[PATCHv2 00/16] kill unnecessary bdi wakeups + cleanups

[PATCHv2 00/16] kill unnecessary bdi wakeups + cleanups

[Artem Bityutskiy]

Hi,

here is v2 of the patch series which clean-ups bdi threads and substantially
lessens amount of unnecessary kernel wake-ups, which is very important on
battery-powered devices.

Changes since v1
Basically, address all requests from Christoph except of 2.

1.  Drop "[PATCH 01/16] writeback: do not self-wakeup"
2.  Add all "Reviewed-by"
3.  Rebase to the latest "linux-2.6-block / for-2.6.36"
4.  Re-order patches so that the independent ones would go first and could
    be picked independently
5.  Do not remove comment about "temporary measure" in the forker thread.
6.  Drop "[PATCH 01/13] writeback: remove redundant list initialization"
    because one of later patches will kill whole function, so this small
    patch is pointless
7.  Merge "[PATCH 03/13] writeback: clean-up the warning about non-registered
    bdi" with the patch which adds bdi threads wake-ups to
    '__mark_inode_dirty()'.
9.  Do not remove bdis from the bdi_list
8.  Drop "[PATCH 09/13] writeback: add to bdi_list in the forker thread"
    because we do not remove bdis from the bdi_list anymore
10. Use less local variables which are not strictly needed

The following Christoph's requests were *not* addressed:

1. Restructure the loop in bdi forker, because we have to drop spinlock
   before forking a thread, see my answer here:
   http://lkml.org/lkml/2010/7/20/92
2. Get rid of 'BDI_pending' and use a per-bdi mutex. We cannot easily
   use a per-bdi mutex, because we would have to take it while holding
   the 'bdi_lock' spinlock. We could turn 'bdi_lock' into a mutex, though,
   and avoid dropping it before the task is created. This would eliminate
   the need in the 'BDI_pending' flag. I can do this change, if needed.


THE PROBLEM
~~~~~~~~~~~

Each block device has corresponding "flusher" thread, which is usually seen as
"flusher-x:y" in your 'ps' output. Flusher threads are responsible for
background write-back and are used in various kernel code paths like memory
reclamation as well as the periodic background write-out.

The flusher threads wake up every 5 seconds and check whether they have to
write anything back or not. In idle systems with good dynamic power-management
this means that they force the system to wake up from deep sleep, find out that
there is nothing to do, and waste power. This hurts small battery-powered
devices, e.g., linux-based phones.

Idle bdi thread wake-ups do not last forever: the threads kill themselves if
nothing useful has been done for 5 minutes.

However, there is the bdi forker thread, seen as 'bdi-default' in your 'ps'
output. This thread also wakes up every 5 seconds and checks whether it has to
fork a bdi flusher thread, in case there is dirty data on the bdi, but bdi
thread was killed. This thread never kills itself, and disturbs the system all
the time. Again, this is bad for battery-powered devices.

THE SOLUTION
~~~~~~~~~~~~

This patch-set makes bdi threads and the forker thread wake-up only if there is
job to do, otherwise they just sleep. The main idea is to wake-up the needed
thread when adding dirty data to the bdi.

To implement this:

1. I address various race conditions in the current bdi code.
2. I move the killing logic from bdi threads to the forker thread, so that we
   would have one central place where we make decisions about killing inactive
   bdi threads. The reason I do this is because otherwise it is difficult to
   kill inactive threads - they never wake-up, so would never kill themselves.
   There are other technical reasons, too.
3. I add a small piece of code to '__mark_inode_dirt()' which wakes up the bdi
   thread when dirty inodes arrive.
4. There are also clean-up patches and nicification patches which I found to be
   good for better code readability.
5. Some patches are just preparations which make the following real patches
   simpler and easier to review.
6. Some patches are just simplifications of current code.

With this patch-set bdi threads wake up considerably less.

v1 can be found here:
http://thread.gmane.org/gmane.linux.file-systems/43306

Artem.

[PATCHv2 01/11] writeback: harmonize writeback threads naming

[Artem Bityutskiy]

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

The write-back code mixes words "thread" and "task" for the same things. This
is not a big deal, but still an inconsistency.

hch: a convention I tend to use and I've seen in various places
is to always use _task for the storage of the task_struct pointer,
and thread everywhere else.  This especially helps with having
foo_thread for the actual thread and foo_task for a global
variable keeping the task_struct pointer

This patch renames:
* 'bdi_add_default_flusher_task()' -> 'bdi_add_default_flusher_thread()'
* 'bdi_forker_task()'              -> 'bdi_forker_thread()'

because bdi threads are 'bdi_writeback_thread()', so these names are more
consistent.

This patch also amends commentaries and makes them refer the forker and bdi
threads as "thread", not "task".

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 fs/fs-writeback.c           |    2 +-
 include/linux/backing-dev.h |    2 +-
 mm/backing-dev.c            |   26 +++++++++++++-------------
 3 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index bf10cbf..5e6b7fc 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -840,7 +840,7 @@ int bdi_writeback_thread(void *data)
 
 			/*
 			 * Longest period of inactivity that we tolerate. If we
-			 * see dirty data again later, the task will get
+			 * see dirty data again later, the thread will get
 			 * recreated automatically.
 			 */
 			max_idle = max(5UL * 60 * HZ, wait_jiffies);
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index e536f3a..f0936f5 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -50,7 +50,7 @@ struct bdi_writeback {
 
 	unsigned long last_old_flush;		/* last old data flush */
 
-	struct task_struct	*task;		/* writeback task */
+	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 */
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index ac78a33..0f7b17c 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -50,7 +50,7 @@ static struct timer_list sync_supers_timer;
 static int bdi_sync_supers(void *);
 static void sync_supers_timer_fn(unsigned long);
 
-static void bdi_add_default_flusher_task(struct backing_dev_info *bdi);
+static void bdi_add_default_flusher_thread(struct backing_dev_info *bdi);
 
 #ifdef CONFIG_DEBUG_FS
 #include <linux/debugfs.h>
@@ -279,10 +279,10 @@ static void bdi_flush_io(struct backing_dev_info *bdi)
 }
 
 /*
- * kupdated() used to do this. We cannot do it from the bdi_forker_task()
+ * kupdated() used to do this. We cannot do it from the bdi_forker_thread()
  * 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.
+ * bdi writeback thread individually.
  */
 static int bdi_sync_supers(void *unused)
 {
@@ -318,7 +318,7 @@ static void sync_supers_timer_fn(unsigned long unused)
 	bdi_arm_supers_timer();
 }
 
-static int bdi_forker_task(void *ptr)
+static int bdi_forker_thread(void *ptr)
 {
 	struct bdi_writeback *me = ptr;
 
@@ -354,7 +354,7 @@ static int bdi_forker_task(void *ptr)
 			    !bdi_has_dirty_io(bdi))
 				continue;
 
-			bdi_add_default_flusher_task(bdi);
+			bdi_add_default_flusher_thread(bdi);
 		}
 
 		set_current_state(TASK_INTERRUPTIBLE);
@@ -376,7 +376,7 @@ static int bdi_forker_task(void *ptr)
 
 		/*
 		 * This is our real job - check for pending entries in
-		 * bdi_pending_list, and create the tasks that got added
+		 * bdi_pending_list, and create the threads that got added
 		 */
 		bdi = list_entry(bdi_pending_list.next, struct backing_dev_info,
 				 bdi_list);
@@ -387,7 +387,7 @@ static int bdi_forker_task(void *ptr)
 		wb->task = kthread_run(bdi_writeback_thread, wb, "flush-%s",
 					dev_name(bdi->dev));
 		/*
-		 * If task creation fails, then readd the bdi to
+		 * If thread creation fails, then readd the bdi to
 		 * the pending list and force writeout of the bdi
 		 * from this forker thread. That will free some memory
 		 * and we can try again.
@@ -430,10 +430,10 @@ static void bdi_add_to_pending(struct rcu_head *head)
 }
 
 /*
- * Add the default flusher task that gets created for any bdi
+ * Add the default flusher thread that gets created for any bdi
  * that has dirty data pending writeout
  */
-void static bdi_add_default_flusher_task(struct backing_dev_info *bdi)
+void static bdi_add_default_flusher_thread(struct backing_dev_info *bdi)
 {
 	if (!bdi_cap_writeback_dirty(bdi))
 		return;
@@ -445,10 +445,10 @@ void static bdi_add_default_flusher_task(struct backing_dev_info *bdi)
 	}
 
 	/*
-	 * Check with the helper whether to proceed adding a task. Will only
+	 * Check with the helper whether to proceed adding a thread. Will only
 	 * abort if we two or more simultanous calls to
-	 * bdi_add_default_flusher_task() occured, further additions will block
-	 * waiting for previous additions to finish.
+	 * bdi_add_default_flusher_thread() occured, further additions will
+	 * block waiting for previous additions to finish.
 	 */
 	if (!test_and_set_bit(BDI_pending, &bdi->state)) {
 		list_del_rcu(&bdi->bdi_list);
@@ -506,7 +506,7 @@ int bdi_register(struct backing_dev_info *bdi, struct device *parent,
 	if (bdi_cap_flush_forker(bdi)) {
 		struct bdi_writeback *wb = &bdi->wb;
 
-		wb->task = kthread_run(bdi_forker_task, wb, "bdi-%s",
+		wb->task = kthread_run(bdi_forker_thread, wb, "bdi-%s",
 						dev_name(dev));
 		if (IS_ERR(wb->task)) {
 			wb->task = NULL;
-- 
1.7.1.1

[PATCHv2 02/11] writeback: fix possible race when creating bdi threads

[Artem Bityutskiy]

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

This patch fixes a very unlikely race condition on the bdi forker thread error
path: when bdi thread creation fails, 'bdi->wb.task' may contain the error code
for a short period of time. If at the same time someone submits a work to this
bdi, we can end up with an oops 'bdi_queue_work()' while executing
'wake_up_process(wb->task)'.

