On Tue, Mar 08, 2011 at 11:31:13PM +0100, Jan Kara wrote:

This patch changes balance_dirty_pages() throttling so that the function does
not submit writes on its own but rather waits for flusher thread to do enough
writes. This has an advantage that we have a single source of IO allowing for
better writeback locality. Also we do not have to reenter filesystems from a
non-trivial context.

The waiting is implemented as follows: Whenever we decide to throttle a task in
balance_dirty_pages(), task adds itself to a list of tasks that are throttled
against that bdi and goes to sleep waiting to receive specified amount of page
IO completions. Once in a while (currently HZ/10, later the interval should be
autotuned based on observed IO completion rate), accumulated page IO
completions are distributed equally among waiting tasks.

This waiting scheme has been chosen so that waiting time in
balance_dirty_pages() is proportional to
  number_waited_pages * number_of_waiters.
In particular it does not depend on the total number of pages being waited for,
thus providing possibly a fairer results. Note that the dependency on the
number of waiters is inevitable, since all the waiters compete for a common
resource so their number has to be somehow reflected in waiting time.

CC: Andrew Morton <akpm@linux-foundation.org>
CC: Christoph Hellwig <hch@infradead.org>
CC: Dave Chinner <david@fromorbit.com>
CC: Wu Fengguang <redacted>
CC: Peter Zijlstra <redacted>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 include/linux/backing-dev.h      |    7 +
 include/linux/writeback.h        |    1 +
 include/trace/events/writeback.h |   65 +++++++-
 mm/backing-dev.c                 |    8 +
 mm/page-writeback.c              |  345 +++++++++++++++++++++++++-------------
 5 files changed, 310 insertions(+), 116 deletions(-)

[..]

+/*
+ * balance_dirty_pages() must be called by processes which are generating dirty
+ * data.  It looks at the number of dirty pages in the machine and will force
+ * the caller to perform writeback if the system is over `vm_dirty_ratio'.
+ * If we're over `background_thresh' then the writeback threads are woken to
+ * perform some writeout.
+ */
+static void balance_dirty_pages(struct address_space *mapping,
+				unsigned long write_chunk)
+{
+	struct backing_dev_info *bdi = mapping->backing_dev_info;
+	struct balance_waiter bw;
+	struct dirty_limit_state st;
+	int dirty_exceeded = check_dirty_limits(bdi, &st);
+
+	if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT ||
+	    (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
+	     !bdi_task_limit_exceeded(&st, current))) {
+		if (bdi->dirty_exceeded &&
+		    dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT)
+			bdi->dirty_exceeded = 0;
 		/*
-		 * Increase the delay for each loop, up to our previous
-		 * default of taking a 100ms nap.
+		 * In laptop mode, we wait until hitting the higher threshold
+		 * before starting background writeout, and then write out all
+		 * the way down to the lower threshold.  So slow writers cause
+		 * minimal disk activity.
+		 *
+		 * In normal mode, we start background writeout at the lower
+		 * background_thresh, to keep the amount of dirty memory low.
 		 */
-		pause <<= 1;
-		if (pause > HZ / 10)
-			pause = HZ / 10;
+		if (!laptop_mode && dirty_exceeded == DIRTY_EXCEED_BACKGROUND)
+			bdi_start_background_writeback(bdi);
+		return;
 	}
 
-	/* Clear dirty_exceeded flag only when no task can exceed the limit */
-	if (!min_dirty_exceeded && bdi->dirty_exceeded)
-		bdi->dirty_exceeded = 0;
+	if (!bdi->dirty_exceeded)
+		bdi->dirty_exceeded = 1;

Will it make sense to move out bdi_task_limit_exceeded() check in a
separate if condition statement as follows. May be this is little
easier to read.

	if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT) {
		if (bdi->dirty_exceeded)
			bdi->dirty_exceeded = 0;

		if (!laptop_mode && dirty_exceeded == DIRTY_EXCEED_BACKGROUND)
			bdi_start_background_writeback(bdi);

		return;
	}

	if (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
	    !bdi_task_limit_exceeded(&st, current))
		return;   

	/* Either task is throttled or we crossed global dirty ratio */
	if (!bdi->dirty_exceeded)
		bdi->dirty_exceeded = 1;

Thanks
Vivek

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