Commit 6467716a authored by Artem Bityutskiy's avatar Artem Bityutskiy Committed by Jens Axboe
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writeback: optimize periodic bdi thread wakeups 



Whe the first inode for a bdi is marked dirty, we wake up the bdi thread which
should take care of the periodic background write-out. However, the write-out
will actually start only 'dirty_writeback_interval' centisecs later, so we can
delay the wake-up.

This change was requested by Nick Piggin who pointed out that if we delay the
wake-up, we weed out 2 unnecessary contex switches, which matters because
'__mark_inode_dirty()' is a hot-path function.

This patch introduces a new function - 'bdi_wakeup_thread_delayed()', which
sets up a timer to wake-up the bdi thread and returns. So the wake-up is
delayed.

We also delete the timer in bdi threads just before writing-back. And
synchronously delete it when unregistering bdi. At the unregister point the bdi
does not have any users, so no one can arm it again.

Since now we take 'bdi->wb_lock' in the timer, which can execute in softirq
context, we have to use 'spin_lock_bh()' for 'bdi->wb_lock'. This patch makes
this change as well.

This patch also moves the 'bdi_wb_init()' function down in the file to avoid
forward-declaration of 'bdi_wakeup_thread_delayed()'.

Signed-off-by: default avatarArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: default avatarJens Axboe <jaxboe@fusionio.com>
parent 253c34e9

fs/fs-writeback.c

+11 −25
Original line number Diff line number Diff line
@@ -76,7 +76,7 @@ static void bdi_queue_work(struct backing_dev_info *bdi, 
{

	trace_writeback_queue(bdi, work);



	spin_lock(&bdi->wb_lock);

	spin_lock_bh(&bdi->wb_lock);

	list_add_tail(&work->list, &bdi->work_list);

	if (bdi->wb.task) {

		wake_up_process(bdi->wb.task);
@@ -88,7 +88,7 @@ static void bdi_queue_work(struct backing_dev_info *bdi, 
		trace_writeback_nothread(bdi, work);

		wake_up_process(default_backing_dev_info.wb.task);

	}

	spin_unlock(&bdi->wb_lock);

	spin_unlock_bh(&bdi->wb_lock);

}



static void
@@ -704,13 +704,13 @@ get_next_work_item(struct backing_dev_info *bdi) 
{

	struct wb_writeback_work *work = NULL;



	spin_lock(&bdi->wb_lock);

	spin_lock_bh(&bdi->wb_lock);

	if (!list_empty(&bdi->work_list)) {

		work = list_entry(bdi->work_list.next,

				  struct wb_writeback_work, list);

		list_del_init(&work->list);

	}

	spin_unlock(&bdi->wb_lock);

	spin_unlock_bh(&bdi->wb_lock);

	return work;

}
@@ -810,6 +810,12 @@ int bdi_writeback_thread(void *data) 
	trace_writeback_thread_start(bdi);



	while (!kthread_should_stop()) {

		/*

		 * Remove own delayed wake-up timer, since we are already awake

		 * and we'll take care of the preriodic write-back.

		 */

		del_timer(&wb->wakeup_timer);



		pages_written = wb_do_writeback(wb, 0);



		trace_writeback_pages_written(pages_written);
@@ -868,26 +874,6 @@ void wakeup_flusher_threads(long nr_pages) 
	rcu_read_unlock();

}



/*

 * This function is used when the first inode for this bdi is marked dirty. It

 * wakes-up the corresponding bdi thread which should then take care of the

 * periodic background write-out of dirty inodes.

 */

static void wakeup_bdi_thread(struct backing_dev_info *bdi)

{

	spin_lock(&bdi->wb_lock);

	if (bdi->wb.task)

		wake_up_process(bdi->wb.task);

	else

		/*

		 * When bdi tasks are inactive for long time, they are killed.

		 * In this case we have to wake-up the forker thread which

		 * should create and run the bdi thread.

		 */

		wake_up_process(default_backing_dev_info.wb.task);

	spin_unlock(&bdi->wb_lock);

}



static noinline void block_dump___mark_inode_dirty(struct inode *inode)

{

	if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {
@@ -1019,7 +1005,7 @@ out: 
	spin_unlock(&inode_lock);



	if (wakeup_bdi)

		wakeup_bdi_thread(bdi);

		bdi_wakeup_thread_delayed(bdi);

}

EXPORT_SYMBOL(__mark_inode_dirty);

include/linux/backing-dev.h

+2 −0
Original line number Diff line number Diff line
@@ -52,6 +52,7 @@ struct bdi_writeback { 
	unsigned long last_active;	/* last time bdi thread was active */



	struct task_struct *task;	/* writeback thread */

	struct timer_list wakeup_timer; /* used for delayed bdi thread wakeup */

	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 */
@@ -105,6 +106,7 @@ void bdi_start_background_writeback(struct backing_dev_info *bdi); 
int bdi_writeback_thread(void *data);

int bdi_has_dirty_io(struct backing_dev_info *bdi);

void bdi_arm_supers_timer(void);

void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi);



extern spinlock_t bdi_lock;

extern struct list_head bdi_list;

mm/backing-dev.c

+57 −16
Original line number Diff line number Diff line
@@ -248,17 +248,6 @@ static int __init default_bdi_init(void) 
}

subsys_initcall(default_bdi_init);



static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)