This patch fixes the issue by introducing a temporary variable 'task' and
storing the possible error code there, so that 'wb->task' would never take
erroneous values.

Note, this race is very unlikely and I never hit it, so it is theoretical, but
nevertheless worth fixing.

This patch also merges 2 comments which were previously separate.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 mm/backing-dev.c |   28 +++++++++++-----------------
 1 files changed, 11 insertions(+), 17 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 0f7b17c..75fae17 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -331,8 +331,8 @@ static int bdi_forker_thread(void *ptr)
 	set_user_nice(current, 0);
 
 	for (;;) {
+		struct task_struct *task;
 		struct backing_dev_info *bdi, *tmp;
-		struct bdi_writeback *wb;
 
 		/*
 		 * Temporary measure, we want to make sure we don't see
@@ -383,29 +383,23 @@ static int bdi_forker_thread(void *ptr)
 		list_del_init(&bdi->bdi_list);
 		spin_unlock_bh(&bdi_lock);
 
-		wb = &bdi->wb;
-		wb->task = kthread_run(bdi_writeback_thread, wb, "flush-%s",
-					dev_name(bdi->dev));
-		/*
-		 * If thread creation fails, then readd the bdi to
-		 * the pending list and force writeout of the bdi
-		 * from this forker thread. That will free some memory
-		 * and we can try again.
-		 */
-		if (IS_ERR(wb->task)) {
-			wb->task = NULL;
-
+		task = kthread_run(bdi_writeback_thread, &bdi->wb, "flush-%s",
+				   dev_name(bdi->dev));
+		if (IS_ERR(task)) {
 			/*
-			 * Add this 'bdi' to the back, so we get
-			 * a chance to flush other bdi's to free
-			 * memory.
+			 * If thread creation fails, then readd the bdi back to
+			 * the list and force writeout of the bdi from this
+			 * forker thread. That will free some memory and we can
+			 * try again. Add it to the tail so we get a chance to
+			 * flush other bdi's to free memory.
 			 */
 			spin_lock_bh(&bdi_lock);
 			list_add_tail(&bdi->bdi_list, &bdi_pending_list);
 			spin_unlock_bh(&bdi_lock);
 
 			bdi_flush_io(bdi);
-		}
+		} else
+			bdi->wb.task = task;
 	}
 
 	return 0;
-- 
1.7.1.1

Re: [PATCHv2 02/11] writeback: fix possible race when creating bdi threads

[Christoph Hellwig]

Looks good,


Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCHv2 03/11] writeback: do not lose wake-ups in the forker thread - 1

[Artem Bityutskiy]

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

Currently the forker thread can lose wake-ups which may lead to unnecessary
delays in processing bdi works. E.g., consider the following scenario.

1. 'bdi_forker_thread()' walks the 'bdi_list', finds out there is nothing to
   do, and is about to finish the loop.
2. A bdi thread decides to exit because it was inactive for long time.
3. 'bdi_queue_work()' adds a work to the bdi which just exited, so it wakes up
   the forker thread.
4. but 'bdi_forker_thread()' executes 'set_current_state(TASK_INTERRUPTIBLE)'
   and goes sleep. We lose a wake-up.

Losing the wake-up is not fatal, but this means that the bdi work processing
will be delayed by up to 5 sec. This race is theoretical, I never hit it, but
it is worth fixing.

The fix is to execute 'set_current_state(TASK_INTERRUPTIBLE)' _before_ walking
'bdi_list', not after.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 mm/backing-dev.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 75fae17..0ea5e4c 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -342,6 +342,7 @@ static int bdi_forker_thread(void *ptr)
 			wb_do_writeback(me, 0);
 
 		spin_lock_bh(&bdi_lock);
+		set_current_state(TASK_INTERRUPTIBLE);
 
 		/*
 		 * Check if any existing bdi's have dirty data without
@@ -357,8 +358,6 @@ static int bdi_forker_thread(void *ptr)
 			bdi_add_default_flusher_thread(bdi);
 		}
 
-		set_current_state(TASK_INTERRUPTIBLE);
-
 		if (list_empty(&bdi_pending_list)) {
 			unsigned long wait;
 
-- 
1.7.1.1

[PATCHv2 04/11] writeback: do not lose wake-ups in the forker thread - 2

[Artem Bityutskiy]

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

Currently, if someone submits jobs for the default bdi, we can lose wake-up
events. E.g., this can happen if 'bdi_queue_work()' is called when
'bdi_forker_thread()' is executing code after 'wb_do_writeback(me, 0)', but
before 'set_current_state(TASK_INTERRUPTIBLE)'.

This situation is unlikely, and the result is not very severe - we'll just
delay the execution of the work, but this is still not very nice.

This patch fixes the issue by checking whether the default bdi has works before
the forker thread goes sleep.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 mm/backing-dev.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 0ea5e4c..ae44417 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -358,6 +358,10 @@ static int bdi_forker_thread(void *ptr)
 			bdi_add_default_flusher_thread(bdi);
 		}
 
+		/* Keep working if default bdi still has things to do */
+		if (!list_empty(&me->bdi->work_list))
+			__set_current_state(TASK_RUNNING);
+
 		if (list_empty(&bdi_pending_list)) {
 			unsigned long wait;
 
-- 
1.7.1.1

Re: [PATCHv2 04/11] writeback: do not lose wake-ups in the forker thread - 2

[Christoph Hellwig]

On Wed, Jul 21, 2010 at 12:31:39PM +0300, Artem Bityutskiy wrote:
> From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
> 
> Currently, if someone submits jobs for the default bdi, we can lose wake-up
> events. E.g., this can happen if 'bdi_queue_work()' is called when
> 'bdi_forker_thread()' is executing code after 'wb_do_writeback(me, 0)', but
> before 'set_current_state(TASK_INTERRUPTIBLE)'.
> 
> This situation is unlikely, and the result is not very severe - we'll just
> delay the execution of the work, but this is still not very nice.
> 
> This patch fixes the issue by checking whether the default bdi has works before
> the forker thread goes sleep.

Looks good,


Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCHv2 05/11] writeback: do not lose wake-ups in bdi threads

[Artem Bityutskiy]

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

Currently, bdi threads ('bdi_writeback_thread()') can lose wake-ups. For
example, if 'bdi_queue_work()' is executed after the bdi thread have had
finished 'wb_do_writeback()' but before it called
'schedule_timeout_interruptible()'.

To fix this issue, we have to check whether we have works to process after we
have changed the task state to 'TASK_INTERRUPTIBLE'.

This patch also clean-ups handling of the cases when 'dirty_writeback_interval'
is zero or non-zero.

Additionally, this patch also removes unneeded 'list_empty_careful()' call.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 fs/fs-writeback.c |   17 +++++++++--------
 1 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 5e6b7fc..8cf53ba 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -848,17 +848,18 @@ int bdi_writeback_thread(void *data)
 				break;
 		}
 
-		if (dirty_writeback_interval) {
-			wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
-			schedule_timeout_interruptible(wait_jiffies);
-		} else {
-			set_current_state(TASK_INTERRUPTIBLE);
-			if (list_empty_careful(&wb->bdi->work_list) &&
-			    !kthread_should_stop())
-				schedule();
+		set_current_state(TASK_INTERRUPTIBLE);
+		if (!list_empty(&bdi->work_list)) {
 			__set_current_state(TASK_RUNNING);
+			continue;
 		}
 
+		if (dirty_writeback_interval) {
+			wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
+			schedule_timeout(wait_jiffies);
+		} else
+			schedule();
+
 		try_to_freeze();
 	}
 
-- 
1.7.1.1

[PATCHv2 06/11] writeback: simplify bdi code a little

[Artem Bityutskiy]

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

This patch simplifies bdi code a little by removing the 'pending_list' which is
redundant. Indeed, currently the forker thread ('bdi_forker_thread()') is
working like this:

1. In a loop, fetch all bdi's which have works but have no writeback thread and
   move them to the 'pending_list'.
2. If the list is empty, sleep for 5 sec.
3. Otherwise, take one bdi from the list, fork the writeback thread for this
   bdi, and repeat the loop.

IOW, it first moves everything to the 'pending_list', then process only one
element, and so on. This patch simplifies the algorithm, which is now as
follows.

1. Find the first bdi which has a work and remove it from the global list of
   bdi's (bdi_list).
2. If there was not such bdi, sleep 5 sec.
3. Fork the writeback thread for this bdi and repeat the loop.

IOW, now we find the first bdi to process, process it, and so on. This is
simpler and involves less lists.

The bonus now is that we can get rid of a couple of functions, as well as
remove complications which involve 'rcu_call()' and 'bdi->rcu_head'.

