL * 60 * HZ, wait_jiffies);
-			if (time_after(jiffies, max_idle + last_active))
+			if (time_after(jiffies, max_idle + wb->last_active))
 				break;
 		}
 

include/linux/backing-dev.h

@@ -45,15 +45,16 @@ enum bdi_stat_item {
 #define BDI_STAT_BATCH (8*(1+ilog2(nr_cpu_ids)))
 
 struct bdi_writeback {
-	struct backing_dev_info *bdi;		/* our parent bdi */
+	struct backing_dev_info *bdi;	/* our parent bdi */
 	unsigned int nr;
 
-	unsigned long last_old_flush;		/* last old data flush */
+	unsigned long last_old_flush;	/* last old data flush */
+	unsigned long last_active;	/* last time bdi thread was active */
 
-	struct task_struct	*task;		/* writeback thread */
-	struct list_head	b_dirty;	/* dirty inodes */
-	struct list_head	b_io;		/* parked for writeback */
-	struct list_head	b_more_io;	/* parked for more writeback */
+	struct task_struct *task;	/* writeback thread */
+	struct list_head b_dirty;	/* dirty inodes */
+	struct list_head b_io;		/* parked for writeback */
+	struct list_head b_more_io;	/* parked for more writeback */
 };
 
 struct backing_dev_info {