{

	memset(wb, 0, sizeof(*wb));



	wb->bdi = bdi;

	wb->last_old_flush = jiffies;

	INIT_LIST_HEAD(&wb->b_dirty);

	INIT_LIST_HEAD(&wb->b_io);

	INIT_LIST_HEAD(&wb->b_more_io);

}



int bdi_has_dirty_io(struct backing_dev_info *bdi)

{

	return wb_has_dirty_io(&bdi->wb);
@@ -316,6 +305,43 @@ static void sync_supers_timer_fn(unsigned long unused) 
	bdi_arm_supers_timer();

}



static void wakeup_timer_fn(unsigned long data)

{

	struct backing_dev_info *bdi = (struct backing_dev_info *)data;



	spin_lock_bh(&bdi->wb_lock);

	if (bdi->wb.task) {

		wake_up_process(bdi->wb.task);

	} else {

		/*

		 * When bdi tasks are inactive for long time, they are killed.

		 * In this case we have to wake-up the forker thread which

		 * should create and run the bdi thread.

		 */

		wake_up_process(default_backing_dev_info.wb.task);

	}

	spin_unlock_bh(&bdi->wb_lock);

}



/*

 * This function is used when the first inode for this bdi is marked dirty. It

 * wakes-up the corresponding bdi thread which should then take care of the

 * periodic background write-out of dirty inodes. Since the write-out would

 * starts only 'dirty_writeback_interval' centisecs from now anyway, we just

 * set up a timer which wakes the bdi thread up later.

 *

 * Note, we wouldn't bother setting up the timer, but this function is on the

 * fast-path (used by '__mark_inode_dirty()'), so we save few context switches

 * by delaying the wake-up.

 */

void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)

{

	unsigned long timeout;



	timeout = msecs_to_jiffies(dirty_writeback_interval * 10);

	mod_timer(&bdi->wb.wakeup_timer, jiffies + timeout);

}



/*

 * Calculate the longest interval (jiffies) bdi threads are allowed to be

 * inactive.
@@ -353,8 +379,10 @@ static int bdi_forker_thread(void *ptr) 
		 * Temporary measure, we want to make sure we don't see

		 * dirty data on the default backing_dev_info

		 */

		if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list))

		if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list)) {

			del_timer(&me->wakeup_timer);

			wb_do_writeback(me, 0);

		}



		spin_lock_bh(&bdi_lock);

		set_current_state(TASK_INTERRUPTIBLE);
@@ -386,7 +414,7 @@ static int bdi_forker_thread(void *ptr) 
				break;

			}



			spin_lock(&bdi->wb_lock);

			spin_lock_bh(&bdi->wb_lock);

			/*

			 * If there is no work to do and the bdi thread was

			 * inactive long enough - kill it. The wb_lock is taken
@@ -403,7 +431,7 @@ static int bdi_forker_thread(void *ptr) 
				action = KILL_THREAD;

				break;

			}

			spin_unlock(&bdi->wb_lock);

			spin_unlock_bh(&bdi->wb_lock);

		}

		spin_unlock_bh(&bdi_lock);
@@ -427,9 +455,9 @@ static int bdi_forker_thread(void *ptr) 
				 * The spinlock makes sure we do not lose

				 * wake-ups when racing with 'bdi_queue_work()'.

				 */

				spin_lock(&bdi->wb_lock);

				spin_lock_bh(&bdi->wb_lock);

				bdi->wb.task = task;

				spin_unlock(&bdi->wb_lock);

				spin_unlock_bh(&bdi->wb_lock);

			}

			break;
@@ -586,6 +614,7 @@ void bdi_unregister(struct backing_dev_info *bdi) 
	if (bdi->dev) {

		trace_writeback_bdi_unregister(bdi);

		bdi_prune_sb(bdi);

		del_timer_sync(&bdi->wb.wakeup_timer);



		if (!bdi_cap_flush_forker(bdi))

			bdi_wb_shutdown(bdi);
@@ -596,6 +625,18 @@ void bdi_unregister(struct backing_dev_info *bdi) 
}

EXPORT_SYMBOL(bdi_unregister);



static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)

{

	memset(wb, 0, sizeof(*wb));



	wb->bdi = bdi;

	wb->last_old_flush = jiffies;

	INIT_LIST_HEAD(&wb->b_dirty);

	INIT_LIST_HEAD(&wb->b_io);

	INIT_LIST_HEAD(&wb->b_more_io);

	setup_timer(&wb->wakeup_timer, wakeup_timer_fn, (unsigned long)bdi);

}



int bdi_init(struct backing_dev_info *bdi)

{

	int i, err;

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