This patch also makes sure we use 'list_add_tail_rcu()', instead of plain
'list_add_tail()', but this piece of code is going to be removed in the next
patch anyway.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 include/linux/backing-dev.h |    1 -
 mm/backing-dev.c            |   82 +++++++++---------------------------------
 2 files changed, 18 insertions(+), 65 deletions(-)

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index f0936f5..95ecb2b 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -58,7 +58,6 @@ struct bdi_writeback {
 
 struct backing_dev_info {
 	struct list_head bdi_list;
-	struct rcu_head rcu_head;
 	unsigned long ra_pages;	/* max readahead in PAGE_CACHE_SIZE units */
 	unsigned long state;	/* Always use atomic bitops on this */
 	unsigned int capabilities; /* Device capabilities */
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index ae44417..dbc6681 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -50,8 +50,6 @@ static struct timer_list sync_supers_timer;
 static int bdi_sync_supers(void *);
 static void sync_supers_timer_fn(unsigned long);
 
-static void bdi_add_default_flusher_thread(struct backing_dev_info *bdi);
-
 #ifdef CONFIG_DEBUG_FS
 #include <linux/debugfs.h>
 #include <linux/seq_file.h>
@@ -331,6 +329,7 @@ static int bdi_forker_thread(void *ptr)
 	set_user_nice(current, 0);
 
 	for (;;) {
+		bool fork = false;
 		struct task_struct *task;
 		struct backing_dev_info *bdi, *tmp;
 
@@ -349,23 +348,30 @@ static int bdi_forker_thread(void *ptr)
 		 * a thread registered. If so, set that up.
 		 */
 		list_for_each_entry_safe(bdi, tmp, &bdi_list, bdi_list) {
+			if (!bdi_cap_writeback_dirty(bdi))
+				continue;
 			if (bdi->wb.task)
 				continue;
 			if (list_empty(&bdi->work_list) &&
 			    !bdi_has_dirty_io(bdi))
 				continue;
 
-			bdi_add_default_flusher_thread(bdi);
+			WARN(!test_bit(BDI_registered, &bdi->state),
+			     "bdi %p/%s is not registered!\n", bdi, bdi->name);
+
+			list_del_rcu(&bdi->bdi_list);
+			fork = true;
+			break;
 		}
+		spin_unlock_bh(&bdi_lock);
 
 		/* Keep working if default bdi still has things to do */
 		if (!list_empty(&me->bdi->work_list))
 			__set_current_state(TASK_RUNNING);
 
-		if (list_empty(&bdi_pending_list)) {
+		if (!fork) {
 			unsigned long wait;
 
-			spin_unlock_bh(&bdi_lock);
 			wait = msecs_to_jiffies(dirty_writeback_interval * 10);
 			if (wait)
 				schedule_timeout(wait);
@@ -378,13 +384,13 @@ static int bdi_forker_thread(void *ptr)
 		__set_current_state(TASK_RUNNING);
 
 		/*
-		 * This is our real job - check for pending entries in
-		 * bdi_pending_list, and create the threads that got added
+		 * Set the pending bit - if someone will try to unregister this
+		 * bdi - it'll wait on this bit.
 		 */
-		bdi = list_entry(bdi_pending_list.next, struct backing_dev_info,
-				 bdi_list);
-		list_del_init(&bdi->bdi_list);
-		spin_unlock_bh(&bdi_lock);
+		set_bit(BDI_pending, &bdi->state);
+
+		/* Make sure no one uses the picked bdi */
+		synchronize_rcu();
 
 		task = kthread_run(bdi_writeback_thread, &bdi->wb, "flush-%s",
 				   dev_name(bdi->dev));
@@ -397,7 +403,7 @@ static int bdi_forker_thread(void *ptr)
 			 * flush other bdi's to free memory.
 			 */
 			spin_lock_bh(&bdi_lock);
-			list_add_tail(&bdi->bdi_list, &bdi_pending_list);
+			list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
 			spin_unlock_bh(&bdi_lock);
 
 			bdi_flush_io(bdi);
@@ -408,57 +414,6 @@ static int bdi_forker_thread(void *ptr)
 	return 0;
 }
 
-static void bdi_add_to_pending(struct rcu_head *head)
-{
-	struct backing_dev_info *bdi;
-
-	bdi = container_of(head, struct backing_dev_info, rcu_head);
-	INIT_LIST_HEAD(&bdi->bdi_list);
-
-	spin_lock(&bdi_lock);
-	list_add_tail(&bdi->bdi_list, &bdi_pending_list);
-	spin_unlock(&bdi_lock);
-
-	/*
-	 * We are now on the pending list, wake up bdi_forker_task()
-	 * to finish the job and add us back to the active bdi_list
-	 */
-	wake_up_process(default_backing_dev_info.wb.task);
-}
-
-/*
- * Add the default flusher thread that gets created for any bdi
- * that has dirty data pending writeout
- */
-void static bdi_add_default_flusher_thread(struct backing_dev_info *bdi)
-{
-	if (!bdi_cap_writeback_dirty(bdi))
-		return;
-
-	if (WARN_ON(!test_bit(BDI_registered, &bdi->state))) {
-		printk(KERN_ERR "bdi %p/%s is not registered!\n",
-							bdi, bdi->name);
-		return;
-	}
-
-	/*
-	 * Check with the helper whether to proceed adding a thread. Will only
-	 * abort if we two or more simultanous calls to
-	 * bdi_add_default_flusher_thread() occured, further additions will
-	 * block waiting for previous additions to finish.
-	 */
-	if (!test_and_set_bit(BDI_pending, &bdi->state)) {
-		list_del_rcu(&bdi->bdi_list);
-
-		/*
-		 * We must wait for the current RCU period to end before
-		 * moving to the pending list. So schedule that operation
-		 * from an RCU callback.
-		 */
-		call_rcu(&bdi->rcu_head, bdi_add_to_pending);
-	}
-}
-
 /*
  * Remove bdi from bdi_list, and ensure that it is no longer visible
  */
@@ -599,7 +554,6 @@ int bdi_init(struct backing_dev_info *bdi)
 	bdi->max_ratio = 100;
 	bdi->max_prop_frac = PROP_FRAC_BASE;
 	spin_lock_init(&bdi->wb_lock);
-	INIT_RCU_HEAD(&bdi->rcu_head);
 	INIT_LIST_HEAD(&bdi->bdi_list);
 	INIT_LIST_HEAD(&bdi->work_list);
 
-- 
1.7.1.1

Re: [PATCHv2 06/11] writeback: simplify bdi code a little

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCHv2 07/11] writeback: do not remove bdi from bdi_list

[Artem Bityutskiy]

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

The forker thread removes bdis from 'bdi_list' before forking the bdi thread.
But this is wrong for at least 2 reasons.

Reason #1: if we temporary remove a bdi from the list, we may miss works which
           would otherwise be given to us.

Reason #2: this is racy; indeed, 'bdi_wb_shutdown()' expects that bdis are
           always in the 'bdi_list' (see 'bdi_remove_from_list()'), and when
           it races with the forker thread, it can shut down the bdi thread
           at the same time as the forker creates it.

This patch makes sure the forker thread never removes bdis from 'bdi_list'
(which was suggested by Christoph Hellwig).

In order to make sure that we do not race with 'bdi_wb_shutdown()', we have to
hold the 'bdi_lock' while walking the 'bdi_list' and setting the 'BDI_pending'
flag.

NOTE! The error path is interesting. Currently, when we fail to create a bdi
thread, we move the bdi to the tail of 'bdi_list'. But if we never remove the
bdi from the list, we cannot move it to the tail either, because then we can
mess up the RCU readers which walk the list. And also, we'll have the race
described above in "Reason #2".

But I not think that adding to the tail is any important so I just do not do
that.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 fs/fs-writeback.c |    7 -------
 mm/backing-dev.c  |   31 ++++++++++---------------------
 2 files changed, 10 insertions(+), 28 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 8cf53ba..eaef5c9 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -804,13 +804,6 @@ int bdi_writeback_thread(void *data)
 	unsigned long wait_jiffies = -1UL;
 	long pages_written;
 
-	/*
-	 * Add us to the active bdi_list
-	 */
-	spin_lock_bh(&bdi_lock);
-	list_add_rcu(&bdi->bdi_list, &bdi_list);
-	spin_unlock_bh(&bdi_lock);
-
 	current->flags |= PF_FLUSHER | PF_SWAPWRITE;
 	set_freezable();
 
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index dbc6681..672c17b 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -331,7 +331,7 @@ static int bdi_forker_thread(void *ptr)
 	for (;;) {
 		bool fork = false;
 		struct task_struct *task;
-		struct backing_dev_info *bdi, *tmp;
+		struct backing_dev_info *bdi;
 
 		/*
 		 * Temporary measure, we want to make sure we don't see
@@ -347,7 +347,7 @@ static int bdi_forker_thread(void *ptr)
 		 * Check if any existing bdi's have dirty data without
 		 * a thread registered. If so, set that up.
 		 */
-		list_for_each_entry_safe(bdi, tmp, &bdi_list, bdi_list) {
+		list_for_each_entry(bdi, &bdi_list, bdi_list) {
 			if (!bdi_cap_writeback_dirty(bdi))
 				continue;
 			if (bdi->wb.task)
@@ -359,8 +359,13 @@ static int bdi_forker_thread(void *ptr)
 			WARN(!test_bit(BDI_registered, &bdi->state),
 			     "bdi %p/%s is not registered!\n", bdi, bdi->name);
 
-			list_del_rcu(&bdi->bdi_list);
 			fork = true;
+
+			/*
+			 * Set the pending bit - if someone will try to
+			 * unregister this bdi - it'll wait on this bit.
+			 */
+			set_bit(BDI_pending, &bdi->state);
 			break;
 		}
 		spin_unlock_bh(&bdi_lock);
@@ -383,29 +388,13 @@ static int bdi_forker_thread(void *ptr)
 
 		__set_current_state(TASK_RUNNING);
 
-		/*
-		 * Set the pending bit - if someone will try to unregister this
-		 * bdi - it'll wait on this bit.
-		 */
-		set_bit(BDI_pending, &bdi->state);
-
-		/* Make sure no one uses the picked bdi */
-		synchronize_rcu();
-
 		task = kthread_run(bdi_writeback_thread, &bdi->wb, "flush-%s",
 				   dev_name(bdi->dev));
 		if (IS_ERR(task)) {
 			/*
-			 * If thread creation fails, then readd the bdi back to
-			 * the list and force writeout of the bdi from this
-			 * forker thread. That will free some memory and we can
-			 * try again. Add it to the tail so we get a chance to
-			 * flush other bdi's to free memory.
+			 * If thread creation fails, force writeout of the bdi
+			 * from the thread.
 			 */
-			spin_lock_bh(&bdi_lock);
-			list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
-			spin_unlock_bh(&bdi_lock);
-
 			bdi_flush_io(bdi);
 		} else
 			bdi->wb.task = task;
-- 
1.7.1.1

Re: [PATCHv2 07/11] writeback: do not remove bdi from bdi_list

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCHv2 08/11] writeback: move last_active to bdi

[Artem Bityutskiy]

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

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>
---
 fs/fs-writeback.c           |    6 +++---
 include/linux/backing-dev.h |   13 +++++++------
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index eaef5c9..53e1028 100644
--- a/fs/fs-writeback.c
+++ b/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;
 		}
 
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 95ecb2b..71b6223 100644
--- a/include/linux/backing-dev.h
+++ b/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 {
-- 
1.7.1.1

[PATCHv2 09/11] writeback: restructure bdi forker loop a little

[Artem Bityutskiy]

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

This patch re-structures the loop which walks bdi a little. This is just a
micro-step towards the coming change where the forker thread will kill the bdi
threads. It should simplify reviewing the following changes, which would
otherwise be larger.

This patch also adds the 'bdi_cap_flush_forker(bdi)' condition check to the
loop. The reason for this is that the forker thread can start _before_ the
'BDI_registered' flag is set (see 'bdi_register()'), so the WARN() statement
will fire for the default bdi. I observed this warning at boot-up.

This patch also amends comments a little.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 mm/backing-dev.c |   28 +++++++++++++++++-----------
 1 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 672c17b..b788b8e 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -348,25 +348,31 @@ static int bdi_forker_thread(void *ptr)
 		 * a thread registered. If so, set that up.
 		 */
 		list_for_each_entry(bdi, &bdi_list, bdi_list) {
-			if (!bdi_cap_writeback_dirty(bdi))
-				continue;
-			if (bdi->wb.task)
-				continue;
-			if (list_empty(&bdi->work_list) &&
-			    !bdi_has_dirty_io(bdi))
+			bool have_dirty_io;
+
+			if (!bdi_cap_writeback_dirty(bdi) ||
+			     bdi_cap_flush_forker(bdi))
 				continue;
 
 			WARN(!test_bit(BDI_registered, &bdi->state),
 			     "bdi %p/%s is not registered!\n", bdi, bdi->name);
 
-			fork = true;
+			have_dirty_io = !list_empty(&bdi->work_list) ||
+					wb_has_dirty_io(&bdi->wb);
 
 			/*
-			 * Set the pending bit - if someone will try to
-			 * unregister this bdi - it'll wait on this bit.
+			 * If the bdi has work to do, but the thread does not
+			 * exist - create it.
 			 */
-			set_bit(BDI_pending, &bdi->state);
-			break;
+			if (!bdi->wb.task && have_dirty_io) {
+				/*
+				 * Set the pending bit - if someone will try to
+				 * unregister this bdi - it'll wait on this bit.
+				 */
+				set_bit(BDI_pending, &bdi->state);
+				fork = true;
+				break;
+			}
 		}
 		spin_unlock_bh(&bdi_lock);
 
-- 
1.7.1.1

Re: [PATCHv2 09/11] writeback: restructure bdi forker loop a little

[Christoph Hellwig]

On Wed, Jul 21, 2010 at 12:31:44PM +0300, Artem Bityutskiy wrote:
> From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
> 
> This patch re-structures the loop which walks bdi a little. This is just a
> micro-step towards the coming change where the forker thread will kill the bdi
> threads. It should simplify reviewing the following changes, which would
> otherwise be larger.
> 
> This patch also adds the 'bdi_cap_flush_forker(bdi)' condition check to the
> loop. The reason for this is that the forker thread can start _before_ the
> 'BDI_registered' flag is set (see 'bdi_register()'), so the WARN() statement
> will fire for the default bdi. I observed this warning at boot-up.
> 
> This patch also amends comments a little.

Looks good,


Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCHv2 10/11] writeback: move bdi threads exiting logic to the forker thread

[Artem Bityutskiy]

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

Currently, bdi threads can decide to exit if there were no useful activities
for 5 minutes. However, this causes nasty races: we can easily oops in the
'bdi_queue_work()' if the bdi thread decides to exit while we are waking it up.

And even if we do not oops, but the bdi tread exits immediately after we wake
it up, we'd lose the wake-up event and have an unnecessary delay (up to 5 secs)
in the bdi work processing.

This patch makes the forker thread to be the central place which not only
creates bdi threads, but also kills them if they were inactive long enough.
This better design-wise.

Another reason why this change was done is to prepare for the further changes
which will prevent the bdi threads from waking up every 5 sec and wasting
power. Indeed, when the task does not wake up periodically anymore, it won't be
able to exit either.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 fs/fs-writeback.c |   38 ++++++--------------------------
 mm/backing-dev.c  |   62 +++++++++++++++++++++++++++++++++++++++-------------
 2 files changed, 53 insertions(+), 47 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 53e1028..9055809 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -78,8 +78,6 @@ static void bdi_queue_work(struct backing_dev_info *bdi,
 
 	spin_lock(&bdi->wb_lock);
 	list_add_tail(&work->list, &bdi->work_list);
-	spin_unlock(&bdi->wb_lock);
-
 	/*
 	 * If the default thread isn't there, make sure we add it. When
 	 * it gets created and wakes up, we'll run this work.
@@ -87,12 +85,9 @@ static void bdi_queue_work(struct backing_dev_info *bdi,
 	if (unlikely(!bdi->wb.task)) {
 		trace_writeback_nothread(bdi, work);
 		wake_up_process(default_backing_dev_info.wb.task);
-	} else {
-		struct bdi_writeback *wb = &bdi->wb;
-
-		if (wb->task)
-			wake_up_process(wb->task);
-	}
+	} else
+		wake_up_process(bdi->wb.task);
+	spin_unlock(&bdi->wb_lock);
 }
 
 static void
@@ -800,7 +795,6 @@ int bdi_writeback_thread(void *data)
 {
 	struct bdi_writeback *wb = data;
 	struct backing_dev_info *bdi = wb->bdi;
-	unsigned long wait_jiffies = -1UL;
 	long pages_written;
 
 	current->flags |= PF_FLUSHER | PF_SWAPWRITE;
@@ -828,18 +822,6 @@ int bdi_writeback_thread(void *data)
 
 		if (pages_written)
 			wb->last_active = jiffies;
-		else if (wait_jiffies != -1UL) {
-			unsigned long max_idle;
-
-			/*
-			 * Longest period of inactivity that we tolerate. If we
-			 * see dirty data again later, the thread will get
-			 * recreated automatically.
-			 */
-			max_idle = max(5UL * 60 * HZ, wait_jiffies);
-			if (time_after(jiffies, max_idle + wb->last_active))
-				break;
-		}
 
 		set_current_state(TASK_INTERRUPTIBLE);
 		if (!list_empty(&bdi->work_list)) {
@@ -847,21 +829,15 @@ int bdi_writeback_thread(void *data)
 			continue;
 		}
 
-		if (dirty_writeback_interval) {
-			wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
-			schedule_timeout(wait_jiffies);
-		} else
+		if (dirty_writeback_interval)
+			schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
+		else
 			schedule();
 
 		try_to_freeze();
 	}
 
-	wb->task = NULL;
-
-	/*
-	 * Flush any work that raced with us exiting. No new work
-	 * will be added, since this bdi isn't discoverable anymore.
-	 */
+	/* Flush any work that raced with us exiting */
 	if (!list_empty(&bdi->work_list))
 		wb_do_writeback(wb, 1);
 
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index b788b8e..3975440 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -316,6 +316,18 @@ static void sync_supers_timer_fn(unsigned long unused)
 	bdi_arm_supers_timer();
 }
 
+/*
+ * Calculate the longest interval (jiffies) bdi threads are allowed to be
+ * inactive.
+ */
+static unsigned long bdi_longest_inactive(void)
+{
+	unsigned long interval;
+
+	interval = msecs_to_jiffies(dirty_writeback_interval * 10);
+	return max(5UL * 60 * HZ, interval);
+}
+
 static int bdi_forker_thread(void *ptr)
 {
 	struct bdi_writeback *me = ptr;
@@ -329,8 +341,8 @@ static int bdi_forker_thread(void *ptr)
 	set_user_nice(current, 0);
 
 	for (;;) {
-		bool fork = false;
-		struct task_struct *task;
+		bool fork = false, kill = false;
+		struct task_struct *task = NULL;
 		struct backing_dev_info *bdi;
 
 		/*
@@ -343,10 +355,6 @@ static int bdi_forker_thread(void *ptr)
 		spin_lock_bh(&bdi_lock);
 		set_current_state(TASK_INTERRUPTIBLE);
 
-		/*
-		 * Check if any existing bdi's have dirty data without
-		 * a thread registered. If so, set that up.
-		 */
 		list_for_each_entry(bdi, &bdi_list, bdi_list) {
 			bool have_dirty_io;
 
@@ -373,6 +381,25 @@ static int bdi_forker_thread(void *ptr)
 				fork = true;
 				break;
 			}
+
+			spin_lock(&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
+			 * to make sure no-one adds more work to this bdi and
+			 * wakes the bdi thread up.
+			 */
+			if (bdi->wb.task && !have_dirty_io &&
+			    time_after(jiffies, bdi->wb.last_active +
+						bdi_longest_inactive())) {
+				task = bdi->wb.task;
+				bdi->wb.task = NULL;
+				spin_unlock(&bdi->wb_lock);
+				set_bit(BDI_pending, &bdi->state);
+				kill = true;
+				break;
+			}
+			spin_unlock(&bdi->wb_lock);
 		}
 		spin_unlock_bh(&bdi_lock);
 
@@ -380,7 +407,7 @@ static int bdi_forker_thread(void *ptr)
 		if (!list_empty(&me->bdi->work_list))
 			__set_current_state(TASK_RUNNING);
 
-		if (!fork) {
+		if (!fork && !kill) {
 			unsigned long wait;
 
 			wait = msecs_to_jiffies(dirty_writeback_interval * 10);
@@ -394,16 +421,19 @@ static int bdi_forker_thread(void *ptr)
 
 		__set_current_state(TASK_RUNNING);
 
-		task = kthread_run(bdi_writeback_thread, &bdi->wb, "flush-%s",
-				   dev_name(bdi->dev));
-		if (IS_ERR(task)) {
-			/*
-			 * If thread creation fails, force writeout of the bdi
-			 * from the thread.
-			 */
-			bdi_flush_io(bdi);
+		if (fork) {
+			task = kthread_run(bdi_writeback_thread, &bdi->wb, "flush-%s",
+					   dev_name(bdi->dev));
+			if (IS_ERR(task)) {
+				/*
+				 * If thread creation fails, force writeout of the bdi
+				 * from the thread.
+				 */
+				bdi_flush_io(bdi);
+			} else
+				bdi->wb.task = task;
 		} else
-			bdi->wb.task = task;
+			kthread_stop(task);
 	}
 
 	return 0;
-- 
1.7.1.1

Re: [PATCHv2 10/11] writeback: move bdi threads exiting logic to the forker thread

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

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

Finally, we can get rid of unnecessary wake-ups in bdi threads, which are very
bad for battery-driven devices.

There are two types of activities bdi threads do:
1. process bdi works from the 'bdi->work_list'
2. periodic write-back

So there are 2 sources of wake-up events for bdi threads:

1. 'bdi_queue_work()' - submits bdi works
2. '__mark_inode_dirty()' - adds dirty I/O to bdi's

The former already has bdi wake-up code. The latter does not, and this patch
adds it.

'__mark_inode_dirty()' is hot-path function, but this patch adds another
'spin_lock(&bdi->wb_lock)' there. However, it is taken only in rare cases when
the bdi has no dirty inodes. So adding this spinlock should be fine and should
not affect performance.

This patch makes sure bdi threads and the forker thread do not wake-up if there
is nothing to do. The forker thread will nevertheless wake up at least every
5 min. to check whether it has to kill a bdi thread. This can also be optimized,
but is not worth it.

This patch also tidies up the warning about unregistered bid, and turns it from
an ugly crocodile to a simple 'WARN()' statement.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
---
 fs/fs-writeback.c |   45 ++++++++++++++++++++++++++++++++++-----------
 mm/backing-dev.c  |   16 ++++++++++++----
 2 files changed, 46 insertions(+), 15 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 9055809..50d9ceb 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -829,10 +829,16 @@ int bdi_writeback_thread(void *data)
 			continue;
 		}
 
-		if (dirty_writeback_interval)
+		if (wb_has_dirty_io(wb) && dirty_writeback_interval)
 			schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
-		else
+		else {
+			/*
+			 * We have nothing to do, so can go sleep without any
+			 * timeout and save power. When a work is queued or
+			 * something is made dirty - we will be woken up.
+			 */
 			schedule();
+		}
 
 		try_to_freeze();
 	}
@@ -920,6 +926,8 @@ static noinline void block_dump___mark_inode_dirty(struct inode *inode)
 void __mark_inode_dirty(struct inode *inode, int flags)
 {
 	struct super_block *sb = inode->i_sb;
+	struct backing_dev_info *bdi = NULL;
+	bool wakeup_bdi = false;
 
 	/*
 	 * Don't do this for I_DIRTY_PAGES - that doesn't actually
@@ -973,22 +981,37 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 		 * reposition it (that would break b_dirty time-ordering).
 		 */
 		if (!was_dirty) {
-			struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
-			struct backing_dev_info *bdi = wb->bdi;
-
-			if (bdi_cap_writeback_dirty(bdi) &&
-			    !test_bit(BDI_registered, &bdi->state)) {
-				WARN_ON(1);
-				printk(KERN_ERR "bdi-%s not registered\n",
-								bdi->name);
+			bdi = inode_to_bdi(inode);
+
+			if (bdi_cap_writeback_dirty(bdi)) {
+				WARN(!test_bit(BDI_registered, &bdi->state),
+				     "bdi-%s not registered\n", bdi->name);
+
+				/*
+				 * If this is the first dirty inode for this
+				 * bdi, we have to wake-up the corresponding
+				 * bdi thread to make sure background
+				 * write-back happens later.
+				 */
+				if (!wb_has_dirty_io(&bdi->wb))
+					wakeup_bdi = true;
 			}
 
 			inode->dirtied_when = jiffies;
-			list_move(&inode->i_list, &wb->b_dirty);
+			list_move(&inode->i_list, &bdi->wb.b_dirty);
 		}
 	}
 out:
 	spin_unlock(&inode_lock);
+
+	if (wakeup_bdi) {
+		spin_lock(&bdi->wb_lock);
+		if (!bdi->wb.task)
+			wake_up_process(default_backing_dev_info.wb.task);
+		else
+			wake_up_process(bdi->wb.task);
+		spin_unlock(&bdi->wb_lock);
+	}
 }
 EXPORT_SYMBOL(__mark_inode_dirty);
 
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 3975440..01e6608 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -411,10 +411,18 @@ static int bdi_forker_thread(void *ptr)
 			unsigned long wait;
 
 			wait = msecs_to_jiffies(dirty_writeback_interval * 10);
-			if (wait)
-				schedule_timeout(wait);
-			else
-				schedule();
+			if (!wb_has_dirty_io(me) || !wait) {
+				/*
+				 * There are no dirty data. The only thing we
+				 * should now care is checking for inactive bdi
+				 * threads and killing them. Thus, let's sleep
+				 * for longer time to avoid unnecessary
+				 * wake-ups, save energy and be friendly for
+				 * battery-driven devices.
+				 */
+				wait = bdi_longest_inactive();
+			}
+			schedule_timeout(wait);
 			try_to_freeze();
 			continue;
 		}
-- 
1.7.1.1

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Wed, 2010-07-21 at 12:31 +0300, Artem Bityutskiy wrote:
>  	spin_unlock(&inode_lock);
> +
> +	if (wakeup_bdi) {
> +		spin_lock(&bdi->wb_lock);
> +		if (!bdi->wb.task)
> +			wake_up_process(default_backing_dev_info.wb.task);
> +		else
> +			wake_up_process(bdi->wb.task);
> +		spin_unlock(&bdi->wb_lock);
> +	}
>  }

Dave,

I do not know whether this stuff will end up in upstream, I did not get
any feed back from Jens so far. But if it will, I'd like to let you know
that the code quoted above is similar to the 'bdi_queue_work()'
function. And the purpose is very similar. But you added a
'trace_writeback_nothread()' call to 'bdi_queue_work()', and I think a
similar call has to be here.

Can I call 'trace_writeback_nothread()'? I guess not. Should I create
another trace point? Any hints/instructions?

Note, the patches are against Jens' tree.

Please, see linux-fsdevel or lkml for the full patch and its purposes.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Dave Chinner]

On Wed, Jul 21, 2010 at 02:45:41PM +0300, Artem Bityutskiy wrote:
> On Wed, 2010-07-21 at 12:31 +0300, Artem Bityutskiy wrote:
> >  	spin_unlock(&inode_lock);
> > +
> > +	if (wakeup_bdi) {
> > +		spin_lock(&bdi->wb_lock);
> > +		if (!bdi->wb.task)
> > +			wake_up_process(default_backing_dev_info.wb.task);
> > +		else
> > +			wake_up_process(bdi->wb.task);
> > +		spin_unlock(&bdi->wb_lock);
> > +	}
> >  }
> 
> Dave,
> 
> I do not know whether this stuff will end up in upstream, I did not get
> any feed back from Jens so far. But if it will, I'd like to let you know
> that the code quoted above is similar to the 'bdi_queue_work()'
> function. And the purpose is very similar. But you added a
> 'trace_writeback_nothread()' call to 'bdi_queue_work()', and I think a
> similar call has to be here.

Yes, that seems like a sane thing to do ;)

> Can I call 'trace_writeback_nothread()'? I guess not. Should I create
> another trace point? Any hints/instructions?

The bdi_queue_work() tracepoints expect a work structure to be
passed in, so you can't use them (or that class of event) if you
don't have a struct wb_writeback_work.

For __mark_inode_dirty(), I'd add two new tracepoints like:

DEFINE_WRITEBACK_EVENT(writeback_wakeup);
DEFINE_WRITEBACK_EVENT(writeback_wakeup_nothread);

and place them as:

	if (wakeup_bdi) {
		trace_writeback_wakeup(bdi)
		spin_lock(&bdi->wb_lock);
		if (!bdi->wb.task) {{
			trace_writeback_wakeup_nothread(bdi);
			wake_up_process(default_backing_dev_info.wb.task);
		} else
			wake_up_process(bdi->wb.task);
		spin_unlock(&bdi->wb_lock);
	}

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 10:41 +1000, Dave Chinner wrote:
> 	if (wakeup_bdi) {
> 		trace_writeback_wakeup(bdi)
> 		spin_lock(&bdi->wb_lock);
> 		if (!bdi->wb.task) {{
> 			trace_writeback_wakeup_nothread(bdi);
> 			wake_up_process(default_backing_dev_info.wb.task);
> 		} else
> 			wake_up_process(bdi->wb.task);
> 		spin_unlock(&bdi->wb_lock);
> 	}

OK, I'll take a look at this and will try to add this to v3, thanks.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 10:41 +1000, Dave Chinner wrote:
> 	if (wakeup_bdi) {
> 		trace_writeback_wakeup(bdi)
> 		spin_lock(&bdi->wb_lock);
> 		if (!bdi->wb.task) {{
> 			trace_writeback_wakeup_nothread(bdi);
> 			wake_up_process(default_backing_dev_info.wb.task);
> 		} else
> 			wake_up_process(bdi->wb.task);
> 		spin_unlock(&bdi->wb_lock);
> 	}

OK, I'll take a look at this and will try to add this to v3, thanks.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Christoph Hellwig]

On Thu, Jul 22, 2010 at 10:41:55AM +1000, Dave Chinner wrote:
> 	if (wakeup_bdi) {
> 		trace_writeback_wakeup(bdi)
> 		spin_lock(&bdi->wb_lock);
> 		if (!bdi->wb.task) {{
> 			trace_writeback_wakeup_nothread(bdi);
> 			wake_up_process(default_backing_dev_info.wb.task);
> 		} else
> 			wake_up_process(bdi->wb.task);
> 		spin_unlock(&bdi->wb_lock);

That gives us duplicate traces for the first case, what about:

	 if (wakeup_bdi) {
	 	spin_lock(&bdi->wb_lock);
		if (bdi->wb.task) {
			trace_writeback_wake_thread(bdi);
			wake_up_process(bdi->wb.task);
		} else {
			trace_writeback_wake_forker_thread(bdi);
			wake_up_process(default_backing_dev_info.wb.task);
		}
		spin_unlock(&bdi->wb_lock);
	}

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 05:00 -0400, Christoph Hellwig wrote:
> 	 if (wakeup_bdi) {
> 	 	spin_lock(&bdi->wb_lock);
> 		if (bdi->wb.task) {
> 			trace_writeback_wake_thread(bdi);
> 			wake_up_process(bdi->wb.task);
> 		} else {
> 			trace_writeback_wake_forker_thread(bdi);
> 			wake_up_process(default_backing_dev_info.wb.task);
> 		}
> 		spin_unlock(&bdi->wb_lock);
> 	}

Side note: I've noticed you've made the optimistic "if" condition first,
which is better, and I'll also amend this in v3.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 05:00 -0400, Christoph Hellwig wrote:
> On Thu, Jul 22, 2010 at 10:41:55AM +1000, Dave Chinner wrote:
> > 	if (wakeup_bdi) {
> > 		trace_writeback_wakeup(bdi)
> > 		spin_lock(&bdi->wb_lock);
> > 		if (!bdi->wb.task) {{
> > 			trace_writeback_wakeup_nothread(bdi);
> > 			wake_up_process(default_backing_dev_info.wb.task);
> > 		} else
> > 			wake_up_process(bdi->wb.task);
> > 		spin_unlock(&bdi->wb_lock);
> 
> That gives us duplicate traces for the first case, what about:
> 
> 	 if (wakeup_bdi) {
> 	 	spin_lock(&bdi->wb_lock);
> 		if (bdi->wb.task) {
> 			trace_writeback_wake_thread(bdi);
> 			wake_up_process(bdi->wb.task);
> 		} else {
> 			trace_writeback_wake_forker_thread(bdi);
> 			wake_up_process(default_backing_dev_info.wb.task);
> 		}
> 		spin_unlock(&bdi->wb_lock);
> 	}

But Dave's version is what we have in 'bdi_queue_work()' as well. I it
is OK - first trace point is triggered every time the bdi thread is
_wanted_ to be woken up.

Then if it does not exist, we need to do something special to wake it up
(ask the forker thread which to create it). This is a different event
and we use a different trace point.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 05:00 -0400, Christoph Hellwig wrote:
> On Thu, Jul 22, 2010 at 10:41:55AM +1000, Dave Chinner wrote:
> > 	if (wakeup_bdi) {
> > 		trace_writeback_wakeup(bdi)
> > 		spin_lock(&bdi->wb_lock);
> > 		if (!bdi->wb.task) {{
> > 			trace_writeback_wakeup_nothread(bdi);
> > 			wake_up_process(default_backing_dev_info.wb.task);
> > 		} else
> > 			wake_up_process(bdi->wb.task);
> > 		spin_unlock(&bdi->wb_lock);
> 
> That gives us duplicate traces for the first case, what about:
> 
> 	 if (wakeup_bdi) {
> 	 	spin_lock(&bdi->wb_lock);
> 		if (bdi->wb.task) {
> 			trace_writeback_wake_thread(bdi);
> 			wake_up_process(bdi->wb.task);
> 		} else {
> 			trace_writeback_wake_forker_thread(bdi);
> 			wake_up_process(default_backing_dev_info.wb.task);
> 		}
> 		spin_unlock(&bdi->wb_lock);
> 	}

But Dave's version is what we have in 'bdi_queue_work()' as well. I it
is OK - first trace point is triggered every time the bdi thread is
_wanted_ to be woken up.

Then if it does not exist, we need to do something special to wake it up
(ask the forker thread which to create it). This is a different event
and we use a different trace point.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Christoph Hellwig]

Looks good,


Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Nick Piggin]

On Wed, Jul 21, 2010 at 12:31:46PM +0300, Artem Bityutskiy wrote:
> @@ -973,22 +981,37 @@ void __mark_inode_dirty(struct inode *inode, int flags)
>  		 * reposition it (that would break b_dirty time-ordering).
>  		 */
>  		if (!was_dirty) {
> -			struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
> -			struct backing_dev_info *bdi = wb->bdi;
> -
> -			if (bdi_cap_writeback_dirty(bdi) &&
> -			    !test_bit(BDI_registered, &bdi->state)) {
> -				WARN_ON(1);
> -				printk(KERN_ERR "bdi-%s not registered\n",
> -								bdi->name);
> +			bdi = inode_to_bdi(inode);
> +
> +			if (bdi_cap_writeback_dirty(bdi)) {
> +				WARN(!test_bit(BDI_registered, &bdi->state),
> +				     "bdi-%s not registered\n", bdi->name);
> +
> +				/*
> +				 * If this is the first dirty inode for this
> +				 * bdi, we have to wake-up the corresponding
> +				 * bdi thread to make sure background
> +				 * write-back happens later.
> +				 */
> +				if (!wb_has_dirty_io(&bdi->wb))
> +					wakeup_bdi = true;
>  			}
>  
>  			inode->dirtied_when = jiffies;
> -			list_move(&inode->i_list, &wb->b_dirty);
> +			list_move(&inode->i_list, &bdi->wb.b_dirty);
>  		}
>  	}
>  out:
>  	spin_unlock(&inode_lock);
> +
> +	if (wakeup_bdi) {
> +		spin_lock(&bdi->wb_lock);
> +		if (!bdi->wb.task)
> +			wake_up_process(default_backing_dev_info.wb.task);
> +		else
> +			wake_up_process(bdi->wb.task);
> +		spin_unlock(&bdi->wb_lock);
> +	}
>  }

We really want to wake up the bdi right away when first dirtying
the inode? I haven't looked at where the state of the bdi code is
now, but isn't it better to have a a delay there?

And rather than spreading details of how bdi tasks are managed
would you consider putting this into its own function?

Other than that, I like your patches. Out of interest, is 5 seconds
very detremental to power usage? What is a reasonable goal for
wakeups? (eg. 95%+ of possible efficiency)

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

Hi Nick,

On Thu, 2010-07-22 at 13:19 +1000, Nick Piggin wrote:
> >  out:
> >  	spin_unlock(&inode_lock);
> > +
> > +	if (wakeup_bdi) {
> > +		spin_lock(&bdi->wb_lock);
> > +		if (!bdi->wb.task)
> > +			wake_up_process(default_backing_dev_info.wb.task);
> > +		else
> > +			wake_up_process(bdi->wb.task);
> > +		spin_unlock(&bdi->wb_lock);
> > +	}
> >  }
> 
> We really want to wake up the bdi right away when first dirtying
> the inode? I haven't looked at where the state of the bdi code is
> now, but isn't it better to have a a delay there?

Yes, I guess we want to wake up the bdi thread after 5 secs (assuming
default settings). I could implement a "wake_up_process_delayed"
function which would use a timer, but I think it is not necessary to
introduce these complications. We can just wake-up the bdi thread, it'll
find out there is nothing to do, and will go sleep for 5 secs. I think
this is good enough.

IOW, delayed wake-up is not worth the trouble.

> And rather than spreading details of how bdi tasks are managed
> would you consider putting this into its own function?

Sure, will do.

> Other than that, I like your patches.

Thanks :-)

>  Out of interest, is 5 seconds
> very detremental to power usage? What is a reasonable goal for
> wakeups? (eg. 95%+ of possible efficiency)

I cannot tell for sure. In Nokia N900 phone we use OMAP3 and we have
dynamic OFF-mode, so we switch off the CPU and peripherals completely
when there is nothing to do, and SDRAM stays in low-power auto-refresh
mode. Every useless wake-up makes us do a lot of job re-constructing the
CPU state. I cannot tell the numbers, but I'm CCing Tero, who is working
on OMAP3 PM and makes a lot of battery current measurements, he can
provide some numbers.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

RE: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Tero.Kristo]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain; charset="utf-8", Size: 3265 bytes --]



>-----Original Message-----
>From: Artem Bityutskiy [mailto:dedekind1@gmail.com]
>Sent: 22 July, 2010 09:48
>To: Nick Piggin; Kristo Tero (Nokia-MS/Tampere)
>Cc: Jens Axboe; linux-fsdevel@vger.kernel.org; linux-
>kernel@vger.kernel.org
>Subject: Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads
>wakeups
>
>Hi Nick,
>
>On Thu, 2010-07-22 at 13:19 +1000, Nick Piggin wrote:
>> >  out:
>> >  	spin_unlock(&inode_lock);
>> > +
>> > +	if (wakeup_bdi) {
>> > +		spin_lock(&bdi->wb_lock);
>> > +		if (!bdi->wb.task)
>> > +			wake_up_process(default_backing_dev_info.wb.task);
>> > +		else
>> > +			wake_up_process(bdi->wb.task);
>> > +		spin_unlock(&bdi->wb_lock);
>> > +	}
>> >  }
>>
>> We really want to wake up the bdi right away when first dirtying
>> the inode? I haven't looked at where the state of the bdi code is
>> now, but isn't it better to have a a delay there?
>
>Yes, I guess we want to wake up the bdi thread after 5 secs (assuming
>default settings). I could implement a "wake_up_process_delayed"
>function which would use a timer, but I think it is not necessary to
>introduce these complications. We can just wake-up the bdi thread, it'll
>find out there is nothing to do, and will go sleep for 5 secs. I think
>this is good enough.
>
>IOW, delayed wake-up is not worth the trouble.
>
>> And rather than spreading details of how bdi tasks are managed
>> would you consider putting this into its own function?
>
>Sure, will do.
>
>> Other than that, I like your patches.
>
>Thanks :-)
>
>>  Out of interest, is 5 seconds
>> very detremental to power usage? What is a reasonable goal for
>> wakeups? (eg. 95%+ of possible efficiency)
>
>I cannot tell for sure. In Nokia N900 phone we use OMAP3 and we have
>dynamic OFF-mode, so we switch off the CPU and peripherals completely
>when there is nothing to do, and SDRAM stays in low-power auto-refresh
>mode. Every useless wake-up makes us do a lot of job re-constructing the
>CPU state. I cannot tell the numbers, but I'm CCing Tero, who is working
>on OMAP3 PM and makes a lot of battery current measurements, he can
>provide some numbers.

Well, it is hard to give any good guidelines here, as it really depends on the device architecture, possible power saving modes etc., but I can give some sort of guestimate. Basically I think kernel should not generate any extra wakeups at all if it is not doing "anything too useful". In ideal world, everything should be interrupt driven as much as possible, and we would only have timers for things that are clearly visible for user, or can cause some sort of failure if neglected. Like if we ignore watchdogs, the device will reset itself.

5 seconds by itself is not that bad, the reason we want to get rid of these is that every single wakeup source cumulates. If we have 2 wakeups occurring at 5 second intervals and they are not synced, we effectively can wakeup every 2.5 seconds and so on. I guess a good target is to have 1 device level wakeup every 30 seconds or so, but due to cumulation, I tend to complain about anything that happens more often than once a minute.

-Tero

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RE: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Tero.Kristo]

>-----Original Message-----
>From: Artem Bityutskiy [mailto:dedekind1@gmail.com]
>Sent: 22 July, 2010 09:48
>To: Nick Piggin; Kristo Tero (Nokia-MS/Tampere)
>Cc: Jens Axboe; linux-fsdevel@vger.kernel.org; linux-
>kernel@vger.kernel.org
>Subject: Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads
>wakeups
>
>Hi Nick,
>
>On Thu, 2010-07-22 at 13:19 +1000, Nick Piggin wrote:
>> >  out:
>> >  	spin_unlock(&inode_lock);
>> > +
>> > +	if (wakeup_bdi) {
>> > +		spin_lock(&bdi->wb_lock);
>> > +		if (!bdi->wb.task)
>> > +			wake_up_process(default_backing_dev_info.wb.task);
>> > +		else
>> > +			wake_up_process(bdi->wb.task);
>> > +		spin_unlock(&bdi->wb_lock);
>> > +	}
>> >  }
>>
>> We really want to wake up the bdi right away when first dirtying
>> the inode? I haven't looked at where the state of the bdi code is
>> now, but isn't it better to have a a delay there?
>
>Yes, I guess we want to wake up the bdi thread after 5 secs (assuming
>default settings). I could implement a "wake_up_process_delayed"
>function which would use a timer, but I think it is not necessary to
>introduce these complications. We can just wake-up the bdi thread, it'll
>find out there is nothing to do, and will go sleep for 5 secs. I think
>this is good enough.
>
>IOW, delayed wake-up is not worth the trouble.
>
>> And rather than spreading details of how bdi tasks are managed
>> would you consider putting this into its own function?
>
>Sure, will do.
>
>> Other than that, I like your patches.
>
>Thanks :-)
>
>>  Out of interest, is 5 seconds
>> very detremental to power usage? What is a reasonable goal for
>> wakeups? (eg. 95%+ of possible efficiency)
>
>I cannot tell for sure. In Nokia N900 phone we use OMAP3 and we have
>dynamic OFF-mode, so we switch off the CPU and peripherals completely
>when there is nothing to do, and SDRAM stays in low-power auto-refresh
>mode. Every useless wake-up makes us do a lot of job re-constructing the
>CPU state. I cannot tell the numbers, but I'm CCing Tero, who is working
>on OMAP3 PM and makes a lot of battery current measurements, he can
>provide some numbers.

Well, it is hard to give any good guidelines here, as it really depends on the device architecture, possible power saving modes etc., but I can give some sort of guestimate. Basically I think kernel should not generate any extra wakeups at all if it is not doing "anything too useful". In ideal world, everything should be interrupt driven as much as possible, and we would only have timers for things that are clearly visible for user, or can cause some sort of failure if neglected. Like if we ignore watchdogs, the device will reset itself.

5 seconds by itself is not that bad, the reason we want to get rid of these is that every single wakeup source cumulates. If we have 2 wakeups occurring at 5 second intervals and they are not synced, we effectively can wakeup every 2.5 seconds and so on. I guess a good target is to have 1 device level wakeup every 30 seconds or so, but due to cumulation, I tend to complain about anything that happens more often than once a minute.

-Tero

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Nick Piggin]

On Thu, Jul 22, 2010 at 09:22:16AM +0200, Tero.Kristo@nokia.com wrote:
> >-----Original Message-----
> >From: Artem Bityutskiy [mailto:dedekind1@gmail.com]
> >Sent: 22 July, 2010 09:48
> >To: Nick Piggin; Kristo Tero (Nokia-MS/Tampere)
> >Cc: Jens Axboe; linux-fsdevel@vger.kernel.org; linux-
> >kernel@vger.kernel.org
> >Subject: Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads
> >wakeups
> >
> >Hi Nick,
> >
> >On Thu, 2010-07-22 at 13:19 +1000, Nick Piggin wrote:
> >> >  out:
> >> >  	spin_unlock(&inode_lock);
> >> > +
> >> > +	if (wakeup_bdi) {
> >> > +		spin_lock(&bdi->wb_lock);
> >> > +		if (!bdi->wb.task)
> >> > +			wake_up_process(default_backing_dev_info.wb.task);
> >> > +		else
> >> > +			wake_up_process(bdi->wb.task);
> >> > +		spin_unlock(&bdi->wb_lock);
> >> > +	}
> >> >  }
> >>
> >> We really want to wake up the bdi right away when first dirtying
> >> the inode? I haven't looked at where the state of the bdi code is
> >> now, but isn't it better to have a a delay there?
> >
> >Yes, I guess we want to wake up the bdi thread after 5 secs (assuming
> >default settings). I could implement a "wake_up_process_delayed"
> >function which would use a timer, but I think it is not necessary to
> >introduce these complications. We can just wake-up the bdi thread, it'll
> >find out there is nothing to do, and will go sleep for 5 secs. I think
> >this is good enough.
> >
> >IOW, delayed wake-up is not worth the trouble.
> >
> >> And rather than spreading details of how bdi tasks are managed
> >> would you consider putting this into its own function?
> >
> >Sure, will do.
> >
> >> Other than that, I like your patches.
> >
> >Thanks :-)
> >
> >>  Out of interest, is 5 seconds
> >> very detremental to power usage? What is a reasonable goal for
> >> wakeups? (eg. 95%+ of possible efficiency)
> >
> >I cannot tell for sure. In Nokia N900 phone we use OMAP3 and we have
> >dynamic OFF-mode, so we switch off the CPU and peripherals completely
> >when there is nothing to do, and SDRAM stays in low-power auto-refresh
> >mode. Every useless wake-up makes us do a lot of job re-constructing the
> >CPU state. I cannot tell the numbers, but I'm CCing Tero, who is working
> >on OMAP3 PM and makes a lot of battery current measurements, he can
> >provide some numbers.
> 
> Well, it is hard to give any good guidelines here, as it really
> depends on the device architecture, possible power saving modes etc.,
> but I can give some sort of guestimate. Basically I think kernel
> should not generate any extra wakeups at all if it is not doing
> "anything too useful". In ideal world, everything should be interrupt
> driven as much as possible, and we would only have timers for things
> that are clearly visible for user, or can cause some sort of failure
> if neglected. Like if we ignore watchdogs, the device will reset
> itself.
> 
> 5 seconds by itself is not that bad, the reason we want to get rid of
> these is that every single wakeup source cumulates. If we have 2
> wakeups occurring at 5 second intervals and they are not synced, we
> effectively can wakeup every 2.5 seconds and so on. I guess a good
> target is to have 1 device level wakeup every 30 seconds or so, but
> due to cumulation, I tend to complain about anything that happens more
> often than once a minute.

Thanks, I'm just interested in a rough idea. I agree that it would be
better to eliminate polling timeouts completely where possible.

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Nick Piggin]

On Thu, Jul 22, 2010 at 09:48:24AM +0300, Artem Bityutskiy wrote:
> Hi Nick,
> 
> On Thu, 2010-07-22 at 13:19 +1000, Nick Piggin wrote:
> > >  out:
> > >  	spin_unlock(&inode_lock);
> > > +
> > > +	if (wakeup_bdi) {
> > > +		spin_lock(&bdi->wb_lock);
> > > +		if (!bdi->wb.task)
> > > +			wake_up_process(default_backing_dev_info.wb.task);
> > > +		else
> > > +			wake_up_process(bdi->wb.task);
> > > +		spin_unlock(&bdi->wb_lock);
> > > +	}
> > >  }
> > 
> > We really want to wake up the bdi right away when first dirtying
> > the inode? I haven't looked at where the state of the bdi code is
> > now, but isn't it better to have a a delay there?
> 
> Yes, I guess we want to wake up the bdi thread after 5 secs (assuming
> default settings). I could implement a "wake_up_process_delayed"
> function which would use a timer, but I think it is not necessary to
> introduce these complications. We can just wake-up the bdi thread, it'll
> find out there is nothing to do, and will go sleep for 5 secs. I think
> this is good enough.
> 
> IOW, delayed wake-up is not worth the trouble.

I can see what you mean, but I think the designs in core code should
be made as efficient as possible _unless_ there is some complication
in doing otherwise (not the other way around).

This is producing 2 unrequired context switches, so I really would
like to see it done properly. Setting up a timer is really pretty
simple (or if you would care to implement a delayed process wakeup
API, I think that would be useful -- I'm surprised there isn't one
already).

 
> > And rather than spreading details of how bdi tasks are managed
> > would you consider putting this into its own function?
> 
> Sure, will do.
> 
> > Other than that, I like your patches.
> 
> Thanks :-)
> 
> >  Out of interest, is 5 seconds
> > very detremental to power usage? What is a reasonable goal for
> > wakeups? (eg. 95%+ of possible efficiency)
> 
> I cannot tell for sure. In Nokia N900 phone we use OMAP3 and we have
> dynamic OFF-mode, so we switch off the CPU and peripherals completely
> when there is nothing to do, and SDRAM stays in low-power auto-refresh
> mode. Every useless wake-up makes us do a lot of job re-constructing the
> CPU state. I cannot tell the numbers, but I'm CCing Tero, who is working
> on OMAP3 PM and makes a lot of battery current measurements, he can
> provide some numbers.

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 18:05 +1000, Nick Piggin wrote:
> I can see what you mean, but I think the designs in core code should
> be made as efficient as possible _unless_ there is some complication
> in doing otherwise (not the other way around).
> 
> This is producing 2 unrequired context switches, so I really would
> like to see it done properly. Setting up a timer is really pretty
> simple (or if you would care to implement a delayed process wakeup
> API, I think that would be useful -- I'm surprised there isn't one
> already).

OK, NP, I'll work on this.

The only problem I see is that it will involve more maintainers and
trees (I guess Ingo?), and make it even more difficult for me to reach
upstream :-) But let's try and see!

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 18:05 +1000, Nick Piggin wrote:
> I can see what you mean, but I think the designs in core code should
> be made as efficient as possible _unless_ there is some complication
> in doing otherwise (not the other way around).
> 
> This is producing 2 unrequired context switches, so I really would
> like to see it done properly. Setting up a timer is really pretty
> simple (or if you would care to implement a delayed process wakeup
> API, I think that would be useful -- I'm surprised there isn't one
> already).

OK, NP, I'll work on this.

The only problem I see is that it will involve more maintainers and
trees (I guess Ingo?), and make it even more difficult for me to reach
upstream :-) But let's try and see!

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Nick Piggin]

On Thu, Jul 22, 2010 at 11:02:19AM +0300, Artem Bityutskiy wrote:
> On Thu, 2010-07-22 at 18:05 +1000, Nick Piggin wrote:
> > I can see what you mean, but I think the designs in core code should
> > be made as efficient as possible _unless_ there is some complication
> > in doing otherwise (not the other way around).
> > 
> > This is producing 2 unrequired context switches, so I really would
> > like to see it done properly. Setting up a timer is really pretty
> > simple (or if you would care to implement a delayed process wakeup
> > API, I think that would be useful -- I'm surprised there isn't one
> > already).
> 
> OK, NP, I'll work on this.

Thanks.

 
> The only problem I see is that it will involve more maintainers and
> trees (I guess Ingo?), and make it even more difficult for me to reach
> upstream :-) But let's try and see!

I wouldn't worry about that. It's so simple that if you end up coding
the helper function to do a timer delayed wakeup, just send it to Jens
in your series, cc Ingo on it if you'd like.

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 18:05 +1000, Nick Piggin wrote:
> This is producing 2 unrequired context switches, so I really would
> like to see it done properly. Setting up a timer is really pretty
> simple (or if you would care to implement a delayed process wakeup
> API, I think that would be useful -- I'm surprised there isn't one
> already).

OK, a generic 'wake_up_process_timeout()' would need a timer, and it
cannot set it up on stack because it has to return immediately, this is
why we do not have such a generic helper, I think.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 18:05 +1000, Nick Piggin wrote:
> This is producing 2 unrequired context switches, so I really would
> like to see it done properly. Setting up a timer is really pretty
> simple (or if you would care to implement a delayed process wakeup
> API, I think that would be useful -- I'm surprised there isn't one
> already).

OK, a generic 'wake_up_process_timeout()' would need a timer, and it
cannot set it up on stack because it has to return immediately, this is
why we do not have such a generic helper, I think.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@vger.kernel.org
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Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 18:05 +1000, Nick Piggin wrote:
> This is producing 2 unrequired context switches, so I really would
> like to see it done properly. Setting up a timer is really pretty
> simple (or if you would care to implement a delayed process wakeup
> API, I think that would be useful -- I'm surprised there isn't one
> already).

It reduces flusher thread wake-up count a lot, BTW. I've just sent v4 of
the patch set which implements this, see patch 12.

Nick, could you please take a look at patch 10 in that v4 series. I've
added memory barriers, they are always tricky, and last time I remember
you helped me to use barriers properly.

Thanks!

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Artem Bityutskiy]

On Thu, 2010-07-22 at 18:05 +1000, Nick Piggin wrote:
> This is producing 2 unrequired context switches, so I really would
> like to see it done properly. Setting up a timer is really pretty
> simple (or if you would care to implement a delayed process wakeup
> API, I think that would be useful -- I'm surprised there isn't one
> already).

It reduces flusher thread wake-up count a lot, BTW. I've just sent v4 of
the patch set which implements this, see patch 12.

Nick, could you please take a look at patch 10 in that v4 series. I've
added memory barriers, they are always tricky, and last time I remember
you helped me to use barriers properly.

Thanks!

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html